Title of Invention	GENERATION OF SPATIAL DOWNMIXES FROM PARAMETRIC REPRESENTATIONS OF MULTI CHANNEL SIGNALS
Abstract	A headphone down mix signal (314) can be efficiently derived from a parametric down mix of a multi-channel signal (312), when modified HRTFs (310) (head related transfer functions) are derived from HRTFs (308) of a multi-channel signal using a level parameter (306) having information on a level relation between two channels of the multi-channel signals such that a modified HRTF (310) is stronger influenced by the HRTF (308) of a channel having a higher level than by the HRTF (308) of a channel having a lower level. Modified HRTFs (310) are derived within the decoding process taking into account the relative strength of the channels associated to the HRTFs (308). The HRTFs (308) are thus modified such that a down mix signal (314) of a parametric representation of a multi-channel signal can directly be used to synthesize the headphone down mix signal (314) without the need of an intermediate full parametric multi-channel reconstruction of the parametric down mix.

Title of Invention

GENERATION OF SPATIAL DOWNMIXES FROM PARAMETRIC REPRESENTATIONS OF MULTI CHANNEL SIGNALS

Abstract

A headphone down mix signal (314) can be efficiently derived from a parametric down mix of a multi-channel signal (312), when modified HRTFs (310) (head related transfer functions) are derived from HRTFs (308) of a multi-channel signal using a level parameter (306) having information on a level relation between two channels of the multi-channel signals such that a modified HRTF (310) is stronger influenced by the HRTF (308) of a channel having a higher level than by the HRTF (308) of a channel having a lower level. Modified HRTFs (310) are derived within the decoding process taking into account the relative strength of the channels associated to the HRTFs (308). The HRTFs (308) are thus modified such that a down mix signal (314) of a parametric representation of a multi-channel signal can directly be used to synthesize the headphone down mix signal (314) without the need of an intermediate full parametric multi-channel reconstruction of the parametric down mix.

