Audio signal processing apparatus

Abstract

To provide an audio signal processing apparatus for processing surround audio signals so that localization feeling of bass is not lost.SOLUTION: An audio signal processing apparatus includes: a surround audio signal generation unit 1 for up-mixing a stereo signal SigIN to generate a 5-channel surround audio signal SigA; an intermediate processing unit 2 for adding harmonics generated using a bass of each channel as a fundamental tone to signals of respective channels of the surround audio signal SigA, and outputting a 5-channel surround audio signal SigB; and a downmix unit 3 for down-mixing the surround audio signal SigB output from the intermediate processing unit 2 and outputting a stereo signal SigOUT. The gain of the harmonics added to the signal of each channel in the intermediate processing unit 2 is set for each channel in accordance with the direction corresponding to the channel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for processing surround audio signals. [Background technology]

[0002] Known surround audio signal processing techniques include upmixing 2-channel stereo signals (L0, R0) to generate 5-channel surround audio signals (L, R, SL, SR, C) (see, for example, Patent Documents 1 and 2). Here, L is the sum of the component of L0 that is correlated with R0 and the component that is uncorrelated with R0, R is the sum of the component of R0 that is correlated with L0 and the component that is uncorrelated with L0, SL is the component of L0 that is uncorrelated with R0, SR is the component of R0 that is uncorrelated with L0, and C is the sum of the components that are correlated with the other of L0 and R0. Furthermore, L is a signal for the speaker diagonally in front of and to the left of the listener, R is a signal for the speaker diagonally in front of and to the right of the listener, SL is a signal for the speaker to the left or diagonally behind the listener, SR is a signal for the speaker to the right or diagonally behind the listener, and C is a signal for the speaker in the center in front of the listener.

[0003] Furthermore, as a technique for processing surround audio signals, a technique for downmixing a multi-channel surround audio signal such as 5.1ch to a 2ch stereo signal is known (for example, Patent Document 3). Furthermore, as a downmixing technique, a virtual surround processing technique is also known that generates a stereo signal that creates a wide sound field when listened to through headphones (for example, Patent Documents 4 and 5). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-126116 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-103768 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-208709 [Patent Document 4] Special Publication No. 2019-508964 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-319802 Summary of the Invention [Problem to be solved by the invention]

[0005] When generating a multi-channel surround audio signal such as 5ch from a 2ch stereo signal, if the surround audio signal is generated to provide a sense of spaciousness in the sound field, there is a problem in that the sense of bass positioning is lost. Furthermore, even when a 2-channel stereo signal is generated by downmixing a multi-channel surround audio signal such as 5-channel, there is a problem in that the bass sound is not properly positioned when listening to the signal through headphones. Therefore, an object of the present invention is to process surround audio signals in a manner that does not lose the sense of bass positioning. [Means for solving the problem]

[0006] To achieve the above object, the present invention provides an audio signal processing device that processes surround audio signals, which is provided with harmonic addition means corresponding to each of a plurality of channels that are at least some of the channels that constitute a first surround audio signal, and causes the harmonic addition means to add harmonic signals within a predetermined band with the bass component of the first surround audio signal as a fundamental tone to the signal of the corresponding channel of the first surround audio signal, and output the resultant signal.However, at least the surround audio signal obtained by replacing the signal of the channel that is corresponding to the harmonic addition means among the channels that constitute the first surround audio signal with the signal output by the corresponding harmonic addition means constitutes a second surround audio signal.

[0007] Here, the audio signal processing device may use the harmonic addition means to generate, for the signal of a channel corresponding to the first surround audio signal, signals for each harmonic within a predetermined band with the bass component of the signal as a fundamental tone, adjust the magnitude of the generated harmonic signals with a gain according to the direction corresponding to the corresponding channel, add the gain-adjusted harmonic signals to the signal of the corresponding channel and output the signal, and may use the direction corresponding to the channel as the direction of the position of the speaker relative to the listener when the signal of that channel is output from the speaker.

