Patent Description:
Reducing the number of channels is essential for achieving multichannel coding at low bitrates, as in <CIT> and <CIT>. For example, parametric stereo coding schemes are based on an appropriate mono downmix from the left and right input channels. The so-obtained mono signal is to be encoded and transmitted by the mono codec along with side-information describing in a parametric form the auditory scene. The side information usually consists of several spatial parameters per frequency sub-band. They could include for example:.

However, a downmix processing is prone to create signal cancellation and coloration due to inter-channel phase misalignment, which leads to undesired quality degradations. As an example, if the channels are coherent and near out-of-phase, the downmix signal is likely to show perceivable spectral bias, such as the characteristics of a comb-filter.

The downmix operation can be performed in time domain simply by a sum of the left and right channels, as expressed by <MAT> where l[n] and r[n] are the left and right channels, n is the time index, and w<NUM>[n] and w<NUM>[n] are weights that determined the mixing. If the weights are constant over time, we speak about passive downmix. It has the disadvantage to be regardless of the input signal and the quality of the obtained downmix signal is highly dependent on input signal characteristics. Adapting the weight over time can reduce this problem to some extent.

However, for solving the main issues, an active downmix is usually performed in the frequency domain using for example a Short-Term Fourier Transform (STFT). Thereby the weights can be made dependent of the frequency index k and time index n and can fit better to the signal characteristics. The downmix signal is then expressed as: <MAT> where M[k,n], L[k,n] and R[k,n] are the STFT components of the downmix signal, the left channel and the right channel, respectively, at frequency index k and time index n. The weights W<NUM>[k, n]and W<NUM>[k, n] can be adaptively adjusted in time and in frequency. It aims at preserving the average energy or amplitude of the two input channels by minimizing spectral bias caused by comb filtering effects.

The most straightforward method for active downmixing is to equalize the energy of the downmix signal to yield for each frequency bin or sub-band the average energy of the two input channels [<NUM>]. The downmix signal as shown in <FIG> can be then formulated as: <MAT> where <MAT>.

Such straight forward solution has several shortcomings. First, the downmix signal is undefined when the two channels have phase inverted time-frequency components of equal amplitude (ILD=0db and IPD=pi). This singularity results from the denominator becoming zero in this case. The output of a simple active downmixing is in this case unpredictable. This behavior is shown in <FIG> for various inter-channel level differences where the phase is plotted as a function of the IPD.

For ILD=0dB, the sum of the two channels is discontinuous at IPD=pi resulting in a step of pi radian. In other conditions, the phase evolves regularly and continuously in modulo 2pi.

The second nature of problems comes from the important variance of the normalization gains for achieving such an energy-equalization. Indeed the normalization gains can fluctuate drastically from frame to frame and between adjacent frequency sub-bands. It leads to an unnatural coloration of the downmix signal and to block effects. The usage of synthesis windows for the STFT and the overlap-add method result in smoothed transitions between processed audio frames. However, a great change in the normalization gains between sequential frames can still lead to audible transition artefacts. Moreover, this drastic equalization can also leads to audible artefacts due to aliasing from the frequency response side lobes of the analysis window of the block transform.

As an alternative, the active downmix can be achieved by performing a phase alignment of the two channels before computing the sum-signal [<NUM>-<NUM>]. The energy-equalization to be done on the new sum signal is then limited, since the two channels are already in-phase before summing them up. In [<NUM>], the phase of the left channel is used as reference for aligning the two channels in phase. If the phases of the left channels are not well conditioned (e.g. zero or low-level noise channel), the downmix signal is directly affected. In [<NUM>], this important issue is solved by taking as reference the phase of the sum signal before rotation. Still the singularity problem at ILD=0dB and IPD= pi is not treated. For this reason, [<NUM>] amends the approach by using a broadband phase difference parameter in order to improve stability in such a case. Nonetheless, none of these approaches considered the second nature of problem related to the instability. The phase rotation of the channels can also lead to an unnatural mixing of the input channels and can create severe instabilities and block effects especially when great changes happen in the processing over time and frequency.

Finally, there are more evolved techniques like [<NUM>] and [<NUM>], which are based on the observations that the signal cancellation during downmixing occurs only on time-frequency components which are coherent between the two channels. In [<NUM>], the coherent components are filtered out before summing-up incoherent parts of the input channels. In [<NUM>], the phase alignment is only computed for the coherent components before summing up the channels. Moreover, the phase alignment is regularized over time and frequency for avoiding problems of stability and discontinuity. Both techniques are computationally demanding since in [<NUM>] filter coefficients need to be identified at every frame and in [<NUM>] a covariance matrix between the channels has to be computed.