Full Text	Generation of spatial downmixes from parametric representations of multi channel signals Field of the Invention The present invention relates to decoding of encoded multi- channel audio signals based on a parametric multi-channel representation and in particular to the generation of 2- channel downmixes providing a spatial listening experience as for example a headphone compatible down mix or a spatial downmix for 2 speaker setups. Background of the Invention in Prior Art Recent development in audio coding has made available the ability to recreate a multi-channel representation of an audio signal based on a stereo (or mono) signal and corresponding control data. These methods differ substantially from older matrix based solutions such as Dolby Prologic, since additional control data is transmitted to control the re-creation, also referred to as up-mix, of the surround channels based on the transmitted mono or stereo channels. Hence, such a parametric multi-channel audio decoder, e.g. MPEG Surround, reconstructs N channels based on M transmitted channels, where N > M, and the additional control data. The additional control data represents a significant lower data rate than transmitting the all N channels, making the coding very efficient while at the same time ensuring compatibility with both M channel devices and N channel devices. These parametric surround coding methods usually comprise a parameterization of the surround signal based on IID (Inter channel intensity Difference) or CLD (Channel Level Difference) and ICC (Inter Channel Coherence). These parameters describe power ratios and correlations, between channel pairs in the up-mix process. Further parameters also used in prior art comprise prediction parameters used to predict intermediate or output channels during the up- mix procedure. Other developments in reproduction of multi-channel audio content have provided means to obtain a spatial listening impression using stereo headphones. To achieve a spatial listening experience using only the two speakers of the headphones, multi-channel signals are down mixed to stereo signals using HRTF (head related transfer functions), intended to take into account the extremely complex transmission characteristics of a human head for providing the spatial listening experience. Another related approach is to use a conventional 2-channel playback environment and to filter the channels of a multi- channel audio signal with appropriate filters to achieve a listening experience close to that of the playback with the original number of speakers. The processing of the signals is similar as in the case of headphone playback to create an appropriate "spatial stereo down mix" having the desired properties. Contrary to the headphone case, the signal of both speakers directly reaches both ears of a listener, causing undesired "crosstalk effects". As this has to be taken into account for optimal reproduction quality, the filters used for signal processing are commonly called crosstalk-cancellation filters. Generally, the aim of this technique is to extend the possible range of sound sources outside the stereo speaker base by cancellation of inherent crosstalk using complex crosstalk-cancellation filters. Because of the complex filtering, HRTF filters are very long, i.e. they may comprise several hundreds of filter taps each. For the same reason, it is hardly possible to find a parameterization of the filters that works well enough not to degrade the perceptual quality when used instead of the actual filter. Thus, on the one hand, bit. saving parametric representations of multi-channel signals do exist that allow for an efficient transport of an encoded multi- channel signal. On the other hand, elegant ways to create a spatial listening experience for a multi-channel signal when using stereo headphones or stereo speakers only are known. However, these require the full number of channels of the multi-channel signal as input for the application of the head related transfer functions that create the headphone down mix signal. Thus, either the full set of multi-channels signals has to be transmitted or a parametric representation has to be fully reconstructed before applying the head related transfer functions or the crosstalk-cancellation filters and thus either the transmission bandwidth or the computational complexity is unacceptably high. Summary of the invention It is the object of the present invention to provide a concept allowing for a more efficient reconstruction of a 2-channel signal providing a spatial listening experience using parametric representations of multi-channel signals. In accordance with a first aspect of the present invention, this object is achieved by a decoder for deriving a headphone down mix signal using a representation of a down mix of a multi-channel signal and using a level parameter having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions related to the two channels of the multi-channel signal, comprising: a filter calculator for deriving modified head-related transfer functions by weighting the head-related transfer functions of the two channels using the level parameter such that a modified head-related transfer function is stronger influenced by the head- related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and a synthesizer for deriving the headphone down mix signal using the modified head-related transfer functions and the representation of the down mix signal. In accordance with a second aspect of the present invention, this object is achieved by a binaural decoder, comprising: a decoder for deriving a headphone down mix signal using a representation of a down mix of a multi- channel signal and using a level parameter having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions related to the two channels of the multi-channel signal, comprising: a filter calculator for deriving modified head-related transfer functions by weighting the head-related transfer functions of the two channels using the level parameter such that a modified head-related transfer function is stronger influenced by the head- related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and a synthesizer for deriving the headphone down mix signal using the modified head-related transfer functions and the representation of the down mix signal; an analysis filterbanJc for deriving the representation of the down mix of the multi-channel signal by subband filtering the downmix of the multi- channel signal; and a synthesis filterbank for deriving a time-domain headphone signal by synthesizing the headphone down mix signal. In accordance with a third aspect of the present invention, this object is achieved by Method of deriving a headphone down mix signal using a representation of a down mix of a multi-channel signal and using a level parameter having formation on a level relation between two channels of the lti-channel signal and using head-related transfer notions related to the two channels of the multi-channel - -gnal, the method comprising: deriving, using the level parameter, modified head-related transfer functions by weighting the head-related transfer functions of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal using the modified head-related transfer functions and the representation of the down mix signal. In accordance with a fourth aspect of the present invention, this object is achieved by a receiver or audio player having a decoder for deriving a headphone down mix signal using a representation of a down mix of a multi- channel signal and using a level parameter having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions related to the two channels of the multi-channel signal, comprising: a filter calculator for deriving modified head-related transfer functions by weighting the head-related transfer functions of the two channels using the level parameter such that a modified head-related transfer function is stronger influenced by the head- related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and a synthesizer for deriving the headphone down mix signal using the modified head-related transfer functions and the representation of the down mix signal. In accordance with a fifth aspect of the present invention, this object is achieved by a method of receiving or audio playing, the method having a method for deriving a headphone down mix signal using a representation of a down mix of a multi-channel signal and using a level parameter having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions related to the two channels of the multi-channel signal, the method comprising: deriving, using the level parameter, modified head-related transfer functions by weighting the head-related transfer functions of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal using the modified head-related transfer functions and the representation of the down mix signal. In accordance with a sixth aspect of the present invention, this object is achieved by a decoder for deriving a spatial stereo down mix signal using a representation of a down mix of a multi-channel signal and using a level parameter having information on a level relation between two channels of the multi-channel signal and using crosstalk cancellation filters related to the two channels of the multi-channel signal, comprising: a filter calculator for deriving modified crosstalk cancellation filters by weighting the crosstalk cancellation filters of the two channels using the level parameter such that a modified crosstalk cancellation filters is stronger influenced by the crosstalk cancellation filter of a channel having a higher level than by the crosstalk cancellation filter of a channel having a lower level; and a synthesizer for deriving the spatial stereo down mix signal using the modified crosstalk cancellation filters and the representation of the down mix signal. The present invention is based on the finding that a headphone down mix signal can be derived from a parametric down mix of a multi-channel signal, when a filter calculator is used for deriving modified HRTFs (head related transfer functions) from original HRTFs of the multi-channel signal and when the filter converter uses a level parameter having information on a level relation between two channels of the multi-channel signal such that modified HRTFs are stronger influenced by the HRTF of a channel having a higher level than by the HRTF of a channel having a lower level. Modified HRTFs are derived during the decoding process taking into account the relative strength of the channels associated to the HRTFs. The original HRTFs are modified such, that a down mix signal of a parametric representation of a multi-channel signal can be directly used to synthesize the headphone down mix signal without the need of a full parametric multi-channel reconstruction of the parametric down mix signal. In one embodiment of the present invention, an inventive decoder is used implementing a parametric multi-channel reconstruction as well as an inventive binaural reconstruction of a transmitted parametric down mix of an original multi-channel signal. According to the present invention, a full reconstruction of the multi-channel signal prior to binaural down mixing is not required, having the obvious great advantage of a strongly reduced computational complexity. This allows, for example, mobile devices having only limited energy reservoirs to extend the playback length significantly. A further advantage is that the same device can serve as provider for complete multi- channel signals (for example 5.1, 7.1, 7.2 signals) as well as for a binaural down mix of the signal having a spatial listening experience even when using only two-speaker headphones. This might, for example, be extremely advantageous in home-entertainment configurations. In a further embodiment of the present invention a filter calculator is used for deriving modified HRTFs not only operative to combine the HRTFs of two channels by applying individual weighting factors to the HRTF but by introducing additional phase factors for each HRTF to be combined. The introduction of the phase factor has the advantage of achieving a delay compensation of two filters prior to their superposition or combination. This leads to a combined response that models a main delay time corresponding to an intermediate position between the front and the back speakers. A second advantage is that a gain factor, which has to be applied during the combination of the filters to ensure energy conservation, is much more stable with respect to its behavior with frequency than without the introduction of the phase factor. This is particular relevant for the inventive concept, as according to an embodiment of the present invention a representation of a down mix of a multi-channel signal is processed within a interbank domain to derive the headphone down mix signal. As such, different frequency bands of the representation of the down mix signal are to be processed separately and therefore, a smooth behavior of the individually applied gain functions is vital. In a further embodiment of the present invention the head- related transfer functions are converted to subband-filters for the subband domains such that the total number of modified HRTFs used in the subband domain is smaller than the total number of original HRTFs. This has the evident advantage that the computational complexity for deriving headphone down mixed signals is even decreased compared to the down mixing using standard HRTF filters. Implementing the inventive concept allows for the use of extremely long HRTFs and thus allows for the reconstruction of headphone down mix signals based on a representation of a parametric down mix of a multi-channel signal with excellent perceptual quality. Furthermore, using the inventive concept on crosstalk- cancellation filters allows for the generation of a spatial stereo down mix to be used with a standard 2 speaker setup based on a representation of a parametric down mix of a multi-channel signal with excellent perceptual quality. One further big advantage.of the inventive decoding concept is that a single inventive binaural decoder implementing the inventive concept may be used to derive a binaural downmix as well as a multi-channel reconstruction of a transmitted down mix taking into account the additionally transmitted spatial parameters. In one embodiment of the present invention an inventive binaural decoder is having an analysis filterbank for deriving the representation of the down mix of the multi- channel signal in a subband domain and an inventive decoder implementing the calculation of the modified HRTFs. The decoder further comprises a synthesis filterbank to finally derive a time domain representation of a headphone down mix signal, which is ready to be played back by any conventional audio playback equipment. In the following paragraphs, prior art parametric multi- channel decoding schemes and binaural decoding schemes are explained in more detail referencing the accompanying drawings, to more clearly outline the great advantages of the inventive concept. Most of the embodiments of the present invention detailed below describe the inventive concept using HRTFs. As previously noted, HRTF processing is similar to the use of crosstalk-cancellation filters. Therefore, all of the embodiments are to be understood as to refer to HRTF processing as well as to crosstalk-cancellation filters. In other words, all HRTF Filters could be replaced by crosstalk-cancellation filters below to apply the inventive concept to the use of crosstalk-cancellation filters. Brief Description of the Drawings Preferred embodiments of the present invention are subsequently described by referring to the enclosed drawings, wherein: Fig. 1 shows a conventional binaural synthesis using HRTFs; Fig. 1b shows a conventional use of crosstalk-cancellation filters; Fig. 2 shows an example of a multi-channel spatial encoder; Fig. 3 shows an example for prior art spatial/binaural- decoders; Fig. 4 shows an example of a parametric nuilti-channel encoder; Fig. 5 shows an example of a parametric multi-channel decoder; Fig. 6 shows an example of an inventive decoder; Fig. 7 shows a block diagram illustrating the concept of transforming filters into the subband domain; Fig. 8 shows an example of an inventive decoder; Fig. 9 shows a further example of an inventive decoder; and Fig. 10 shows an example for an inventive receiver or audio player. Detailed Description of Preferred Embodiments The below-described embodiments are merely illustrative for the principles of the present invention for Binaural Decoding of Multi-Channel Signals By Morphed HRTF Filtering. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein. In order to better outline the features and advantages of the present invention a more elaborate description of prior art will be given now. A conventional binaural synthesis algorithm is outlined in Fig. 1. A set of input channels (left front (LF} , right front (RF), left surround (LS), right surround (RS) and center (C) ), 10a, 10b, 10c, lOd and lOe is filtered by a set of HRTFs 12a to 12j. Each input signal is split into two signals (a left "L" and a right "R" component! wherein each of these signal components is subsequently filtered by an HRTF corresponding to the desired sound position. Finally, all left ear signals are summed by a summer 14a to generate the left binaural output signal L and the right- ear signals are summed by a summer 14b to generate the right binaural output signal R. It may be noted that HR'EF convolution can principally be performed in the time domain, but it is often preferred to perform filtering in the frequency domain due to the increased computational efficiency. That means that, the summation shown in Fig- 1 is also performed in the frequency domain and a subsequent transformation into a time domain is additionally required. Fig. 1b illustrates crosstalk cancellation processing intended to achieve a spatial listening impression using only two speakers of a standard stereo playback environment. The aim is reproduction of a multi-channel signal by means of a stereo playback system having only two speakers 16a and 16b such that a listener 18 experiences a spatial listening experience. Am major difference with respect to headphone reproduction is that signals of both speakers 16a and 16b directly reach both ears of listener 18. The signals indicated by dashed lines (crosstalk) therefore have to be taken into account additionally. For ease of explanation only a 3 channel input signal having 3 sources 20a to 20c is illustrated in Fig. 1b. It goes without saying that the scenario can in principle be extended to arbitrary number of channels. To derive the stereo signal to be played back, each input source is processed by 2 of the crosstalk cancellation filters 21a to 2lf, one filter for each channel of the playback signal. Finally, all filtered signals for the left playback channel 16a and the right playback channel 16b are summed up for playback. It is evident that the crosstalk cancellation filters will in general be different for each source 20a and 20b (depending on its desired perceived position) and that they could furthermore even depend on the listener. Owing to the high flexibility of the inventive concept, one benefits from high flexibility in the design and application of the crosstalk cancellation filters such that filters can be optimized for each application or playback device individually. One further advantage is that the method is computationally extremely efficient, since only 2 synthesis filterbanks are required. A principle sketch of a spatial audio encoder is shown in Fig. 2. In such a basic encoding scenario., a spatial audio decoder 40 comprises a spatial encoder 42, a down mix encoder 44 and a multiplexer 46. A multi-channel input signal 50 is analyzed by the spatial encoder 42, extracting spatial parameters describing spatial properties of the multi-channel input signal that have to be transmitted to the decoder side. The down mixed signal generated by the spatial encoder 42 may for example be a monophonic or a stereo signal depending on different encoding scenarios. The down mix encoder 4 4 may then encode the monophonic or stereo down mix signal using any conventional mono or stereo audio coding scheme. The multiplexer 46 creates an output bit stream by combining the spatial parameters and the encoded down mix signal into the output bit stream. Fig. 3 shows a possible direct combination of a multi- channel decoder corresponding to the encoder of Fig. 2 and a binaural synthesis method as, for example, outlined in Fig. 1. As can be seen, the prior art approach of combining the features is simple and straight forward. The set-up comprises a de-multiplexer 60, a down mix decoder 62, a spatial decoder 64 and a binaural synthesizer 66. An input bit stream 68 is de-multiplexed resulting in spatial parameters 70 and a down mix signal bit stream. The latter down-mix signal bit stream is decoded by the down mix decoder 62 using a conventional mono or stereo decoder. The decoded down mix is input, together with the spatial parameters 70, into the spatial decoder 64 that generates a multi-channel output signal 72 having the spatial properties indicated by the spatial parameters 70. Having a multi-channel signal 72 completely reconstructed, the approach of simply adding a binaural synthesizer 66 to implement the binaural synthesis concept of Fig. 1 is straight-forward. Therefore, the multi-channel output signal 72 is used as an input for the binaural synthesizer 66 which processes the multi-channel output signal to derive the resulting binaural output signal 74. The approach shown in Fig. 3 has at least three disadvantages: a complete multi-channel signal representation has to be computed as an intermediate step, followed by HRTF convolution and down mixing in the binaural synthesis. Although HRTF convolution should be performed on a per channel basis, given the fact that each audio channel can have a different spatial position, this is an undesirable situation from a complexity point of view. Thus, computational complexity is high and energy is wasted. The spatial decoder operates in a filterbank (QMF) domain. HRTF convolution, on the other hand, is typically applied in the FFT domain. Therefore, a cascade of a multi-channel QMF synthesis filterbank, a multi-channel DFT transform, and a stereo inverse DFT transform is necessary, resulting in a system with high computational demands. Coding artefacts created by the spatial decoder to create a multi-channel reconstruction will be audible, and possibly enhanced in the (stereo) binaural output. An even more detailed description of multi-channel encoding and decoding is given in Figs. 4 and 5. The spatial encoder 100 shown in Fig. 4 comprises a first OTT (1-to-2-encoder) 102a, a second OTT 102b and a TTT box (3-to-2-encoder) 104. A multi-channel input signal 106 consisting of LF, LS, C, RF, RS (left-front, left- surround, center, right-front and right-surround) channels is processed by the spatial encoder 100. The OTT boxes receive two input audio channels each, and derive a single monophonic audio output channel and associated spatial parameters, the parameters having information on the spatial properties of the original channels with respect to one another or with respect to the output channel (for example CLD, ICC, parameters) . In the encoder 100, the LF and the LS channels are processed by OTT encoder 102a and the RF and RS channels are processed by the OTT encoder 102b. Two signals, L and R are generated, the one only having information on the left side and the other only having information on the right side. The signals L, R and C are further processed by the TTT encoder 104, generating a stereo down mix and additional parameters. The parameters resulting from the TTT encoder typically consist of a pair of prediction coefficients for each parameter band, or a pair of level differences to describe the energy ratios of the three input signals. The parameters of the 'OTT' encoders consist of level differences and coherence or cross-correlation values between the input signals for each frequency band. It may be noted that although the schematic sketch of the spatial encoder 100 points to a sequential processing of the individual channels of the down mix signal during the encoding, it is also possible to implement the complete down mixing process of the encoder 100 within one single matrix operation. Fig. 5 shows a corresponding spatial decoder, receiving as an input the down mix signals as provided by the encoder of Fig. 4 and the corresponding spatial parameters. The spatial decoder 120 comprises a 2-to-3-decoder 122 and 1-to-2-decoders 124a to 124c. The down mix signals Lo and Ro are input into the 2-to-3-decoder 122 that recreates a center channel C, a right channel R and a left channel L. These three channels are further processed by the OTT- decoders 124a to 124c yielding six output channels. It may be noted that the derivation of a low-frequency enhancement channel LFE is not mandatory and can be omitted such that one single OTT-encoder may be saved within the surround decoder 120 shown in Fig. 5. According to one embodiment of the present invention the ihventive concept is applied in a decoder as shown in Fig. 6. The inventive decoder 200 comprises a 2-to-3-decoder 104 and six HRTF-filters 106a to 106f. A stereo input signal (Lo, Ro) is processed by the TTT-decoder 104, deriving three signals L, C and R. It may be noted, that the stereo input signal is assumed to be delivered within a subband domain, since the TTT-encoder may be the same encoder as shown in Fig. 5 and hence adapted to be operative on subband signals. The signals L, R and C are subject to HRTF parameter processing by the HRTF filters 106a to 106f. The resulting 6 channels are summed to generate the stereo binaural output pair (Lb, Rb) . The TTT decoder, 106, can be described as the following matrix operation: with matrix entries mxy dependent on the spatial parameters. The relation of spatial parameters and matrix entries is identical to those relations as in the 5.1- multichannel MPEG surround decoder. Each of the three resulting signals L, R, and C are split in two and processed with HRTF parameters corresponding to the desired (perceived) position of these sound sources. For the center channel (C), the spatial parameters of the sound source position can be applied directly, resulting in two output signals for the center, LB(C) and RB(C): For the left (L) channel, the HFTF parameters from the left-front and left-surround channels are combined into a single HRTF parameter set, using the weights wlf and wrt• The resulting 'composite' HRTF parameters simulate the effect of both the front and surround channels in a statistical sense. The following equations are used to generate the binaural output pair (IB, RB) for the left channel: In a similar fashion, the binaural output for the right channel is obtained according to: Given the above definitions of LA{C), RB(C), LB(L), RB{L), LB(R) and RS(R), the complete LB and RB signals can be derived from a single 2 by 2 matrix given the stereo input signal: In the above it was assumed that the Hr(X) elements, for Y= L0,R0 and X = L,R,C, were complex scalars. However, the present invention teaches how to extend the approach of a 2 by 2 matrix binaural decoder to handle arbitrary length HRTF filters. In order to achieve this, the present invention comprises the following steps: • Transform the HRTF filter responses to a filterbank domain; • Overall delay difference or phase difference extraction from HRTF filter pairs; • Morph the responses of the HRTF filter pair as a function- of the CLD parameters • Gain adjustment This is achieved by replacing the six complex gainsHy(X) for Y= L0,R0 and X = L,R,C with six filters. These filters are derived from the ten filters Hr(X) for T = L0,i?oand X = Lf,LS,Rf, Rs, C, which describe the given HRTF filter responses in the QMF domain. These QMF representations can be achieved according to the method described in one of the subsequent paragraphs. In other words, the present invention teaches a concept for deriving modified HRTFs as by modifying (morphing) of the front end surround channel filters using a complex linear combination according to As it can be seen from the above formula, deriving of the modified HRTFs is a weighted superposition of the original HRTFs, additionally applying phase factors. The weights ws, Wf depend on the CLD parameters intended to be used by the OTT decoders 124a and 124b of Fig. 5. The weights wrf and wrs depend on the CLD parameter of the 'OTT' box for Rf and Rs: The weights wlf and w1s depend on the CLD parameter of the 'OTT'box for Lf and Ls: The phase parameter xy can be derived from the main delay time difference txy between the front and back HRTF filters and the subband index nof the QMF bank: The role of this phase parameter in the morphing of filters is twofold. First, it realizes a delay compensation of the two filters prior to superposition which leads to a combined response which models a main delay time corresponding to a source position between the front and the back speakers. Second, it makes the necessary gain compensation factor g much more stable and slowly varying over frequency than in the case of simple superposition with xy=0. The gain factor g is determined by the incoherent addition power rule, and pxy is the real value of the normalized complex cross correlation between the filters For the above equations, P denotes a parameter describing an average level per frequency band for the impulse response of the filter specified by the indexes. This mean intensity is of course easily derived, once the filter response function are known. In the case of simple superposition with xy=0, the value of Pxy varies in an erratic and oscillatory manner as a function of frequency, which leads to the need for extensive gain adjustment. In practical implementation it is necessary to limit the value of the gaing and a remaining spectral colorization of the signal cannot be avoided. In contrast, the use of morphing with a delay based phase compensation as taught by the present invention leads to a smooth behaviour of pxy as a function of frequency. This value is often even close to one for natural HRTF derived filter pairs since they differ mainly in delay and amplitude, and the purpose of the phase parameter is to take the delay difference into account in the QMF filterbank domain. An alternative beneficial choice of phase parameter xy taught by the present invention is given by the phase angle of the normalized complex cross correlation between the filters and unwrapping the phase values with standard unwrapping techniques as a function of the subband index n of the QMF bank. This choice has the consequence that pxy is never negative and hence the compensation gain gsatisfies 1/2 parameter enables the morphing of the front and surround channel filters in situations where a main delay time difference txy is not available. For the embodiment of the present invention as described above/ it is taught to accurately transform the HRTFs into an efficient representation of the HRTF filters within the QMF domain. Fig. 7 gives a principle sketch of the concept to accurately transform time-domain filters into filters within the subband domain having the same net effect on ,a reconstructed signal. Fig. 7 shows a complex analysis bank 300, a synthesis bank 302 corresponding to the analysis bank 300, a filter converter 304 and a subband filter 306. An input signal 310 is provided for which a filter 312 is known having desired properties. The aim of the implementation of the filter converter 304 is that the output signal 314 has the same characteristics after analysis by the analysis filterbank 300, subsequent subband filtering 306 and synthesis 302 as if it would have when filtered by filter 312 in the time domain. The task of providing a number of subband filters corresponding to the number of subbands used is fulfilled by filter converter 304. The following description outlines a method for implementing a given FIR filter A(v)in the complex QMF subband domain. The principle of operation is shown in Figure 7. Here, the subband filtering is simply the application of one complex valued FIR filter for each subband, n =0,1,.,.,L-1 to transform the original indices cn into their filtered counterparts dn according to the following formula: Observe that this is different from well known methods developed for critically sampled filterbanks, since those methods require multiband filtering with longer responses. The key component is the filter converter, which converts any time domain FIR filter into the complex subband domain filters. Since the complex QMF subband domain is oversampled, there is no canonical set of subband filters for a given time domain filter. Different subband filters can have the same net effect of the time domain signal. What will be described here is a particularly attractive approximate solution, which is obtained by restricting the filter converter to be a complex analysis bank similar to the QMF. Assuming that the filter converter prototype is of length 64Kq, a real 64KH tap FIR filter is transformed into a set of 64 complex KH+KQ-l tap subband filters. ForKQ=3, a FIR filter of 1024 taps is converted into 18 tap subband filtering with an approximation quality of 50 dB. The subband filter taps are computed from the; formula where q(V) is a FIR prototype filter derived from the QMF prototype filter- As it can be seen, this is just a complex filterbank analysis of the given filterh(V)• In the following, the inventive concept will be outlined for a further embodiment of the present invention, where a multi-channel parametric representation for a multi-channel signal having five channels is available. Please note that in this particular embodiment of the present invention, original 10 HRTF filters VY,X (as for example given by a QMF representation of the filters 12a to 12j of Fig 1) are morphed into six filters hV/X for Y = L, R and X = L,R,C The ten filters vyx for Y=L,R and X = FL,BL,FR,BR,C describe the given HRTF filter responses in a hybrid QMF domain - the combination of the front and surround channel filters is performed with a complex linear combination according to equal to n forA: = 0,1 In cases where there are two choices, ±n , for the increment, the sign of the increment for a phase measurement in the interval ]-,]is chosen. Finally, normalized phase compensated cross correlations are defined for Y=LJEt and X=L,R by Please note that in the case where the multi-channel processing is performed within a hybrid subband domain, i.e. in a domain where subbands are further decomposed into different frequency bands, a mapping of the HRTF responses to the hybrid band filters may for example be performed as follows: As in the case without an hybrid filterbank, the ten given HRTF impulse responses from source X = FL,BL,FR,BR,C to target Y = L,R are all converted into QMF subband filters according to the method outlined above. The result is the ten subband filters vyx with components for QMF subband m=0,l,...,63and QMF time slot / = 0,1,...,Lq-1. Let the index mapping from the hybrid band it to QMF band mbe denoted by m=Q(k) . Then the HRTF filters vyxin the hybrid band domain are defined by For the specific embodiment described in the previous paragraphs, the filter conversion of HRTF filters into the QMF domain can be implemented as follows, given a FIR filter h(y) of length Nh to be transferred to the complex QMF subband domain: The subband filtering consists of the separate application of one complex valued FIR filter hm(l) for each QMF subband, m' = 0,V..,63 • The key component is the filter converter, which converts the given time domain FIR filter h(v)±nto the complex subband domain filters hm(l). The filter converter is a complex analysis bank similar to the QMF analysis bank. Its prototype