[0008] In this case, the left diagonal forward direction is defined as the first direction, the right diagonal forward direction is defined as the second direction, the left or left diagonal rear direction is defined as the third direction, and the right or right diagonal rear direction is defined as the fourth direction, and the channels constituting the first surround audio signal include at least four channels: a channel corresponding to the first direction, a channel corresponding to the second direction, a channel corresponding to the third direction, and a channel corresponding to the fourth direction; and the harmonic adding means is provided corresponding to each of the four channels, and the gain corresponding to the first direction and the second direction may be larger than the gain corresponding to the third direction and the fourth direction.

[0009] Furthermore, the forward front direction may be defined as a fifth direction, and the channels constituting the first surround audio signal may include a channel whose corresponding direction is the fifth direction, and the harmonic addition means may be provided for the channel whose corresponding direction is the fifth direction, and the gain corresponding to the third direction and the fourth direction may be made larger than the gain corresponding to the fifth direction.

[0010] In the above audio signal processing device, the predetermined band is preferably set to include a band between 2 kHz and 3 kHz. The above audio signal processing device may further include a downmixing means for generating a stereo audio signal from the second surround audio signal. The above audio signal processing device may further include a surround audio signal generating means for generating the first surround audio signal from a stereo audio signal. Furthermore, the above audio signal processing device may be provided with both a surround audio signal generating means for generating the first surround audio signal from the first stereo audio signal, and a downmixing means for generating a second stereo audio signal from the second surround audio signal.

[0011] According to the audio signal processing device described above, for each of the multiple channels of the first surround audio signal, a harmonic signal with the bass component of that channel as the fundamental tone is added to the signal of that channel and then output. This allows the listener to get a sense of the positioning of the bass, which is the fundamental tone, by relying on the harmonic signals, which have a higher frequency band than the bass components and therefore provide better directional recognition than the bass components.

[0012] Furthermore, when the gain of the harmonic signals to be added is adjusted with a gain according to the direction corresponding to the channel, the signals can be adjusted and output so that a gain difference that matches the direction-gain characteristics of the sound reaching the outer ear occurs between the harmonic signals of each channel, and in this way the sense of positioning of the bass sounds achieved by adding harmonic signals can be further enhanced.

[0013] In this case, if the band of harmonic signals to be added by adjusting the gain is set to a band including the 2kHz-3kHz band where the gain difference between directions becomes large in the direction-gain characteristics of sounds reaching the outer ear, the sense of positioning of low-frequency sounds can be reliably and effectively increased by adjusting the gain. [Effects of the Invention]

[0014] As described above, according to the present invention, surround audio signals can be processed without losing the sense of bass positioning. [Brief explanation of the drawings]

[0015] [Figure 1]1 is a diagram illustrating a configuration of an audio signal processing device according to an embodiment of the present invention. [Figure 2] 10 is a diagram showing the configuration of a component separation unit that can be used in a surround audio signal generation unit according to an embodiment of the present invention. FIG. [Figure 3] FIG. 2 is a diagram showing a configuration of a harmonic addition block according to an embodiment of the present invention. [Figure 4] FIG. 10 is a diagram showing frequency-gain characteristics for each direction of sound reaching the outer ear. [Figure 5] FIG. 10 is an explanatory diagram of another configuration example of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described. FIG. 1a shows the configuration of an audio signal processing device according to this embodiment. As shown in the figure, the audio signal processing device includes a surround audio signal generation unit 1 that upmixes an input 2-channel stereo signal SigIN of Lin and Rin to generate a 5-channel surround audio signal SigA of L, R, SL, SR, and C, an intermediate processing unit 2 that processes the surround audio signal SigA generated by the surround audio signal generation unit 1 and outputs it as a 5-channel surround audio signal SigB of L', R', SL', SR', and C', and a downmix unit 3 that combines and downmixes L', R', SL', SR', and C' of the surround audio signal SigB output by the intermediate processing unit 2, and outputs a 2-channel stereo signal SigOUT of Lout and Rout.

[0017] Here, the stereo signal SigOUT output by the downmixer 3 is a stereo signal for an audio output device equipped with a pair of left and right speakers, such as stereo headphones or stereo earphones. The audio signal processing apparatus may be configured so that the surround audio signal SigA generated by the surround audio signal generating unit 1 can be output as is to a speaker system consisting of five actual physical speakers. Next, L of the surround audio signal SigA generated by the surround audio signal generating unit 1 is the sum of the component of Lin that is correlated with Rin and the component that is uncorrelated with Rin, R is the sum of the component of Rin that is correlated with Lin and the component that is uncorrelated with Lin, SL is the component of Lin that is uncorrelated with Rin, SR is the component of Rin that is uncorrelated with Lin, and C is the sum of the components of Lin or Rin that are correlated with the other.