It is the object of the present invention to provide an improved concept for downmixing or multichannel processing.

This object is achieved by a downmixer of claim <NUM>, a method of downmixing of claim <NUM>, a multichannel encoder of claim <NUM>, a method of multichannel encoding of claim <NUM>, an audio processing system of claim <NUM>, a method of processing an audio signal of claim <NUM> or a computer program of claim <NUM>.

The present invention is based on the finding that a downmixer for downmixing at least two channel of a multichannel signal having the two or more channels not only performs an addition of the at least two channels for calculating a downmix signal from the at least two channels, but the downmixer additionally comprises a complementary signal calculator for calculating a complementary signal from the multichannel signal, wherein the complementary signal is different from the partial downmix signal. Furthermore, the downmixer comprises an adder for adding the partial downmix signal and the complementary signal to obtain a downmix signal of the multichannel signal. This procedure is advantageous, since the complementary signal, being different from the partial downmix signal fills any time domain or spectral domain holes within the downmix signal that may occur due to certain phase constellations of the at least two channels. Particularly, when the two channels are in phase, then typically no problem should occur when a straight-forward adding together of the two channels is performed. When, however, the two channels are out of phase, then the adding together of these two channels results in a signal with a very low energy even approaching zero energy. Due to the fact, however, that the complementary signal is now added to the partial downmix signal, the finally obtained downmix signal still has significant energy or at least does not show such serious energy fluctuations.

The present invention is advantageous, since it introduces a procedure for downmixing two or more channels aiming to minimize typical signal cancellation and instabilities observed in conventional downmixing.

Furthermore, embodiments are advantageous, since they represent a low complex procedure that has the potential to minimize usual problems from multichannel downmixing.

Preferred embodiments rely on a controlled energy or amplitude-equalization of the sum signal mixed with the complementary signal that is also derived from the input signals, but is different from the partial downmix signal. The energy-equalization of the sum signal is controlled for avoiding problems at the singularity point, but also to minimize significant signal impairments due to large fluctuations of the gain. Preferably, the complementary signal is there to compensate a remaining energy loss or to compensate at least a part of this remaining energy loss.

In an embodiment, the processor is configured to calculate the partial downmix signal so that the predefined energy related or amplitude related relation between the at least two channels and the partial downmix channel is fulfilled, when the at least two channels are in phase, and so that an energy loss is created in the partial downmix signal, when the at least two channels are out of phase. In this embodiment, the complementary signal calculator is configured to calculate the complementary signal so that the energy loss of the partial downmix signal is partly or fully compensated by adding the partial downmix signal and the complementary signal together.

In an embodiment, the complementary signal calculator is configured for calculating the complementary signal so that the complementary signal has a coherence index of <NUM> with respect to the partial downmix signal, where a coherence index of <NUM> shows a full incoherence and a coherence index of <NUM> shows a full coherence. Thus, it is made sure that the partial downmix signal on the one hand and the complementary signal on the other hand are sufficiently different from each other.

Preferably, the downmixing generates the sum signal of the two channels such as L+R as it is done in conventional passive or active downmixing approaches. The gains applied to this sum signal that are subsequently called W<NUM> aim at equalizing the energy of the sum channel for either matching the average energy or the average amplitude of the input channels. However, in contrast to conventional active downmixing approaches, W<NUM> values are limited to avoid instability problems and to avoid that the energy relations are restored based on an impaired sum signal.

A second mixing is done with the complementary signal. The complementary signal is chosen such that its energy does not vanish when L and R are out-of-phase. The weighting factors W<NUM> compensate the energy equalization due to the limitation introduced into W<NUM> values.

Preferred embodiments are subsequently discussed with respect to the accompanying drawings, in which:.