filter q(v) is of length 192. An extension with zeros of the time domain FIR filter is defined by Although the inventive concept has been detailed with respect to a down mix signal having two channels, i.e. a transmitted stereo signal, the application of the inventive concept is by no means restricted to a scenario having a stereo-down mix signal. Summarizing, the present invention relates to the problem of using long HRTF or crosstalk cancellation filters for binaural rendering of parametric multi-channel signals. The invention teaches new ways to extend the parametric HRTF approach to arbitrary length of HRTF filters. The present invention comprises the following features: Multiplying the stereo down mix signal by a 2 by 2 matrix where every matrix element is a FIR filter or arbitrary length (as given by the HRTF filter); Deriving the filters in the 2 by 2 matrix by morphing the original HRTF filters based on the transmitted multi-channel parameters; - Calculation of the morphing of the HRTF filters so that the correct spectral envelope and overall energy is obtained. Fig. 8 shows an example for an inventive decoder 300 for deriving a headphone down mix signal. The decoder comprises a filter calculator 302 and a synthesizer 304. The filter calculator receives as a first input level parameters 306 and as a second input HRTFs (head-related transfer functions) 308 to derive modified HRTFs 310 that have the same net effect on a signal when applied to the signal in the subband domain than the head-related transfer functions 308 applied in the time domain. The modified HRTFs 310 serve as first input to the synthesizer 304 that receives as a second input a representation of a down-mix signal 312 within a subband domain. The representation of the down-mix signal 312 is derived by a parametric multi-channel encoder and intended to be used as a basis for reconstruction of a full multi-channel signal by a multi-channel decoder. The synthesizer 404 is thus able to derive a headphone down-mix signal 314 using the modified HRTFs 310 and the representation of the down-mix signal 312. It may be noted, that the HRTFs could be provided in any possible parametric representation, for example as the transfer function associated to the filter, as the impulse response of the filter or as a series of tap coefficients for an FIR-filter. The previous examples assume, that the representation of the down-mix signal is already supplied as a filterbank representation, i.e. as samples derived by a filterbank. In practical applications, however, a time-domain down-mix signal is typically supplied and transmitted to allow also for a direct playback of the submitted signal in simple playback environments. Therefore, in Fig. 9 in a further embodiment of the present invention, where a binaural compatible decoder 400 comprises an analysis filterbank 402 and a synthesis filterbank 404 and an inventive decoder, which could, for example, be the decoder 300 of Fig. 8. Decoder functionalities and their descriptions are applicable in Fig. 9 as well as in Fig. 8 and the description of the decoder 300 will be omitted within the following paragraph. The analysis filterbank 402 receives a downmix of a multi- channel signal 406 as created by a multi-channel parametric encoder. The analysis filterbank 402 derives the filterbank representation of the received down mix signal 406 which is then input into decoder 300 that derives a headphone downmix signal 408, still within the filterbank domain. That is, the down mix is represented by a multitude of samples or coefficients within the frequency bands introduced by the analysis filterbank 402. Therefore, to provide a final headphone down mix signal 410 in the time domain the headphone downmix signal 408 is input into synthesis filterbank 404 that derives the headphone down mix signal 410, which is ready to be played back by stereo reproduction equipment. Fig. 10 shows an inventive receiver or audio player 500, having an inventive audio decoder 501, a bit stream input 502, and an audio output 504. A bit stream can be input at the input 502 of the inventive receiver/audio player 500. The bit stream then is decoded by the decoder 501 and the decoded signal is output or played at the output 504 of the inventive receiver/audio player 500. Although examples have been derived in the preceding paragraphs to implement the inventive concept relying on a transmitted stereo down mix, the inventive concept may also be applied in configurations based on a single monophonic down mix channel or on more than two down mix channels. One particular implementation of the transfer of head- related transfer functions into the subband domain is given in the description of the present invention. However, other techniques of deriving the subband filters may also be used without limiting the inventive concept. The phase factors introduced in the derivation of the modified HRTFs can be derived also by other computations than the ones previously presented. Therefore, deriving those factors in a different way does not limit the scope of the invention. Even as the inventive concept is shown particularly for HRTF and crosstalk cancellation filters, it can be used for other filters defined for one or more individual channels of a multi channel signal to allow for a computationally efficient generation of a high quality stereo playback signal. The filters are furthermore not only restricted to filters intended to model a listening environment. Even filters adding "artificial" components to a signal can be used, such as for example reverberation or other distortion filters. Depending on certain implementation requirements of the inventive methods, the inventive methods can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, in particular a disk, DVD or a CD having electronically readable control signals stored thereon, which cooperate with a programmable computer system such that the inventive methods are performed. Generally, the present invention is, therefore, a computer program product with a program code stored on a machine readable carrier, the program code being operative for performing the inventive methods when the computer program product runs on a computer. In other words, the inventive methods are, therefore, a computer program having a program code for performing at least one of the inventive methods when the computer program runs on a computer. While the foregoing has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope thereof. It is to be understood that various changes may be made in adapting to different embodiments without departing from the broader concepts disclosed herein and comprehended by the claims that follow. What is claimed is: 1. Decoder for deriving a headphone down mix signal (314) using a representation of a down mix of a multi- channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head- related transfer functions (308) related to the two channels of the multi-channel signal, comprising: a filter calculator (302) for deriving modified head- related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels using the level parameter (306) such that a modified head-related transfer function (310) is stronger influenced by the head-related transfer function (308) of a channel having a higher level than, by the head-related transfer function (308) of a channel having a lower level; and a synthesizer (304) for deriving the headphone down mix signal (314) using the modified head-related transfer functions (310) and the representation of the down mix signal (312) . 2. Decoder in accordance with claim 1, in which the filter calculator (302) is operative to derive the modified head-related transfer functions (310) further applying phase shifts to the head-related transfer functions (308) of the two channels such that the head-related transfer function (308) of a channel having a lower level is shifted closer to a mean phase of the head-related transfer functions (308) of the two channels than a channel having a higher level. 3. Decoder in accordance with claim 1 in which the filter calculator (302) is operative such that the number of modified head-related transfer functions (310) derived is smaller than the number of associated head-related transfer functions (308) of the two channels. 4. Decoder in accordance with claim 1 in which the filter calculator (302) is operative to derive modified head- related transfer functions (310) adapted to be applied to a filterbank representation of the down mix signal. 5. Decoder in accordance with claim 1, adapted to use a representation of the down mix signal derived in a filterbank domain. 6. Decoder in accordance with claim 1, in which the filter calculator (302) is operative to derive modified head-related transfer functions (310) using head-related transfer functions (308) characterized by more than three parameters. 7. Decoder in accordance with claim 1, in which the filter calculator (302) is operative to derive the weighting factors for the head-related transfer functions (308) of the two channels using the same level parameter (306). 8. Decoder in accordance with claim 1, in which the filter calculator (302) is operative to derive a first weighting factor wlf for a first channel f and a second weighting factor Wls for a second channel s using the level parameter CLD1 according to the following formulas: 9. Decoder in accordance with claim 1, in which the filter calculator (302) is operative to derive the modified head-related transfer functions (310) applying a common gain factor to the head-related transfer functions (308) of the two channels such that energy is preserved when deriving the modified head- related transfer functions (310). 10. Decoder in accordance with claim 9, in which the common gain factor is within the interval [1/V2,1]. 11. Decoder in accordance with claim 2, in which the filter calculator (302) is operative to derive the mean phase using a delay time between impulse responses of head-related transfer functions (308) of the two channels. 12. Decoder in accordance with claim 11, in which the filter calculator (302) is operative in a filterbank domain having n frequency bands and to derive individual mean phase shifts for each frequency band using the delay time. 13. Decoder in accordance with claim 11, in which the filter calculator (302) is operative in a filterbank domain having more than 2 frequency bands and to derive individual mean phase shifts xy for each frequency band using the delay time txy according to the following formula : 14. Decoder in accordance with claim 2, in which the filter calculator (302) is operative to derive the mean phase using the phase angle of the normalized complex cross correlation between the impulse responses of head-related transfer functions (308) of the first and the second channel. 15. Decoder in accordance with claim 1, in which the first channel of the two channels is a front channel of the left or the right side of the multi-channel signal and the second channel of the two channels is a back channel of the same side. 16. Decoder in accordance with claim 15, in which the filter calculator is operative to derive the modified head-related transfer function Hr(X) (310) using the front channel head-related transfer function HY(Xf) and the back channel head-related transfer function Hr(Xs) using the following complex linear combination: xy is a mean phase, ws and wf are weighting factors derived using the level parameter (306) and g is a common gain factor derived using the level parameter (306). 17. Decoder in accordance with claim 1, adapted to use a representation of a down mix signal (312) having a left and a right channel derived from a multi-channel signal having a left-front, a left-surround, a right- front, a right-surround and a center channel. 18. Decoder in accordance with claim 1, in which the synthesizer is operative to derive channels of the headphone down mix signal (314) applying a linear combination of the modified head-related transfer functions (310) to the representation of the dovm mix (312) of the multi-channel signal. 19. Decoder in accordance with claim 18, in which the synthesizer is operative to use coefficients for the linear combination depending on the level parameter (306). 20. Decoder in accordance with claim 18, in which the synthesizer (304) is operative to use coefficients for the linear combination depending on additional multi- channel parameters related to additional spatial properties of the multi-channel signal. 21. Binaural decoder, comprising: a decoder in accordance with claim 1; an analysis filterbank ( ) for deriving the representation of the down mix of the multi-channel signal (312) by subband filtering the downmix of the multi-channel signal; and a synthesis filterbank ( ) for deriving a time-domain headphone signal by synthesizing the headphone down mix signal (314). 22. Decoder for deriving a spatial stereo down mix signal using a representation of a down mix of a multi- channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using crosstalk cancellation filters related to the tyro channels of the multi-channel signal, comprising: a filter calculator (302) for deriving modified crosstalk cancellation filters by weighting the crosstalk cancellation filters of the two channels using the level parameter (306) such that a modified crosstalk cancellation filter is stronger influenced by the crosstalk cancellation filter of a channel having a higher level than by the crosstalk cancellation filter of a channel having a lower level; and a synthesizer (304) for deriving the spatial stereo down mix signal using the modified crosstalk cancellation filters and the representation of the down mix signal (312) . 23. Method of deriving a headphone down mix signal (314) using a representation of a down mix of a multi- channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head- related transfer functions (308) related to the two channels of the multi-channel signal, the method comprising: deriving, using the level parameter (306), modified head-related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal (314) using the modified head-related transfer functions (310} and the representation of the down mix signal. 24. Receiver or audio player having a decoder for deriving a headphone down mix signal (314) using a representation of a down mix of a multi-channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions (308) related to the two channels of the multi-channel signal, comprising: a filter calculator for deriving modified head-related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels using the level parameter (306) such that a modified head- related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and a synthesizer for deriving the headphone down mix signal (314) using the modified head-related transfer functions (310) and the representation of the down mix signal. 25. Method of receiving or audio playing, the method having a method for deriving a headphone down mix signal (314) using a representation of a down mix of a multi-channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions (308) related to the two channels of the multi-channel signal, the method comprising: deriving, using the level parameter (306), modified head-related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal (314) using the modified head-related transfer functions (310) and the representation of the down mix signal. 26- Computer program having a program code for performing, when running on a computer, a method for deriving a headphone down mix signal (314) using a representation of a downmix of a multi-channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions (308) related to the two channels of the multi-channel signal, the method comprising: deriving, using the level parameter (306), modified head-related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal (314) using the modified head-related transfer functions (310) and the representation of the down mix signal. 27. Computer program having a program code for performing, when running on a computer, a method for receiving or audio playing, the method having a method for deriving a headphone down mix signal (314) using a representation of a down mix of a multi-channel signal (312) and using a level parameter (306) having information on a level relation between two channels of the multi-channel signal and using head-related transfer functions (308) related to the two channels of the multi-channel signal, the method comprising: deriving, using the level parameter (306), modified head-related transfer functions (310) by weighting the head-related transfer functions (308) of the two channels such that a modified head-related transfer function is stronger influenced by the head-related transfer function of a channel having a higher level than by the head-related transfer function of a channel having a lower level; and deriving the headphone down mix signal (314) using the modified head-related transfer functions (310) and the representation of the down mix signal. A headphone down mix signal (314) can be efficiently derived from a parametric down mix of a multi-channel signal (312), when modified HRTFs (310) (head related transfer functions) are derived from HRTFs (308) of a multi-channel signal using a level parameter (306) having information on a level relation between two channels of the multi-channel signals such that a modified HRTF (310) is stronger influenced by the HRTF (308) of a channel having a higher level than by the HRTF (308) of a channel having a lower level. Modified HRTFs (310) are derived within the decoding process taking into account the relative strength of the channels associated to the HRTFs (308). The HRTFs (308) are thus modified such that a down mix signal (314) of a parametric representation of a multi-channel signal can directly be used to synthesize the headphone down mix signal (314) without the need of an intermediate full parametric multi-channel reconstruction of the parametric down mix.