[0018] Furthermore, the surround audio signal SigA is a signal for a speaker system that has a speaker FL 45° to the left of the listener, a speaker FR 45° to the right of the listener, a speaker RL 90° to the left of the listener, a speaker RR 90° to the right of the listener, and a speaker CS in front of the listener, with the front direction of the listener being the 0° direction, as shown in FIG. 1b, in this case, L of the surround audio signal SigA is a signal for speaker FL, R is a signal for speaker FR, SL is a signal for speaker RL, SR is a signal for speaker RR, and C is a signal for speaker CS.

[0019] The specific configuration of the surround audio signal generating unit 1 for generating the surround audio signal SigA can vary depending on the application, but basically, the L, R, SL, SR, and C of the surround audio signal SigA are generated by separating the Lin and Rin of the stereo signal SigIN into components that are correlated with Lin and Rin, components that are uncorrelated with Lin and Rin, components that are correlated with Rin and Rin and components that are uncorrelated with Rin and Lin, and combining each of the separated components and each component with Lin and Rin.

[0020] Here, such separation into correlated and uncorrelated components can be performed using a component separation unit having the configuration shown in FIG. The component separator shown in FIG. 2 separates, from signals A and B, a component CA of signal B that is correlated with signal A and a component SB of signal B that is uncorrelated with signal A. As shown in the figure, the component separation unit includes a variable filter 11, an update unit 12 that updates the transfer function (filter coefficient) W of the variable filter 11 using an adaptive algorithm such as an LMS algorithm, and an adder 13, and the variable filter 11, update unit 12, and adder 13 form an adaptive filter.

[0021] Variable filter 11 receives signal A as input, and adder 13 subtracts the output of variable filter 11 from signal B and outputs the result. Update unit 12 executes an adaptive algorithm using the output of adder 13 as error E, and updates transfer function W of variable filter 11 so that the power of error E is minimized.

[0022] The power of the output of adder 13 is minimum when the output of variable filter 11 matches component CA of signal B that is correlated with signal A, and at this time, the output of adder 13, which is obtained by subtracting the output of variable filter 11 from signal B, represents component SB of signal B that is uncorrelated with signal A. Therefore, a component CA of signal B that is correlated with signal A can be separated as the output of variable filter 11, and a component SB of signal B that is uncorrelated with signal A can be separated as the output of adder 13. Similarly, by swapping signal A of signal B, a component CB of signal A that is correlated with signal B and a component SA of signal A that is uncorrelated with signal B can be separated.

[0023] Returning to FIG. 1, the intermediate processing unit 2 includes five harmonic addition blocks 21, each corresponding to L, R, SL, SR, and C of the surround audio signal SigA. A corresponding signal from among L, R, SL, SR, and C is input to each harmonic addition block 21. The output of the harmonic addition block 21 corresponding to L becomes L' of the surround audio signal SigB, the output of the harmonic addition block 21 corresponding to R becomes R' of the surround audio signal SigB, the output of the harmonic addition block 21 corresponding to SL becomes SL' of the surround audio signal SigB, the output of the harmonic addition block 21 corresponding to SR becomes SR' of the surround audio signal SigB, and the output of the harmonic addition block 21 corresponding to C becomes C' of the surround audio signal SigB.

[0024] FIG. 3 shows the configuration of the harmonic addition block 21 of the intermediate processing section 2. Since the five harmonic addition blocks 21 all have the same configuration, the configuration of the harmonic addition block 21 corresponding to L of the surround audio signal SigA will be shown here as a representative example. As shown in the figure, the harmonic addition block 21 includes a first LPF 211 , a harmonic generation section 212 , a second LPF 213 , an HPF 214 , an amplifier 215 , and a synthesis section 216 . The first LPF 211 is a low-pass filter that extracts the L bass components of the surround audio signal SigA, which is the input to the harmonic addition block. The cutoff frequency of the first LPF 211 is set to, for example, 250 Hz, and extracts and outputs the L bass components that are approximately lower than 250 Hz.