<FIG> illustrates a downmixer for downmixing at least two channels of a multichannel signal <NUM> having the two or more channels. Particularly, the multichannel signal can only be a stereo signal with a left channel L and a right channel R, or the multichannel signal can have three or even more channels. The channels can also include or consist of audio objects. The downmixer comprises a processor <NUM> for calculating a partial downmix signal <NUM> from the at least two channels from the multichannel signal <NUM>. Furthermore, the downmixer comprises a complementary signal calculator <NUM> for calculating a complementary signal from the multichannel signal <NUM>, wherein the complementary signal <NUM> is output by block <NUM> is different from the partial downmix signal <NUM> output by block <NUM>. Additionally, the downmixer comprises an adder <NUM> for adding the partial downmix signal and the complementary signal to obtain a downmix signal <NUM> of the multichannel signal <NUM>. Generally, the downmix signal <NUM> has only a single channel or, alternatively, has more than one channel. Generally, however, the downmix signal has fewer channels than are included in the multichannel signal <NUM>. Thus, when the multichannel signal has, for example, five channels, the downmix signal may have four channels, three channels, two channels or a single channel. The downmix signal with one or two channels is preferred over a downmix signal having more than two channels. In the case of a two channel signal as the multichannel signal <NUM>, the downmix signal <NUM> only has a single channel.

In an embodiment, the processor <NUM> is configured to calculate the partial downmix signal <NUM> so that the predefined energy-related or amplitude-related relation between the at least two channels and the partial downmix signal is fulfilled, when the at least two channels are in phase and so that an energy loss is created in the partial downmix signal with respect to the at least two channels, when the at least two channels are out of phase. Embodiments and examples for the predefined relation are that the amplitudes of the downmix signal are in a certain relation to the amplitudes of the input signals or the subband-wise energies, for example, of the downmix signal are in a predefined relation to the energies of the input signals. One particularly interesting relation is that the energy of the downmix signal either over the full bandwidth or in subbands is equal to an average energy of the two input signals or the more than two input signals. Thus, the relation can be with respect to energy, or with respect to amplitude. Furthermore, the complementary signal calculator <NUM> of <FIG> is configured to calculate the complementary signal <NUM> so that the energy loss of the partial downmix signal as illustrated at <NUM> in <FIG> is partly or fully compensated by adding the partial downmix signal <NUM> and the complementary signal <NUM> in the adder <NUM> of <FIG> to obtain the downmix signal.

Generally, embodiments are based on the controlled energy or amplitude-equalization of the sum signal mixed with the complementary signal also derived from the input channels.

Embodiments are based on a controlled energy or amplitude-equalization of the sum signal mixed with a complementary signal also derived from the input channels. The energy-equalization of the sum signal is controlled for avoiding problems at the singularity point but also to minimize significantly signal impairments due to large fluctuations of the gain. The complementary signal is there to compensate the remaining energy loss or at least a part of it. The general form of the new downmix can be expressed as <MAT> where the complementary signal S[k,n] must be ideally orthogonal as much as possible to the sum signal, but can be in practice chosen as <MAT> or <MAT> or <MAT>.

In all cases, the downmixing generates first the sum channel L+R as it is done in conventional passive and active downmixing approaches. The gain W<NUM>[k, n] aims at equalizing the energy of the sum channel for either matching the average energy or the average amplitude of the input channels. However, unlike conventional active downmixing approaches, W<NUM>[k, n] is limited to avoid instability problems and to avoid that the energy relations are restored based on an impaired sum signal.

A second mixing is done with the complementary signal. The complementary signal is chosen such that its energy doesn't vanish when L[k, n] and R[k, n] are out-of-phase. W<NUM>[k, n] compensates the energy-equalization due to the limitation introduced in W<NUM>[k, n].

As illustrated, the complementary signal calculator <NUM> is configured to calculate the complementary signal so that the complementary signal is different from the partial downmix signal. In quantities, it is preferred that a coherence index of the complementary signal is less than <NUM> with respect to the partial downmix signal. In this scale, a coherence index of <NUM> shows a full incoherence and a coherence index of <NUM> shows a full coherence. Thus, a coherence index of less than <NUM> has proven to be useful so that the partial downmix signal and the complementary signal are sufficiently different from each other. However, coherence indices of less than <NUM> and even less than <NUM> are more preferred.

<FIG> illustrates a procedure performed by the processor. Particularly, as illustrated in item <NUM> of <FIG>, the processor calculates the partial downmix signal with an energy loss with respect the at least two channels that represent the input into the processor. Furthermore, the complementary signal calculator <NUM> calculates the complementary signal <NUM> of <FIG> to partly or fully compensate for the energy loss.