Full Text

Generation of spatial downmixes from parametric
representations of multi channel signals
Field of the Invention
The present invention relates to decoding of encoded multi-
channel audio signals based on a parametric multi-channel
representation and in particular to the generation of 2-
channel downmixes providing a spatial listening experience
as for example a headphone compatible down mix or a spatial
downmix for 2 speaker setups.
Background of the Invention in Prior Art
Recent development in audio coding has made available the
ability to recreate a multi-channel representation of an
audio signal based on a stereo (or mono) signal and
corresponding control data. These methods differ
substantially from older matrix based solutions such as
Dolby Prologic, since additional control data is
transmitted to control the re-creation, also referred to
as up-mix, of the surround channels based on the
transmitted mono or stereo channels.
Hence, such a parametric multi-channel audio decoder, e.g.
MPEG Surround, reconstructs N channels based on M
transmitted channels, where N > M, and the additional
control data. The additional control data represents a
significant lower data rate than transmitting the all N
channels, making the coding very efficient while at the
same time ensuring compatibility with both M channel
devices and N channel devices.
These parametric surround coding methods usually comprise
a parameterization of the surround signal based on IID
(Inter channel intensity Difference) or CLD (Channel Level

Difference) and ICC (Inter Channel Coherence). These
parameters describe power ratios and correlations, between
channel pairs in the up-mix process. Further parameters
also used in prior art comprise prediction parameters used
to predict intermediate or output channels during the up-
mix procedure.
Other developments in reproduction of multi-channel audio
content have provided means to obtain a spatial listening
impression using stereo headphones. To achieve a spatial
listening experience using only the two speakers of the
headphones, multi-channel signals are down mixed to stereo
signals using HRTF (head related transfer functions),
intended to take into account the extremely complex
transmission characteristics of a human head for providing
the spatial listening experience.
Another related approach is to use a conventional 2-channel
playback environment and to filter the channels of a multi-
channel audio signal with appropriate filters to achieve a
listening experience close to that of the playback with the
original number of speakers. The processing of the signals
is similar as in the case of headphone playback to create
an appropriate "spatial stereo down mix" having the desired
properties. Contrary to the headphone case, the signal of
both speakers directly reaches both ears of a listener,
causing undesired "crosstalk effects". As this has to be
taken into account for optimal reproduction quality, the
filters used for signal processing are commonly called
crosstalk-cancellation filters. Generally, the aim of this
technique is to extend the possible range of sound sources
outside the stereo speaker base by cancellation of inherent
crosstalk using complex crosstalk-cancellation filters.
Because of the complex filtering, HRTF filters are very
long, i.e. they may comprise several hundreds of filter
taps each. For the same reason, it is hardly possible to
find a parameterization of the filters that works well

enough not to degrade the perceptual quality when used
instead of the actual filter.
Thus, on the one hand, bit. saving parametric
representations of multi-channel signals do exist that
allow for an efficient transport of an encoded multi-
channel signal. On the other hand, elegant ways to create a
spatial listening experience for a multi-channel signal
when using stereo headphones or stereo speakers only are
known. However, these require the full number of channels
of the multi-channel signal as input for the application of
the head related transfer functions that create the
headphone down mix signal. Thus, either the full set of
multi-channels signals has to be transmitted or a
parametric representation has to be fully reconstructed
before applying the head related transfer functions or the
crosstalk-cancellation filters and thus either the
transmission bandwidth or the computational complexity is
unacceptably high.
Summary of the invention
It is the object of the present invention to provide a
concept allowing for a more efficient reconstruction of a
2-channel signal providing a spatial listening experience
using parametric representations of multi-channel signals.
In accordance with a first aspect of the present invention,
this object is achieved by a decoder for deriving a
headphone down mix signal using a representation of a down
mix of a multi-channel signal and using a level parameter
having information on a level relation between two channels
of the multi-channel signal and using head-related transfer
functions related to the two channels of the multi-channel
signal, comprising: a filter calculator for deriving
modified head-related transfer functions by weighting the
head-related transfer functions of the two channels using

the level parameter such that a modified head-related
transfer function is stronger influenced by the head-
related transfer function of a channel having a higher
level than by the head-related transfer function of a
channel having a lower level; and a synthesizer for
deriving the headphone down mix signal using the modified
head-related transfer functions and the representation of
the down mix signal.
In accordance with a second aspect of the present
invention, this object is achieved by a binaural decoder,
comprising: a decoder for deriving a headphone down mix
signal using a representation of a down mix of a multi-
channel signal and using a level parameter having
information on a level relation between two channels of the
multi-channel signal and using head-related transfer
functions related to the two channels of the multi-channel
signal, comprising: a filter calculator for deriving
modified head-related transfer functions by weighting the
head-related transfer functions of the two channels using
the level parameter such that a modified head-related
transfer function is stronger influenced by the head-
related transfer function of a channel having a higher
level than by the head-related transfer function of a
channel having a lower level; and a synthesizer for
deriving the headphone down mix signal using the modified
head-related transfer functions and the representation of
the down mix signal; an analysis filterbanJc for deriving
the representation of the down mix of the multi-channel
signal by subband filtering the downmix of the multi-
channel signal; and a synthesis filterbank for deriving a
time-domain headphone signal by synthesizing the headphone
down mix signal.
In accordance with a third aspect of the present invention,
this object is achieved by Method of deriving a headphone
down mix signal using a representation of a down mix of a
multi-channel signal and using a level parameter having

formation on a level relation between two channels of the
lti-channel signal and using head-related transfer
notions related to the two channels of the multi-channel
- -gnal, the method comprising: deriving, using the level
parameter, modified head-related transfer functions by
weighting the head-related transfer functions of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level than
by the head-related transfer function of a channel having a
lower level; and deriving the headphone down mix signal
using the modified head-related transfer functions and the
representation of the down mix signal.
In accordance with a fourth aspect of the present
invention, this object is achieved by a receiver or audio
player having a decoder for deriving a headphone down mix
signal using a representation of a down mix of a multi-
channel signal and using a level parameter having
information on a level relation between two channels of the
multi-channel signal and using head-related transfer
functions related to the two channels of the multi-channel
signal, comprising: a filter calculator for deriving
modified head-related transfer functions by weighting the
head-related transfer functions of the two channels using
the level parameter such that a modified head-related
transfer function is stronger influenced by the head-
related transfer function of a channel having a higher
level than by the head-related transfer function of a
channel having a lower level; and a synthesizer for
deriving the headphone down mix signal using the modified
head-related transfer functions and the representation of
the down mix signal.
In accordance with a fifth aspect of the present invention,
this object is achieved by a method of receiving or audio
playing, the method having a method for deriving a
headphone down mix signal using a representation of a down

mix of a multi-channel signal and using a level parameter
having information on a level relation between two channels
of the multi-channel signal and using head-related transfer
functions related to the two channels of the multi-channel
signal, the method comprising: deriving, using the level
parameter, modified head-related transfer functions by
weighting the head-related transfer functions of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level than
by the head-related transfer function of a channel having a
lower level; and deriving the headphone down mix signal
using the modified head-related transfer functions and the
representation of the down mix signal.
In accordance with a sixth aspect of the present invention,
this object is achieved by a decoder for deriving a spatial
stereo down mix signal using a representation of a down mix
of a multi-channel signal and using a level parameter
having information on a level relation between two channels
of the multi-channel signal and using crosstalk
cancellation filters related to the two channels of the
multi-channel signal, comprising: a filter calculator for
deriving modified crosstalk cancellation filters by
weighting the crosstalk cancellation filters of the two
channels using the level parameter such that a modified
crosstalk cancellation filters is stronger influenced by
the crosstalk cancellation filter of a channel having a
higher level than by the crosstalk cancellation filter of a
channel having a lower level; and a synthesizer for
deriving the spatial stereo down mix signal using the
modified crosstalk cancellation filters and the
representation of the down mix signal.
The present invention is based on the finding that a
headphone down mix signal can be derived from a parametric
down mix of a multi-channel signal, when a filter
calculator is used for deriving modified HRTFs (head

related transfer functions) from original HRTFs of the
multi-channel signal and when the filter converter uses a
level parameter having information on a level relation
between two channels of the multi-channel signal such that
modified HRTFs are stronger influenced by the HRTF of a
channel having a higher level than by the HRTF of a channel
having a lower level. Modified HRTFs are derived during the
decoding process taking into account the relative strength
of the channels associated to the HRTFs. The original HRTFs
are modified such, that a down mix signal of a parametric
representation of a multi-channel signal can be directly
used to synthesize the headphone down mix signal without
the need of a full parametric multi-channel reconstruction
of the parametric down mix signal.
In one embodiment of the present invention, an inventive
decoder is used implementing a parametric multi-channel
reconstruction as well as an inventive binaural
reconstruction of a transmitted parametric down mix of an
original multi-channel signal. According to the present
invention, a full reconstruction of the multi-channel
signal prior to binaural down mixing is not required,
having the obvious great advantage of a strongly reduced
computational complexity. This allows, for example, mobile
devices having only limited energy reservoirs to extend the
playback length significantly. A further advantage is that
the same device can serve as provider for complete multi-
channel signals (for example 5.1, 7.1, 7.2 signals) as well
as for a binaural down mix of the signal having a spatial
listening experience even when using only two-speaker
headphones. This might, for example, be extremely
advantageous in home-entertainment configurations.
In a further embodiment of the present invention a filter
calculator is used for deriving modified HRTFs not only
operative to combine the HRTFs of two channels by applying
individual weighting factors to the HRTF but by introducing
additional phase factors for each HRTF to be combined. The

introduction of the phase factor has the advantage of
achieving a delay compensation of two filters prior to
their superposition or combination. This leads to a
combined response that models a main delay time
corresponding to an intermediate position between the front
and the back speakers.
A second advantage is that a gain factor, which has to be
applied during the combination of the filters to ensure
energy conservation, is much more stable with respect to
its behavior with frequency than without the introduction
of the phase factor. This is particular relevant for the
inventive concept, as according to an embodiment of the
present invention a representation of a down mix of a
multi-channel signal is processed within a interbank
domain to derive the headphone down mix signal. As such,
different frequency bands of the representation of the down
mix signal are to be processed separately and therefore, a
smooth behavior of the individually applied gain functions
is vital.
In a further embodiment of the present invention the head-
related transfer functions are converted to subband-filters
for the subband domains such that the total number of
modified HRTFs used in the subband domain is smaller than
the total number of original HRTFs. This has the evident
advantage that the computational complexity for deriving
headphone down mixed signals is even decreased compared to
the down mixing using standard HRTF filters.
Implementing the inventive concept allows for the use of
extremely long HRTFs and thus allows for the reconstruction
of headphone down mix signals based on a representation of
a parametric down mix of a multi-channel signal with
excellent perceptual quality.
Furthermore, using the inventive concept on crosstalk-
cancellation filters allows for the generation of a spatial

stereo down mix to be used with a standard 2 speaker setup
based on a representation of a parametric down mix of a
multi-channel signal with excellent perceptual quality.
One further big advantage.of the inventive decoding concept
is that a single inventive binaural decoder implementing
the inventive concept may be used to derive a binaural
downmix as well as a multi-channel reconstruction of a
transmitted down mix taking into account the additionally
transmitted spatial parameters.
In one embodiment of the present invention an inventive
binaural decoder is having an analysis filterbank for
deriving the representation of the down mix of the multi-
channel signal in a subband domain and an inventive decoder
implementing the calculation of the modified HRTFs. The
decoder further comprises a synthesis filterbank to
finally derive a time domain representation of a headphone
down mix signal, which is ready to be played back by any
conventional audio playback equipment.
In the following paragraphs, prior art parametric multi-
channel decoding schemes and binaural decoding schemes are
explained in more detail referencing the accompanying
drawings, to more clearly outline the great advantages of
the inventive concept.
Most of the embodiments of the present invention detailed
below describe the inventive concept using HRTFs. As
previously noted, HRTF processing is similar to the use of
crosstalk-cancellation filters. Therefore, all of the
embodiments are to be understood as to refer to HRTF
processing as well as to crosstalk-cancellation filters. In
other words, all HRTF Filters could be replaced by
crosstalk-cancellation filters below to apply the inventive
concept to the use of crosstalk-cancellation filters.