[0025] The harmonic generation unit 212 generates harmonics from n times to m times (m>n) times the fundamental tone of the low frequency component of L extracted by the first LPF 211, and outputs the harmonics as harmonic signals. The harmonic signal output from the harmonic generation unit 212 passes through the second LPF 213 and the HPF 214 and is input to the amplifier 215 . The second LPF 213 and the HPF 214 are a low-pass filter and a high-pass filter that constitute a band-pass filter for limiting the band of the harmonic signal. The band limitation performed by the second LPF 213 and the HPF 214 is performed by extracting a band that includes a band of approximately 2 kHz to 3 kHz.

[0026] That is, for example, by setting the cutoff frequency of the second LPF 213 to 3 kHz and the cutoff frequency of the HPF 214 to 2 kHz, harmonic signals in the band of 2 kHz to 3 kHz are extracted and output to the amplifier 215 . Here, the range of multiples (the above n and m multiples) of the harmonics generated by the harmonic generation unit 212 is set so that the harmonics within the band extracted by the second LPF 213 and HPF 214 are included in the generated harmonics relative to the cutoff frequency of the first LPF 211. Next, the amplifier 215 adjusts the level of the harmonic signal band-limited by the second LPF 213 and the HPF 214 with a gain set for each harmonic addition block 21. The gain of the amplifier 215 set for each harmonic addition block 21 will be described later. The synthesis unit 216 then synthesizes the harmonic signal output by the amplifier 215 with the L of the surround audio signal SigA that is the input of the harmonic addition block 21, and outputs the result as the L' of the surround audio signal SigB. For confirmation, the configuration of the overtone addition block 21 corresponding to R is obtained by replacing L with R and L' with R' in Figure 3, the configuration of the overtone addition block 21 corresponding to SL is obtained by replacing L with SL and L' with SL' in Figure 3, the configuration of the overtone addition block 21 corresponding to SR is obtained by replacing L with SR and L' with SR' in Figure 3, and the configuration of the overtone addition block 21 corresponding to C is obtained by replacing L with C and L' with C' in Figure 3.

[0027] However, although the five harmonic addition blocks 21 have the same configuration as described above, the gain of the amplifier 215 is set for each harmonic addition block 21 as described above. The gain of the amplifier 215 for each harmonic addition block 21 will be explained below. First, FIG. 4 shows the frequency-gain characteristics for each direction of sound reaching the outer ear. As shown in the figure, the gain of the sound reaching the outer ear varies depending on the frequency of the sound and also on the direction of the sound source. Furthermore, in the 2kHz-3kHz band, the difference in gain between the 0°, 45°, and 90° directions is evident. In the 2kHz-3kHz band, the gain is greatest in the 45° direction, followed by the 90° direction and the 0° direction, with the difference between the 45° and 90° directions being approximately 2dB on average, and the difference between the 90° and 0° directions being approximately 2dB on average. Therefore, as shown in Figure 4, the difference in gain between the 45° direction and the 0° direction in the 2kHz-3kHz band is set to d1, and the difference in gain between the 90° direction and the 0° direction in the 2kHz-3kHz band is set to d2, and the gain of the amplifier 215 for each harmonic addition block 21 is set so that the gain of the amplifier 215 of the harmonic addition block 21 corresponding to L and R of the surround audio signal SigA is d1 larger than the gain of the amplifier 215 of the harmonic addition block 21 corresponding to C of the surround audio signal SigA, and the gain of the amplifier 215 of the harmonic addition block 21 corresponding to SL and SR of the surround audio signal SigA is d2 larger than the gain of the amplifier 215 of the harmonic addition block 21 corresponding to C of the surround audio signal SigA.

[0028] However, the gain of the amplifier 215 of each harmonic addition block 21 may be determined by further taking into consideration other conditions that affect the gain of the harmonic signals before they are output as Lout and Rout of the stereo signal SigOUT, such as the gains given to L', R', SL', SR', and C' by the downmix unit 3 when synthesizing to generate Lout and Rout of the stereo signal SigOUT.