In an embodiment illustrated in <FIG>, the complementary signal calculator comprises a complementary signal selector or complementary signal determiner <NUM>, a weighting factor calculator <NUM> and a weighter <NUM> to finally obtain the complementary signal <NUM>. Particularly, the complementary signal selector or complementary signal determiner <NUM> is configured to use, for calculating the complementary signal, one signal of a group of signals consisting of a first channel such as L, a second channel such as R, a difference between the first channel and the second channel as indicated L-R in <FIG>. Alternatively, the difference can also be R-L. A further signal used by the complementary signal selector <NUM> can be a further channel of the multichannel signal, i.e., a channel that is not selected to be by the processor for calculating the partial downmix signal. This channel can, for example, be a center channel, or a surround channel or any other additional channel comprising an object. In other embodiments, the signal used by the complementary signal selector is a decorrelated first channel, a decorrelated second channel, a decorrelated further channel or even the decorrelated partial downmix signal as calculated by the processor <NUM>. In preferred embodiments, however, either the first channel such as L or the second channel such as R or, even more preferably, the difference between the left channel and the right channel or the difference between the right channel and the left channel are preferred for calculating the complementary signal.

The output of the complementary signal selector <NUM> is input into a weighting factor calculator <NUM>. The weighting factor calculator additionally typically receives the two or more signals to be combined by the processor <NUM> and the weighting factor calculator calculates weights W<NUM> illustrated at <NUM>. Those weights together with the signal used and determined by the complementary signal selector <NUM> are input into the weighter <NUM>, and the weighter then weights the corresponding signal output from block <NUM> using the weighting factors from block <NUM> to finally obtain the complementary signal <NUM>.

The weighting factors can only be time-dependent, so that for a certain block or frame in time, a single weighting factor W<NUM> is calculated. In other embodiments, however, it is preferred to use time and frequency dependent weighting factors W<NUM> so that, for a certain block or frame of the complementary signal, not only a single weighting factor for this time block is available, but a set of weighting factors W<NUM> for a set of different frequency values or spectral bins of the signal generated or selected by block <NUM>.

A corresponding embodiment for time and frequency dependent weighting factors not only for usage of the complementary signal calculator <NUM>, but also for usage of the processor <NUM> is illustrated in <FIG>.

Particularly, <FIG> illustrates a downmixer in a preferred embodiment that comprises a time-spectrum converted <NUM> for converting time domain input channels into frequency domain input channels, where each frequency domain input channel has a sequence of spectra. Each spectrum has a separate time index n and, within each spectrum, a certain frequency index k refers to a frequency component uniquely associated with the frequency index. Thus, in an example, when a block has <NUM> spectral values, then the frequency k runs from <NUM> to <NUM> in order to uniquely identify each one of the <NUM> different frequency indices.

The time-spectrum converter <NUM> is configured for applying an FFT and, preferably, an overlapping FFT so that the sequence of spectra obtained by block <NUM> are related to overlapping blocks of the input channels. However, non-overlapping spectral conversion algorithms and other conversions apart from an FFT such as DCT or so can be used as well.

Particularly, the processor <NUM> of <FIG> comprises a first weighting factor calculator <NUM> for calculating weights W<NUM> for individual spectral indices k or weighting factors W<NUM> for subbands b, where a subband is broader than a spectral value with respect to frequency, and typically, comprises two or more spectral values.

The complementary signal calculator <NUM> of <FIG> comprises a second weighting factor calculator that calculates the weighting factors W<NUM>. Thus, item <NUM> can be similarly constructed as item <NUM> of <FIG>.

Furthermore, the processor <NUM> of <FIG> calculating the partial downmix signal comprises a downmix weighter <NUM> that receives, as an input, the weighting factors W<NUM> and that outputs the partial downmix signal <NUM> that is forwarded to the adder <NUM>. Furthermore, the embodiment illustrated in <FIG> additionally comprises the weighter <NUM> already described with respect <FIG> that receives, as an input, the second weighting factors W<NUM>.

The adder <NUM> outputs the downmix signal <NUM>. The downmix <NUM> can be used in several different occurrences. One way to use the downmix signal <NUM> is to input it into a frequency domain downmix encoder <NUM> illustrated in <FIG> that outputs an encoded downmix signal. An alternative procedure is to insert the frequency domain representation of the downmix signal <NUM> into a spectrum-time converter <NUM> in order to obtain, at the output of block <NUM>, a time domain downmix signal. A further embodiment is to feed the downmix signal <NUM> into a further downmix processor <NUM> that generates some kind of process downmix channel such as a transmitted downmix channel, a stored downmix channel, or a downmix channel that has performed some kind of equalization, a gain variation etc..