Brief Description of the Drawings
Preferred embodiments of the present invention are
subsequently described by referring to the enclosed
drawings, wherein:
Fig. 1 shows a conventional binaural synthesis using HRTFs;
Fig. 1b shows a conventional use of crosstalk-cancellation
filters;
Fig. 2 shows an example of a multi-channel spatial encoder;
Fig. 3 shows an example for prior art spatial/binaural-
decoders;
Fig. 4 shows an example of a parametric nuilti-channel
encoder;
Fig. 5 shows an example of a parametric multi-channel
decoder;
Fig. 6 shows an example of an inventive decoder;
Fig. 7 shows a block diagram illustrating the concept of
transforming filters into the subband domain;
Fig. 8 shows an example of an inventive decoder;
Fig. 9 shows a further example of an inventive decoder; and
Fig. 10 shows an example for an inventive receiver or audio
player.
Detailed Description of Preferred Embodiments
The below-described embodiments are merely illustrative
for the principles of the present invention for Binaural

Decoding of Multi-Channel Signals By Morphed HRTF
Filtering. It is understood that modifications and
variations of the arrangements and the details described
herein will be apparent to others skilled in the art. It
is the intent, therefore, to be limited only by the scope
of the impending patent claims and not by the specific
details presented by way of description and explanation of
the embodiments herein.
In order to better outline the features and advantages of
the present invention a more elaborate description of prior
art will be given now.
A conventional binaural synthesis algorithm is outlined in
Fig. 1. A set of input channels (left front (LF} , right
front (RF), left surround (LS), right surround (RS) and
center (C) ), 10a, 10b, 10c, lOd and lOe is filtered by a
set of HRTFs 12a to 12j. Each input signal is split into
two signals (a left "L" and a right "R" component! wherein
each of these signal components is subsequently filtered by
an HRTF corresponding to the desired sound position.
Finally, all left ear signals are summed by a summer 14a to
generate the left binaural output signal L and the right-
ear signals are summed by a summer 14b to generate the
right binaural output signal R. It may be noted that HR'EF
convolution can principally be performed in the time
domain, but it is often preferred to perform filtering in
the frequency domain due to the increased computational
efficiency. That means that, the summation shown in Fig- 1
is also performed in the frequency domain and a subsequent
transformation into a time domain is additionally required.
Fig. 1b illustrates crosstalk cancellation processing
intended to achieve a spatial listening impression using
only two speakers of a standard stereo playback
environment.

The aim is reproduction of a multi-channel signal by means
of a stereo playback system having only two speakers 16a
and 16b such that a listener 18 experiences a spatial
listening experience. Am major difference with respect to
headphone reproduction is that signals of both speakers 16a
and 16b directly reach both ears of listener 18. The
signals indicated by dashed lines (crosstalk) therefore
have to be taken into account additionally.
For ease of explanation only a 3 channel input signal
having 3 sources 20a to 20c is illustrated in Fig. 1b. It
goes without saying that the scenario can in principle be
extended to arbitrary number of channels.
To derive the stereo signal to be played back, each input
source is processed by 2 of the crosstalk cancellation
filters 21a to 2lf, one filter for each channel of the
playback signal. Finally, all filtered signals for the left
playback channel 16a and the right playback channel 16b are
summed up for playback. It is evident that the crosstalk
cancellation filters will in general be different for each
source 20a and 20b (depending on its desired perceived
position) and that they could furthermore even depend on
the listener.
Owing to the high flexibility of the inventive concept, one
benefits from high flexibility in the design and
application of the crosstalk cancellation filters such
that filters can be optimized for each application or
playback device individually. One further advantage is that
the method is computationally extremely efficient, since
only 2 synthesis filterbanks are required.
A principle sketch of a spatial audio encoder is shown in
Fig. 2. In such a basic encoding scenario., a spatial audio
decoder 40 comprises a spatial encoder 42, a down mix
encoder 44 and a multiplexer 46.

A multi-channel input signal 50 is analyzed by the spatial
encoder 42, extracting spatial parameters describing
spatial properties of the multi-channel input signal that
have to be transmitted to the decoder side. The down mixed
signal generated by the spatial encoder 42 may for example
be a monophonic or a stereo signal depending on different
encoding scenarios. The down mix encoder 4 4 may then encode
the monophonic or stereo down mix signal using any
conventional mono or stereo audio coding scheme. The
multiplexer 46 creates an output bit stream by combining
the spatial parameters and the encoded down mix signal into
the output bit stream.
Fig. 3 shows a possible direct combination of a multi-
channel decoder corresponding to the encoder of Fig. 2 and
a binaural synthesis method as, for example, outlined in
Fig. 1. As can be seen, the prior art approach of combining
the features is simple and straight forward. The set-up
comprises a de-multiplexer 60, a down mix decoder 62, a
spatial decoder 64 and a binaural synthesizer 66. An input
bit stream 68 is de-multiplexed resulting in spatial
parameters 70 and a down mix signal bit stream. The latter
down-mix signal bit stream is decoded by the down mix
decoder 62 using a conventional mono or stereo decoder. The
decoded down mix is input, together with the spatial
parameters 70, into the spatial decoder 64 that generates a
multi-channel output signal 72 having the spatial
properties indicated by the spatial parameters 70. Having a
multi-channel signal 72 completely reconstructed, the
approach of simply adding a binaural synthesizer 66 to
implement the binaural synthesis concept of Fig. 1 is
straight-forward. Therefore, the multi-channel output
signal 72 is used as an input for the binaural synthesizer
66 which processes the multi-channel output signal to
derive the resulting binaural output signal 74. The
approach shown in Fig. 3 has at least three disadvantages:

a complete multi-channel signal representation has to
be computed as an intermediate step, followed by HRTF
convolution and down mixing in the binaural synthesis.
Although HRTF convolution should be performed on a per
channel basis, given the fact that each audio channel
can have a different spatial position, this is an
undesirable situation from a complexity point of view.
Thus, computational complexity is high and energy is
wasted.
The spatial decoder operates in a filterbank (QMF)
domain. HRTF convolution, on the other hand, is
typically applied in the FFT domain. Therefore, a
cascade of a multi-channel QMF synthesis filterbank, a
multi-channel DFT transform, and a stereo inverse DFT
transform is necessary, resulting in a system with
high computational demands.
Coding artefacts created by the spatial decoder to
create a multi-channel reconstruction will be audible,
and possibly enhanced in the (stereo) binaural output.
An even more detailed description of multi-channel encoding
and decoding is given in Figs. 4 and 5.
The spatial encoder 100 shown in Fig. 4 comprises a first
OTT (1-to-2-encoder) 102a, a second OTT 102b and a TTT box
(3-to-2-encoder) 104. A multi-channel input signal 106
consisting of LF, LS, C, RF, RS (left-front, left-
surround, center, right-front and right-surround) channels
is processed by the spatial encoder 100. The OTT boxes
receive two input audio channels each, and derive a single
monophonic audio output channel and associated spatial
parameters, the parameters having information on the
spatial properties of the original channels with respect to
one another or with respect to the output channel (for
example CLD, ICC, parameters) . In the encoder 100, the LF
and the LS channels are processed by OTT encoder 102a and

the RF and RS channels are processed by the OTT encoder
102b. Two signals, L and R are generated, the one only
having information on the left side and the other only
having information on the right side. The signals L, R and
C are further processed by the TTT encoder 104, generating
a stereo down mix and additional parameters.
The parameters resulting from the TTT encoder typically
consist of a pair of prediction coefficients for each
parameter band, or a pair of level differences to describe
the energy ratios of the three input signals. The
parameters of the 'OTT' encoders consist of level
differences and coherence or cross-correlation values
between the input signals for each frequency band.
It may be noted that although the schematic sketch of the
spatial encoder 100 points to a sequential processing of
the individual channels of the down mix signal during the
encoding, it is also possible to implement the complete
down mixing process of the encoder 100 within one single
matrix operation.
Fig. 5 shows a corresponding spatial decoder, receiving as
an input the down mix signals as provided by the encoder of
Fig. 4 and the corresponding spatial parameters.
The spatial decoder 120 comprises a 2-to-3-decoder 122 and
1-to-2-decoders 124a to 124c. The down mix signals Lo and Ro
are input into the 2-to-3-decoder 122 that recreates a
center channel C, a right channel R and a left channel L.
These three channels are further processed by the OTT-
decoders 124a to 124c yielding six output channels. It may
be noted that the derivation of a low-frequency enhancement
channel LFE is not mandatory and can be omitted such that
one single OTT-encoder may be saved within the surround
decoder 120 shown in Fig. 5.

According to one embodiment of the present invention the
ihventive concept is applied in a decoder as shown in Fig.
6. The inventive decoder 200 comprises a 2-to-3-decoder 104
and six HRTF-filters 106a to 106f. A stereo input signal
(Lo, Ro) is processed by the TTT-decoder 104, deriving
three signals L, C and R. It may be noted, that the stereo
input signal is assumed to be delivered within a subband
domain, since the TTT-encoder may be the same encoder as
shown in Fig. 5 and hence adapted to be operative on
subband signals. The signals L, R and C are subject to HRTF
parameter processing by the HRTF filters 106a to 106f.
The resulting 6 channels are summed to generate the stereo
binaural output pair (Lb, Rb) .
The TTT decoder, 106, can be described as the following
matrix operation:

with matrix entries mxy dependent on the spatial
parameters. The relation of spatial parameters and matrix
entries is identical to those relations as in the 5.1-
multichannel MPEG surround decoder. Each of the three
resulting signals L, R, and C are split in two and
processed with HRTF parameters corresponding to the desired
(perceived) position of these sound sources. For the center
channel (C), the spatial parameters of the sound source
position can be applied directly, resulting in two output
signals for the center, LB(C) and RB(C):

For the left (L) channel, the HFTF parameters from the
left-front and left-surround channels are combined into a
single HRTF parameter set, using the weights wlf and wrt•

The resulting 'composite' HRTF parameters simulate the
effect of both the front and surround channels in a
statistical sense. The following equations are used to
generate the binaural output pair (IB, RB) for the left
channel:

In a similar fashion, the binaural output for the right
channel is obtained according to:

Given the above definitions of LA{C), RB(C), LB(L), RB{L),
LB(R) and RS(R), the complete LB and RB signals can be
derived from a single 2 by 2 matrix given the stereo input
signal:

In the above it was assumed that the Hr(X) elements, for
Y= L0,R0 and X = L,R,C, were complex scalars. However, the
present invention teaches how to extend the approach of a 2
by 2 matrix binaural decoder to handle arbitrary length
HRTF filters. In order to achieve this, the present
invention comprises the following steps:

• Transform the HRTF filter responses to a filterbank
domain;
• Overall delay difference or phase difference
extraction from HRTF filter pairs;
• Morph the responses of the HRTF filter pair as a
function- of the CLD parameters
• Gain adjustment
This is achieved by replacing the six complex gainsHy(X)
for Y= L0,R0 and X = L,R,C with six filters. These filters
are derived from the ten filters Hr(X) for T = L0,i?oand
X = Lf,LS,Rf, Rs, C, which describe the given HRTF filter
responses in the QMF domain. These QMF representations can
be achieved according to the method described in one of the
subsequent paragraphs.
In other words, the present invention teaches a concept for
deriving modified HRTFs as by modifying (morphing) of the
front end surround channel filters using a complex linear
combination according to

As it can be seen from the above formula, deriving of the
modified HRTFs is a weighted superposition of the original
HRTFs, additionally applying phase factors. The weights ws,
Wf depend on the CLD parameters intended to be used by the
OTT decoders 124a and 124b of Fig. 5.