[0029] Furthermore, when the downmix unit 3 adjusts and combines the gains of each channel, if the difference in gain used for the adjustment is the same as the difference in gain of the harmonic signals of each channel achieved by the gain adjustment by the amplifier 215 in the above embodiment, it is possible to eliminate the amplifier 215 and not adjust the gain of the harmonic signals.

[0030] The embodiments of the present invention have been described above. In this embodiment, harmonic signals with the bass components of each channel of the surround audio signal SigA as the fundamental tone are added to the signals of each channel and output, so that the listener can get a sense of the positioning of the bass, which is the fundamental tone, by relying on the harmonic signals, which have a higher frequency band than the bass components and therefore provide better directional recognition than the bass components.

[0031] In addition, the gain of the harmonic signals added to each channel is adjusted and output so that a gain difference that matches the direction-gain characteristics of the sound reaching the outer ear is generated between the harmonic signals of each channel, thereby further enhancing the sense of positioning of bass sounds achieved by adding harmonic signals. Furthermore, the band of harmonic signals that are added to each channel by adjusting the gain is set to the 2kHz-3kHz band, where the gain difference between directions becomes large in the direction-gain characteristics of sounds reaching the outer ear, so that the sense of positioning of low-frequency sounds can be reliably and effectively increased by adjusting the gain. In the above embodiment, the bass band used as the fundamental tone of the harmonic signal to be added is set to 250 Hz or less, but this band may be set to another band, such as 100 Hz or less or 100 Hz to 200 Hz, depending on the bass band in which the sense of positioning is desired to be enhanced in actual use or application.

[0032] Furthermore, in the above embodiment, the intermediate processing unit 2 receives the 5-channel surround audio signal SigA generated by the surround audio signal generating unit 1 as input and generates a 5-channel surround audio signal SigB by adding harmonic signals to each of the 5 channels of the surround audio signal SigA. However, n may be any number greater than 4, and the surround audio signal generating unit 1 may generate an n-channel or n.1-channel surround audio signal SigA, and the intermediate processing unit 2 may generate a surround audio signal SigB by adding harmonic signals to all or some of the channels of the surround audio signal SigA.

[0033] Furthermore, without providing the surround audio signal generating unit 1, the intermediate processing unit 2 may generate a surround audio signal SigB by adding harmonic signals to predetermined channels, including L, R, SL, and SR, of the surround audio signal SigS of n channels or n.1 channels, which serves as an audio source.

[0034] For example, in the case where the audio source is an 8-channel surround audio signal SigS as shown in FIG. 5a, in which L is assumed to be output from a speaker 30° to the left of the listener, R is assumed to be output from a speaker 30° to the right of the listener, SL is assumed to be output from a speaker 90° to the left of the listener, SR is assumed to be output from a speaker 90° to the right of the listener, C is assumed to be output from a speaker in front of the listener, SBL is assumed to be output from a speaker 135° diagonally behind the left of the listener, and SBR is assumed to be output from a speaker 135° diagonally behind the right of the listener, the intermediate processing unit 2 may add harmonic signals only to L, R, SL, and SR of the surround audio signal SigS in the harmonic addition block 21 as described above to generate the surround audio signal SigB.

[0035] Furthermore, in the above embodiment, the surround audio signal SigB generated by the intermediate processing unit 2 may be output to a speaker system consisting of a plurality of real physical speakers corresponding to the respective channels of the surround audio signal. In this case, the gain of the amplifier 215 of each harmonic addition block 21 in the intermediate processing section 2 may be determined taking into consideration the placement of each speaker in the speaker system. For example, as shown in FIG. 5b, if a speaker system is configured with a speaker 30° to the left of the listener that outputs L of a 5-channel surround audio signal, a speaker 30° to the right of the listener that outputs R, a speaker 120° diagonally behind the left of the listener that outputs SL, a speaker 120° diagonally behind the right of the listener that outputs SR, and a speaker in front of the listener that outputs C, and the output of each harmonic addition block 21 is output from the speaker that outputs the channel corresponding to that harmonic addition block 21, The gains of the amplifiers 215 of the harmonic addition block 21 corresponding to L and R of the surround audio signal SigA output from the speaker diagonally behind the listener are set to be larger than the gain of the amplifiers 215 of the harmonic addition block 21 corresponding to C of the surround audio signal SigA, and the gains of the amplifiers 215 of the harmonic addition block 21 corresponding to SL and SR of the surround audio signal SigA output from the speaker diagonally behind the listener are set to be smaller than the gain of the amplifiers 215 of the harmonic addition block 21 corresponding to C of the surround audio signal SigA.