In embodiments, the processor <NUM> is configured for calculating time or frequency-dependent weighting factors W<NUM> as illustrated by block <NUM> in <FIG> for a weighting a sum of the at least two channels in accordance with a predefined energy or amplitude relation between the at least two channels and a sum signal of the at least two channels. Furthermore, subsequent to this procedure that is also illustrated in item <NUM> of <FIG>, the processor is configured to compare a calculated weighting factor W<NUM> for a certain frequency index k and a certain time index n or for a certain spectral subband b and a certain time index n to a predefined threshold as indicated at block <NUM> of <FIG>. This comparison is performed preferably for each spectral index k or for each subband index b or for each time index n and preferably for one spectrum index k or b and for each time index n. When the calculated weighting factor is in a first relation to the predefined threshold such as below the threshold as illustrated at <NUM>, then the calculated weighting factor W<NUM> is used as indicated at <NUM> in <FIG>. When, however, the calculated weighting factor is in a second relation to the predefined threshold that is different from the first relation to the predefined threshold such as above the threshold as indicated at <NUM>, the predefined threshold is used instead of the calculated weighting factor for calculating the partial downmix signal in block <NUM> of <FIG> for example. This is a "hard" limitation of W<NUM>. In other embodiments, a kind of a "soft limitation" is performed. In this embodiment, a modified weighting factor is derived using a modification function, wherein the modification function is so that the modified weighting factor is closer to the predefined threshold then the calculated weighting factor.

The embodiment in <FIG> uses a hard limitation, while the embodiment in <FIG> and the embodiment in <FIG> use a soft limitation, i.e., a modification function.

In a further embodiment, the procedure in <FIG> is performed with respect to block <NUM> and block <NUM>, but a comparison to a threshold as discussed with respect to block <NUM> is not performed. Subsequent to the calculation in block <NUM>, a modified weighting factor is derived using the modification function of the above description of block <NUM>, wherein the modification function is so that a modified weighting factor results in an energy of the partial downmix signal being smaller than an energy of the predefined energy relation. Preferably, the modification function that is applied without a specific comparison is so that it limits, for high values of W<NUM> the manipulated or modified weighting factor to a certain limit or only has a very small increase such as a log or In function or so that, though not being limited to a certain value only has a very slow increase anymore so that stability problems as discussed before are substantially avoided or at least reduced.

In a preferred embodiment illustrated in <FIG>, the downmix is given by: <MAT> where <MAT> <MAT>.

In the above equation, A is a real valued constant preferably being equal to the square root of <NUM>, but A can have different values between <NUM> or <NUM> as well. Depending on the application, even values different from the above mentioned values can be used as well.

W<NUM>[k, n] and W<NUM>[k, n]are always positive and W<NUM>[k, n] is limited to <MAT> or e.g. <NUM>.

The mixing gains can be computed bin-wise for each index k of the STFT as described in the previous formulas or can be computed band-wise for each non-overlapping sub-band gathering a set of indices b of the STFT. The gains are calculated based on the following equation: <MAT> <MAT>.

Since the energy preservation during the equalization is not a hard constraint, the energy of the resulting downmix signal varies compared the average energy of the input channel. The energy relation depends on the ILD and IPD as illustrated in <FIG>.

In contrast to the simple active downmixing method, which preserves a constant relation between the output energy and the average energy of the input channels, the new downmix signal does not show any singularity as illustrated in <FIG>. Indeed, in <FIG> a jump of a magnitude Pi (<NUM>°), can be observed at IP=Pi and ILD=0dB, while in <FIG>, the jump is of 2Pi (<NUM>°), which corresponds to a continuous change in the unwrapped phase domain.

Listening test results confirm that the new down-mix method results in significantly less instabilities and impairments for a large range of stereo signals than conventional active downmixing.