The weights wrf and wrs depend on the CLD parameter of the
'OTT' box for Rf and Rs:
The weights wlf and w1s depend on the CLD parameter of the
'OTT'box for Lf and Ls:

The phase parameter xy can be derived from the main delay
time difference txy between the front and back HRTF filters
and the subband index nof the QMF bank:

The role of this phase parameter in the morphing of
filters is twofold. First, it realizes a delay
compensation of the two filters prior to superposition
which leads to a combined response which models a main
delay time corresponding to a source position between the
front and the back speakers. Second, it makes the
necessary gain compensation factor g much more stable and
slowly varying over frequency than in the case of simple
superposition with xy=0.
The gain factor g is determined by the incoherent addition
power rule,

and pxy is the real value of the normalized complex cross
correlation between the filters

For the above equations, P denotes a parameter describing
an average level per frequency band for the impulse
response of the filter specified by the indexes. This mean
intensity is of course easily derived, once the filter
response function are known.

In the case of simple superposition with xy=0, the value
of Pxy varies in an erratic and oscillatory manner as a
function of frequency, which leads to the need for
extensive gain adjustment. In practical implementation it
is necessary to limit the value of the gaing and a
remaining spectral colorization of the signal cannot be
avoided.
In contrast, the use of morphing with a delay based phase
compensation as taught by the present invention leads to a
smooth behaviour of pxy as a function of frequency. This
value is often even close to one for natural HRTF derived
filter pairs since they differ mainly in delay and
amplitude, and the purpose of the phase parameter is to
take the delay difference into account in the QMF
filterbank domain.
An alternative beneficial choice of phase parameter
xy taught by the present invention is given by the phase
angle of the normalized complex cross correlation between
the filters

and unwrapping the phase values with standard unwrapping
techniques as a function of the subband index n of the QMF
bank. This choice has the consequence that pxy is never
negative and hence the compensation gain gsatisfies
1/2 parameter enables the morphing of the front and surround
channel filters in situations where a main delay time
difference txy is not available.
For the embodiment of the present invention as described
above/ it is taught to accurately transform the HRTFs into
an efficient representation of the HRTF filters within the
QMF domain.

Fig. 7 gives a principle sketch of the concept to
accurately transform time-domain filters into filters
within the subband domain having the same net effect on ,a
reconstructed signal. Fig. 7 shows a complex analysis bank
300, a synthesis bank 302 corresponding to the analysis
bank 300, a filter converter 304 and a subband filter 306.
An input signal 310 is provided for which a filter 312 is
known having desired properties. The aim of the
implementation of the filter converter 304 is that the
output signal 314 has the same characteristics after
analysis by the analysis filterbank 300, subsequent subband
filtering 306 and synthesis 302 as if it would have when
filtered by filter 312 in the time domain. The task of
providing a number of subband filters corresponding to the
number of subbands used is fulfilled by filter converter
304.
The following description outlines a method for
implementing a given FIR filter A(v)in the complex QMF
subband domain. The principle of operation is shown in
Figure 7.
Here, the subband filtering is simply the application of
one complex valued FIR filter for each subband, n =0,1,.,.,L-1
to transform the original indices cn into their filtered
counterparts dn according to the following formula:

Observe that this is different from well known methods
developed for critically sampled filterbanks, since those
methods require multiband filtering with longer responses.
The key component is the filter converter, which converts
any time domain FIR filter into the complex subband domain
filters. Since the complex QMF subband domain is
oversampled, there is no canonical set of subband filters

for a given time domain filter. Different subband filters
can have the same net effect of the time domain signal.
What will be described here is a particularly attractive
approximate solution, which is obtained by restricting the
filter converter to be a complex analysis bank similar to
the QMF.
Assuming that the filter converter prototype is of length
64Kq, a real 64KH tap FIR filter is transformed into a set
of 64 complex KH+KQ-l tap subband filters. ForKQ=3, a
FIR filter of 1024 taps is converted into 18 tap subband
filtering with an approximation quality of 50 dB.
The subband filter taps are computed from the; formula

where q(V) is a FIR prototype filter derived from the QMF
prototype filter- As it can be seen, this is just a complex
filterbank analysis of the given filterh(V)•
In the following, the inventive concept will be outlined
for a further embodiment of the present invention, where a
multi-channel parametric representation for a multi-channel
signal having five channels is available. Please note that
in this particular embodiment of the present invention,
original 10 HRTF filters VY,X (as for example given by a QMF
representation of the filters 12a to 12j of Fig 1) are
morphed into six filters hV/X for Y = L, R and X = L,R,C
The ten filters vyx for Y=L,R and X = FL,BL,FR,BR,C
describe the given HRTF filter responses in a hybrid QMF
domain -
the combination of the front and surround channel filters
is performed with a complex linear combination according to

equal to n forA: = 0,1 In cases where there are two
choices, ±n , for the increment, the sign of the increment
for a phase measurement in the interval ]-,]is chosen.
Finally, normalized phase compensated cross correlations
are defined for Y=LJEt and X=L,R by

Please note that in the case where the multi-channel
processing is performed within a hybrid subband domain,
i.e. in a domain where subbands are further decomposed
into different frequency bands, a mapping of the HRTF
responses to the hybrid band filters may for example be
performed as follows:
As in the case without an hybrid filterbank, the ten given
HRTF impulse responses from source X = FL,BL,FR,BR,C to
target Y = L,R are all converted into QMF subband filters
according to the method outlined above. The result is the
ten subband filters vyx with components

for QMF subband m=0,l,...,63and QMF time slot / = 0,1,...,Lq-1. Let
the index mapping from the hybrid band it to QMF band mbe
denoted by m=Q(k) .
Then the HRTF filters vyxin the hybrid band domain are
defined by

For the specific embodiment described in the previous
paragraphs, the filter conversion of HRTF filters into the
QMF domain can be implemented as follows, given a FIR
filter h(y) of length Nh to be transferred to the complex
QMF subband domain:

The subband filtering consists of the separate application
of one complex valued FIR filter hm(l) for each QMF subband,
m' = 0,V..,63 • The key component is the filter converter, which
converts the given time domain FIR filter h(v)±nto the
complex subband domain filters hm(l). The filter converter
is a complex analysis bank similar to the QMF analysis
bank. Its prototype filter q(v) is of length 192. An
extension with zeros of the time domain FIR filter is
defined by

Although the inventive concept has been detailed with
respect to a down mix signal having two channels, i.e. a
transmitted stereo signal, the application of the inventive
concept is by no means restricted to a scenario having a
stereo-down mix signal.
Summarizing, the present invention relates to the problem
of using long HRTF or crosstalk cancellation filters for
binaural rendering of parametric multi-channel signals. The
invention teaches new ways to extend the parametric HRTF
approach to arbitrary length of HRTF filters.
The present invention comprises the following features:
Multiplying the stereo down mix signal by a 2 by 2
matrix where every matrix element is a FIR filter or
arbitrary length (as given by the HRTF filter);

Deriving the filters in the 2 by 2 matrix by morphing
the original HRTF filters based on the transmitted
multi-channel parameters;
- Calculation of the morphing of the HRTF filters so
that the correct spectral envelope and overall energy
is obtained.
Fig. 8 shows an example for an inventive decoder 300 for
deriving a headphone down mix signal. The decoder comprises
a filter calculator 302 and a synthesizer 304. The filter
calculator receives as a first input level parameters 306
and as a second input HRTFs (head-related transfer
functions) 308 to derive modified HRTFs 310 that have the
same net effect on a signal when applied to the signal in
the subband domain than the head-related transfer functions
308 applied in the time domain. The modified HRTFs 310
serve as first input to the synthesizer 304 that receives
as a second input a representation of a down-mix signal 312
within a subband domain. The representation of the down-mix
signal 312 is derived by a parametric multi-channel encoder
and intended to be used as a basis for reconstruction of a
full multi-channel signal by a multi-channel decoder. The
synthesizer 404 is thus able to derive a headphone down-mix
signal 314 using the modified HRTFs 310 and the
representation of the down-mix signal 312.
It may be noted, that the HRTFs could be provided in any
possible parametric representation, for example as the
transfer function associated to the filter, as the impulse
response of the filter or as a series of tap coefficients
for an FIR-filter.
The previous examples assume, that the representation of
the down-mix signal is already supplied as a filterbank
representation, i.e. as samples derived by a filterbank. In
practical applications, however, a time-domain down-mix
signal is typically supplied and transmitted to allow also

for a direct playback of the submitted signal in simple
playback environments. Therefore, in Fig. 9 in a further
embodiment of the present invention, where a binaural
compatible decoder 400 comprises an analysis filterbank 402
and a synthesis filterbank 404 and an inventive decoder,
which could, for example, be the decoder 300 of Fig. 8.
Decoder functionalities and their descriptions are
applicable in Fig. 9 as well as in Fig. 8 and the
description of the decoder 300 will be omitted within the
following paragraph.
The analysis filterbank 402 receives a downmix of a multi-
channel signal 406 as created by a multi-channel parametric
encoder. The analysis filterbank 402 derives the filterbank
representation of the received down mix signal 406 which is
then input into decoder 300 that derives a headphone
downmix signal 408, still within the filterbank domain.
That is, the down mix is represented by a multitude of
samples or coefficients within the frequency bands
introduced by the analysis filterbank 402. Therefore, to
provide a final headphone down mix signal 410 in the time
domain the headphone downmix signal 408 is input into
synthesis filterbank 404 that derives the headphone down
mix signal 410, which is ready to be played back by stereo
reproduction equipment.
Fig. 10 shows an inventive receiver or audio player 500,
having an inventive audio decoder 501, a bit stream
input 502, and an audio output 504.
A bit stream can be input at the input 502 of the inventive
receiver/audio player 500. The bit stream then is decoded
by the decoder 501 and the decoded signal is output or
played at the output 504 of the inventive receiver/audio
player 500.
Although examples have been derived in the preceding
paragraphs to implement the inventive concept relying on a

transmitted stereo down mix, the inventive concept may also
be applied in configurations based on a single monophonic
down mix channel or on more than two down mix channels.
One particular implementation of the transfer of head-
related transfer functions into the subband domain is given
in the description of the present invention. However, other
techniques of deriving the subband filters may also be used
without limiting the inventive concept.
The phase factors introduced in the derivation of the
modified HRTFs can be derived also by other computations
than the ones previously presented. Therefore, deriving
those factors in a different way does not limit the scope
of the invention.
Even as the inventive concept is shown particularly for
HRTF and crosstalk cancellation filters, it can be used for
other filters defined for one or more individual channels
of a multi channel signal to allow for a computationally
efficient generation of a high quality stereo playback
signal. The filters are furthermore not only restricted to
filters intended to model a listening environment. Even
filters adding "artificial" components to a signal can be
used, such as for example reverberation or other distortion
filters.
Depending on certain implementation requirements of the
inventive methods, the inventive methods can be implemented
in hardware or in software. The implementation can be
performed using a digital storage medium, in particular a
disk, DVD or a CD having electronically readable control
signals stored thereon, which cooperate with a programmable
computer system such that the inventive methods are
performed. Generally, the present invention is, therefore,
a computer program product with a program code stored on a
machine readable carrier, the program code being operative
for performing the inventive methods when the computer

program product runs on a computer. In other words, the
inventive methods are, therefore, a computer program having
a program code for performing at least one of the inventive
methods when the computer program runs on a computer.
While the foregoing has been particularly shown and
described with reference to particular embodiments thereof,
it will be understood by those skilled in the art that
various other changes in the form and details may be made
without departing from the spirit and scope thereof. It is
to be understood that various changes may be made in
adapting to different embodiments without departing from
the broader concepts disclosed herein and comprehended by
the claims that follow.