[0036] However, the gain of the amplifier 215 of each harmonic addition block 21 may be determined by further taking into consideration other conditions that affect the gain of the harmonic signal before it reaches the listener's outer ear, such as differences in the direction and distance of each speaker from the listener.

[0037] Furthermore, if the difference in gain of the sound of each channel that reaches the listener's outer ear from each speaker, which is caused by differences in the actual direction of each speaker relative to the listener, is the same as the difference in gain of the harmonic signals of each channel achieved by adjusting the gain using amplifier 215 in the above embodiment, then amplifier 215 can be eliminated and the gain of the harmonic signals can be unadjusted. [Explanation of symbols]

[0038] 1...Surround audio signal generation unit, 2...Intermediate processing unit, 3...Downmix unit, 11...Variable filter, 12...Update unit, 13...Adder, 21...Harmonic addition block, 211...First LPF, 212...Harmonic generation unit, 213...Second LPF, 214...HPF, 215...Amplifier, 216...Synthesis unit.

Claims

1. An audio signal processing device for processing a surround audio signal, a harmonic adding means corresponding to each of a plurality of channels that are at least a part of the channels that constitute the first surround audio signal; the harmonic addition means adds a harmonic signal within a predetermined band having a bass component of the first surround audio signal as a fundamental tone to the signal of the corresponding channel of the first surround audio signal, and outputs the result; An audio signal processing device characterized in that a surround audio signal obtained by replacing at least the signal of a channel corresponding to said harmonic addition means among the channels constituting the first surround audio signal with the signal output by said corresponding harmonic addition means constitutes a second surround audio signal.

2. 2. The audio signal processing device according to claim 1, the harmonic addition means generates, for the signal of a channel corresponding to the first surround audio signal, a signal of each harmonic within a predetermined band having a bass component of the signal as a fundamental tone, adjusts the magnitude of the generated harmonic signal by a gain according to the direction corresponding to the corresponding channel, and adds the gain-adjusted harmonic signal to the signal of the corresponding channel and outputs the signal; 10. The audio signal processing apparatus according to claim 9, wherein the direction corresponding to a channel is the direction of a position of a speaker relative to a listener when the speaker outputs a signal of the channel.

3. 3. An audio signal processing apparatus according to claim 2, The left diagonal forward direction is the first direction, the right diagonal forward direction is the second direction, the left or left diagonal backward direction is the third direction, and the right or right diagonal backward direction is the fourth direction. the channels constituting the first surround audio signal include at least four channels: a channel corresponding to the first direction, a channel corresponding to the second direction, a channel corresponding to the third direction, and a channel corresponding to the fourth direction; the harmonic adding means is provided corresponding to each of the four channels, The audio signal processing device, wherein the gains according to the first direction and the second direction are greater than the gains according to the third direction and the fourth direction.

4. 4. The audio signal processing device according to claim 3, The forward direction is the fifth direction, the channels constituting the first surround audio signal include a channel whose corresponding direction is the fifth direction; the harmonic addition means having a corresponding direction corresponding to the channel in the fifth direction, The audio signal processing device according to claim 1, wherein the gains according to the third direction and the fourth direction are greater than the gain according to the fifth direction.

5. 5. An audio signal processing apparatus according to claim 1, 2, 4 or 4, 10. An audio signal processing apparatus, wherein the predetermined band includes a band between 2 kHz and 3 kHz.

6. 5. An audio signal processing apparatus according to claim 1, 2, 3 or 4, 10. An audio signal processing apparatus comprising: a downmixing means for generating a stereo audio signal from the second surround audio signal.

7. 5. An audio signal processing apparatus according to claim 1, 2, 3 or 4, 10. An audio signal processing apparatus comprising: a surround audio signal generating means for generating the first surround audio signal from a stereo audio signal.

8. 5. An audio signal processing apparatus according to claim 1, 2, 3 or 4, a surround audio signal generating means for generating the first surround audio signal from a first stereo audio signal; and downmixing means for generating a second stereo audio signal from the second surround audio signal.

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