In this context, <FIG> illustrates, along the x-axis, the inter-channel level difference between an original left and an original right channel in dB. Furthermore, the downmix energy is indicated in a relative scale between <NUM> and <NUM> along the y-axis and the parameter is the inter-channel phase difference IPD. Particularly, it appears that the energy of the resulting downmix signal varies particularly dependent on the phase between the channels and, for a phase of Pi (<NUM>°), i.e., for an out of phase situation, the energy variation is, at least for positive inter-channel level differences, in good shape. <FIG> illustrates equations for calculating the downmix signal M and it also becomes clear that, as the complementary signal, the left channel is selected. <FIG> illustrates weighting factors W<NUM> and W<NUM> not only for individual spectral indices, but for subbands where a set of indices from the STFT, i.e., at least two spectral values k are added together to obtain a certain subband.

Compared to the prior art illustrated in <FIG>, any singularity is not included anymore when <FIG> is compared to <FIG>.

<FIG> illustrates a further embodiment, where the downmix is calculated using the difference between left and right signals L and R as the basis for the complementary signal. Particularly, in this embodiment, <MAT> where the set of gains W<NUM>[k, n] and W<NUM>[k, n] are computed such that the energy relation between the down-mixed signal and the input channels holds in every condition.

First the gain W<NUM>[k, n] is computed for equalizing the energy till a given limit, where A is again a real valued number equal to <MAT> or different from this value: <MAT> <MAT>.

As a consequence, the gain W<NUM>[k, n] of the sum signal is limited to the range [<NUM>, <NUM>] as shown in <FIG>. In the equation for x, an alternative implementation is to use the denominator without a square root.

If the two channels have an IPD greater than pi/<NUM>, W<NUM> can no more compensate for the loss of energy, and it will be then coming from the gain W<NUM>. W<NUM> is computed as one of the roots of the following quadratic equation: <MAT>.

The roots of the equation are given by: <MAT> where <MAT> <MAT>.

One of the two roots can be then selected. For both roots, the energy relation is preserved for all conditions as shown in <FIG>.

Preferably, the root with the minimum absolute value is adaptively selected for W<NUM>[k, n]. Such an adaptive selection will result in a switch from one root to another for ILD=0dB, which once again can create a discontinuity.

In contrast to the state-of-the art, this approach solves the comb-filtering effect of the downmix and spectral bias without introducing any singularity. It maintains the energy relations in all conditions but introduces more instabilities compared to the preferred embodiment.

Thus, <FIG> illustrates a comparison of the gain limitation obtained by the factors W<NUM> of the sum signal in the calculation of the partial downmix signal of this embodiment. Particularly, the straight line is the situation before normalization or before modification of the value as discussed before with respect to block <NUM> of <FIG>. And, the other line that approaches a value of <NUM> for the modification function as a function of the weighting factor W<NUM>. It becomes clear that an influence of the modification function occurs at values above <NUM> but the deviation only becomes really visible for values W<NUM> of about <NUM> and greater.

<FIG> illustrates the equation implemented by the <FIG> block diagram for this embodiment.

Furthermore, <FIG> illustrates how the values W<NUM> are calculated and, therefore, <FIG> illustrates the functional situation of <FIG>. Finally, <FIG> illustrates the calculation of W<NUM>, i.e., the weighting factors used by the complementary signal generator <NUM> of <FIG>.

<FIG> illustrates that the downmix energy is always the same and equal to <NUM> for all phase differences between the first and the second channels and for all level differences ALD between the first and the second channels.

However, <FIG> illustrates the discontinuities incurred by the calculations of the rules of the equation for EM of <FIG> due to the fact there is a denominator in the equation for p and the equation for q illustrated in <FIG> that can become <NUM>.

<FIG> illustrate a further embodiment that can be seen as a compromise between the two earlier described alternatives.

In the equation for x, an alternative implementation is to use the denominator without a square root.

In this case the quadratic equation to solve is: <MAT>.

This time the gain W<NUM> is not exactly taken as one of the roots of the quadratic equation but rather: <MAT> where <MAT> <MAT>.

As a result, the energy relation is not preserved all the time as shown in <FIG>. On the other hand the gain W<NUM> doesn't show any discontinuities in <FIG> and compared to the second embodiment instability problems are reduced.

Thus, <FIG> illustrates the energy relation of this embodiment illustrated by <FIG> where, once again, the downmix energy is illustrated at the y-axis and the inter-channel level difference is illustrated at the x-axis. <FIG> illustrates the equations applied by <FIG> and the procedures performed for calculating the first weighting factors W<NUM> as illustrated with respect to block <NUM>. Furthermore, <FIG> illustrates the alternative calculation of W<NUM> with respect to the embodiment of <FIG>. Particularly, p is subjected to an absolute value function which appears when comparing <FIG> to the similar equation in <FIG>.