What is claimed is:
1. Decoder for deriving a headphone down mix signal (314)
using a representation of a down mix of a multi-
channel signal (312) and using a level parameter (306)
having information on a level relation between two
channels of the multi-channel signal and using head-
related transfer functions (308) related to the two
channels of the multi-channel signal, comprising:
a filter calculator (302) for deriving modified head-
related transfer functions (310) by weighting the
head-related transfer functions (308) of the two
channels using the level parameter (306) such that a
modified head-related transfer function (310) is
stronger influenced by the head-related transfer
function (308) of a channel having a higher level than,
by the head-related transfer function (308) of a
channel having a lower level; and
a synthesizer (304) for deriving the headphone down
mix signal (314) using the modified head-related
transfer functions (310) and the representation of the
down mix signal (312) .
2. Decoder in accordance with claim 1, in which the
filter calculator (302) is operative to derive the
modified head-related transfer functions (310) further
applying phase shifts to the head-related transfer
functions (308) of the two channels such that the
head-related transfer function (308) of a channel
having a lower level is shifted closer to a mean phase
of the head-related transfer functions (308) of the
two channels than a channel having a higher level.
3. Decoder in accordance with claim 1 in which the filter
calculator (302) is operative such that the number of
modified head-related transfer functions (310) derived

is smaller than the number of associated head-related
transfer functions (308) of the two channels.
4. Decoder in accordance with claim 1 in which the filter
calculator (302) is operative to derive modified head-
related transfer functions (310) adapted to be applied
to a filterbank representation of the down mix signal.
5. Decoder in accordance with claim 1, adapted to use a
representation of the down mix signal derived in a
filterbank domain.
6. Decoder in accordance with claim 1, in which the
filter calculator (302) is operative to derive
modified head-related transfer functions (310) using
head-related transfer functions (308) characterized by
more than three parameters.
7. Decoder in accordance with claim 1, in which the
filter calculator (302) is operative to derive the
weighting factors for the head-related transfer
functions (308) of the two channels using the same
level parameter (306).
8. Decoder in accordance with claim 1, in which the
filter calculator (302) is operative to derive a first
weighting factor wlf for a first channel f and a
second weighting factor Wls for a second channel s
using the level parameter CLD1 according to the
following formulas:

9. Decoder in accordance with claim 1, in which the
filter calculator (302) is operative to derive the
modified head-related transfer functions (310)
applying a common gain factor to the head-related

transfer functions (308) of the two channels such that
energy is preserved when deriving the modified head-
related transfer functions (310).
10. Decoder in accordance with claim 9, in which the
common gain factor is within the interval [1/V2,1].
11. Decoder in accordance with claim 2, in which the
filter calculator (302) is operative to derive the
mean phase using a delay time between impulse
responses of head-related transfer functions (308) of
the two channels.
12. Decoder in accordance with claim 11, in which the
filter calculator (302) is operative in a filterbank
domain having n frequency bands and to derive
individual mean phase shifts for each frequency band
using the delay time.
13. Decoder in accordance with claim 11, in which the
filter calculator (302) is operative in a filterbank
domain having more than 2 frequency bands and to
derive individual mean phase shifts xy for each
frequency band using the delay time txy according to
the following formula :

14. Decoder in accordance with claim 2, in which the
filter calculator (302) is operative to derive the
mean phase using the phase angle of the normalized
complex cross correlation between the impulse
responses of head-related transfer functions (308) of
the first and the second channel.
15. Decoder in accordance with claim 1, in which the first
channel of the two channels is a front channel of the

left or the right side of the multi-channel signal and
the second channel of the two channels is a back
channel of the same side.
16. Decoder in accordance with claim 15, in which the
filter calculator is operative to derive the modified
head-related transfer function Hr(X) (310) using the
front channel head-related transfer function HY(Xf)
and the back channel head-related transfer function
Hr(Xs) using the following complex linear combination:

xy is a mean phase, ws and wf are weighting factors
derived using the level parameter (306) and g is a
common gain factor derived using the level parameter
(306).
17. Decoder in accordance with claim 1, adapted to use a
representation of a down mix signal (312) having a
left and a right channel derived from a multi-channel
signal having a left-front, a left-surround, a right-
front, a right-surround and a center channel.
18. Decoder in accordance with claim 1, in which the
synthesizer is operative to derive channels of the
headphone down mix signal (314) applying a linear
combination of the modified head-related transfer
functions (310) to the representation of the dovm mix
(312) of the multi-channel signal.
19. Decoder in accordance with claim 18, in which the
synthesizer is operative to use coefficients for the
linear combination depending on the level parameter
(306).
20. Decoder in accordance with claim 18, in which the
synthesizer (304) is operative to use coefficients for

the linear combination depending on additional multi-
channel parameters related to additional spatial
properties of the multi-channel signal.
21. Binaural decoder, comprising:
a decoder in accordance with claim 1;
an analysis filterbank ( ) for deriving the
representation of the down mix of the multi-channel
signal (312) by subband filtering the downmix of the
multi-channel signal; and
a synthesis filterbank ( ) for deriving a time-domain
headphone signal by synthesizing the headphone down
mix signal (314).
22. Decoder for deriving a spatial stereo down mix signal
using a representation of a down mix of a multi-
channel signal (312) and using a level parameter (306)
having information on a level relation between two
channels of the multi-channel signal and using
crosstalk cancellation filters related to the tyro
channels of the multi-channel signal, comprising:
a filter calculator (302) for deriving modified
crosstalk cancellation filters by weighting the
crosstalk cancellation filters of the two channels
using the level parameter (306) such that a modified
crosstalk cancellation filter is stronger influenced
by the crosstalk cancellation filter of a channel
having a higher level than by the crosstalk
cancellation filter of a channel having a lower level;
and
a synthesizer (304) for deriving the spatial stereo
down mix signal using the modified crosstalk

cancellation filters and the representation of the
down mix signal (312) .
23. Method of deriving a headphone down mix signal (314)
using a representation of a down mix of a multi-
channel signal (312) and using a level parameter (306)
having information on a level relation between two
channels of the multi-channel signal and using head-
related transfer functions (308) related to the two
channels of the multi-channel signal, the method
comprising:
deriving, using the level parameter (306), modified
head-related transfer functions (310) by weighting the
head-related transfer functions (308) of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level
than by the head-related transfer function of a
channel having a lower level; and
deriving the headphone down mix signal (314) using the
modified head-related transfer functions (310} and the
representation of the down mix signal.
24. Receiver or audio player having a decoder for deriving
a headphone down mix signal (314) using a
representation of a down mix of a multi-channel signal
(312) and using a level parameter (306) having
information on a level relation between two channels
of the multi-channel signal and using head-related
transfer functions (308) related to the two channels
of the multi-channel signal, comprising:
a filter calculator for deriving modified head-related
transfer functions (310) by weighting the head-related
transfer functions (308) of the two channels using the
level parameter (306) such that a modified head-

related transfer function is stronger influenced by
the head-related transfer function of a channel having
a higher level than by the head-related transfer
function of a channel having a lower level; and
a synthesizer for deriving the headphone down mix
signal (314) using the modified head-related transfer
functions (310) and the representation of the down mix
signal.
25. Method of receiving or audio playing, the method
having a method for deriving a headphone down mix
signal (314) using a representation of a down mix of a
multi-channel signal (312) and using a level parameter
(306) having information on a level relation between
two channels of the multi-channel signal and using
head-related transfer functions (308) related to the
two channels of the multi-channel signal, the method
comprising:
deriving, using the level parameter (306), modified
head-related transfer functions (310) by weighting the
head-related transfer functions (308) of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level
than by the head-related transfer function of a
channel having a lower level; and
deriving the headphone down mix signal (314) using the
modified head-related transfer functions (310) and the
representation of the down mix signal.
26- Computer program having a program code for performing,
when running on a computer, a method for deriving a
headphone down mix signal (314) using a representation
of a downmix of a multi-channel signal (312) and using
a level parameter (306) having information on a level

relation between two channels of the multi-channel
signal and using head-related transfer functions (308)
related to the two channels of the multi-channel
signal, the method comprising:
deriving, using the level parameter (306), modified
head-related transfer functions (310) by weighting the
head-related transfer functions (308) of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level
than by the head-related transfer function of a
channel having a lower level; and
deriving the headphone down mix signal (314) using the
modified head-related transfer functions (310) and the
representation of the down mix signal.
27. Computer program having a program code for performing,
when running on a computer, a method for receiving or
audio playing, the method having a method for deriving
a headphone down mix signal (314) using a
representation of a down mix of a multi-channel signal
(312) and using a level parameter (306) having
information on a level relation between two channels
of the multi-channel signal and using head-related
transfer functions (308) related to the two channels
of the multi-channel signal, the method comprising:
deriving, using the level parameter (306), modified
head-related transfer functions (310) by weighting the
head-related transfer functions (308) of the two
channels such that a modified head-related transfer
function is stronger influenced by the head-related
transfer function of a channel having a higher level
than by the head-related transfer function of a
channel having a lower level; and

deriving the headphone down mix signal (314) using the
modified head-related transfer functions (310) and the
representation of the down mix signal.

A headphone down mix signal (314) can be efficiently derived from a parametric down mix of a multi-channel signal (312), when modified HRTFs (310) (head related transfer functions) are derived from HRTFs (308) of a multi-channel signal using a level parameter (306) having
information on a level relation between two channels of the multi-channel signals such that a modified HRTF (310) is stronger influenced by the HRTF (308) of a channel having a higher level than by the HRTF (308) of a channel having a lower level. Modified HRTFs (310) are derived within the decoding process taking into account the relative strength of the channels associated to the HRTFs (308). The HRTFs (308) are thus modified such that a down mix signal (314)
of a parametric representation of a multi-channel signal can directly be used to synthesize the headphone down mix signal (314) without the need of an intermediate full
parametric multi-channel reconstruction of the parametric down mix.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=Tr0BDoSs+1uF5eWCvF55UQ==&loc=wDBSZCsAt7zoiVrqcFJsRw==

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Patent Number

279410

Indian Patent Application Number

3747/KOLNP/2008

PG Journal Number

04/2017

Publication Date

27-Jan-2017

Grant Date

20-Jan-2017

Date of Filing

12-Sep-2008

Name of Patentee

See attached documents

Applicant Address

See attached documents

Inventors:

#	Inventor's Name	Inventor's Address
1	KJOERLING, KRISTOFER	LOSTIGEN 10 170 75 SOLNA
2	VILLEMOES, LARS	MANDOLINVAEGEN 22 175 56 JAERFAELLA/SE
3	BREEBAART, JEROEN	GROENEWOUDSEWEG 1 5621 BA EINDHOVEN

PCT International Classification Number

H04S 3/00

PCT International Application Number

PCT/EP2006/008566

PCT International Filing date

2006-09-01

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	0600674-6	2006-03-24	Sweden
2	60/744,555	2006-04-10	Sweden