<FIG> then once again shows the calculation of p and q and <FIG> roughly corresponds to the equations in <FIG> at the bottom.

<FIG> illustrates the energy relation of this new downmixing in accordance with the embodiment illustrated in <FIG>, and it appears that the gain W<NUM> only approaches a maximum value of <NUM>.

Although the preceding description and certain Figs. provide detailed equations, it is to be noted that advantages are already obtained even when the equations are not calculated exactly, but when the equations are calculated, but the results are modified. Particularly, the functionalities of the first weighting factor calculator <NUM> and the second weighting factor calculator <NUM> of <FIG> are performed so that the first weighting factors or the second weighting factors have values being in a range of ± <NUM>% of values determined based on the above given equations. In the preferred embodiment, the weighting factors are determined to have values being in a range of ± <NUM>% of the values determined by the above equations. In even more preferred embodiments, the deviation is only ± <NUM>% and in the most preferred embodiments, the results of the equations are exactly taken. But, as stated, advantages of the present invention are even obtained, when deviations of ± <NUM>% from the above described equations are applied.

<FIG> illustrates an embodiment of a multichannel encoder, in which the inventive downmixer as discussed before with respect to <FIG>, <FIG> can be used. Particularly, the multichannel encoder comprises a parameter calculator <NUM> for calculating multichannel parameters <NUM> from at least two channels of the multichannel signal <NUM> having the two or more channels. Furthermore, the multichannel encoder comprises the downmixer <NUM> that can be implemented as discussed before and that provides one or more downmix channels <NUM>. Both, the multichannel parameters <NUM> and the one or more downmix channels <NUM> are input into an output interface <NUM> for outputting an encoded multichannel signal comprising the one or more downmix channels and/or the multichannel parameters. Alternatively, the output interface can be configured for storing or transmitting the encoded multichannel signal to, for example, a multichannel decoder illustrated in <FIG>. The multichannel decoder illustrated in <FIG> receives, as an input, the encoded multichannel signal <NUM>. This signal is input into an input interface <NUM>, and the input interface <NUM> outputs, on the first hand, the multichannel parameters <NUM> and, on the other hand, the one or more downmix channels <NUM>. Both data items, i.e., the multichannel parameters <NUM> and downmix channels <NUM> are input into a multichannel reconstructor <NUM> that reconstructs, at its output, an approximation of the original input channels and, in general, outputs output channels that may comprise or consist of output audio objects or anything like that as indicated by reference numeral <NUM>. Particularly, the multichannel encoder in <FIG> and the multichannel decoder in <FIG> together represent an audio processing system where the multichannel encoder is operative as discussed with respect to <FIG> and where the multichannel decoder is, for example, implemented as illustrated in <FIG> and is, in general, configured for decoding the encoded multichannel signal to obtain a reconstructed audio signal illustrated at <NUM> in <FIG>. Thus, the procedures illustrated with respect to <FIG> additionally represent a method of processing an audio signal comprising a method of multichannel encoding and a corresponding method of multichannel decoding.

An inventively encoded audio signal can be stored on a digital storage medium or a non-transitory storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system.

Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier or a non-transitory storage medium.

In other words, an embodiment of the inventive method is, therefore, a computer program having a program code.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program.

A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program.

Claim 1:
Downmixer for downmixing at least two channels of a multichannel audio signal (<NUM>) having the at least two channels, comprising:
a processor (<NUM>) for calculating a partial downmix signal (<NUM>) from the at least two channels using adding the at least two channels;
a complementary signal calculator (<NUM>) for calculating a complementary signal from the multichannel audio signal (<NUM>), the complementary signal (<NUM>) being different from the partial downmix signal (<NUM>), wherein the complementary signal calculator (<NUM>) is configured to use, for calculating the complementary signal, one signal of the following group of signals comprising a first channel of the at least two channels, a second channel of the at least two channels, a difference between the first channel and the second channel, a difference between the second channel and the first channel, a decorrelated first channel, a decorrelated second channel, a decorrelated further channel, a decorrelated difference involving the first channel and the second channel, and a decorrelated partial downmix signal (<NUM>); and
an adder (<NUM>) for adding the partial downmix signal (<NUM>) and the complementary signal (<NUM>) to obtain a downmix signal (<NUM>) of the multichannel audio signal (<NUM>).