Adaptive rematrixing of matrixed audio signals

• US 5,291,557 A
• Filed: 10/13/1992
• Issued: 03/01/1994
• Est. Priority Date: 10/13/1992
• Status: Expired due to Term

• Current Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Original Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Inventors: Vernon, Stephen D., Davis, Mark F.
• Primary Examiner(s): Isen, Forester W.
• Assistant Examiner(s): Kelly, Mark D.
• Application Number:
  US07/959,730
• Time in Patent Office: 504 Days
• Field of Search: 381/22, 381/23, 381/18, 381/20, 381/21
• US Class Current: 381/22
• CPC Class Codes: H04S 3/02 of the matrix type, i.e. in which input signals are combined algebraically, e.g. after having been phase shifted with respect to each other

Abstract

In a system in which a low-bit rate encoder and decoder carries matrixed audio signals, an adaptive rematrix rematrixes matrixed signals from an unmodified 4:2 matrix encoder to separate and isolate quiet components from loud ones, thereby avoiding the corruption of quiet signals with the low-bit-rate coding quantization noise of loud signals. The decoder is similarly equipped with a rematrix, which tracks the encoder rematrix and restores the signals to the form required by the unmodified 2:4 matrix decoder. The encoder adaptive rematrix selects the matrix output signals or the amplitude weighted sum and difference of the matrix output signals. The choice of whether the matrix output signals or the sum and difference of the matrix output signals are selected is based on a determination of which results in fewer undesirable artifacts when the output audio signals are recovered in the decoder. The adaptive rematrix may operate on frequency component representations of signals rather than the time-domain signals themselves.

Claims

1. Apparatus for adaptively rematrixing the audio output signals of a 4:

2 audio signal matrix for coding, transmission, or storage and retrieval in a system in which the noise level varies with signal amplitude level, comprising

means for determining which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and

means for applying the matrix output signals to the coding, transmission, or storage and retrieval if one of the matrix output signals has the smallest amplitude and for applying the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude.

2. The apparatus of claim 1 wherein the sum of the matrix output signals is an amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

3. Apparatus for adaptively matrixing four audio input signals into two signals for coding, transmission, or storage and retrieval in a system in which the noise level varies with signal amplitude level, comprising

4:

2 audio matrix means receiving said four audio input signals for providing two matrix output signals, and

adaptive rematrixing means for selectively applying the matrix output signals or the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval.

4. The apparatus of claim 3 wherein said adaptive rematrixing means determines which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and applies the matrix output signals to the coding, transmission, or storage and retrieval if one of the matrix output signals has the smallest amplitude and applies the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude.

5. The apparatus of claim 3 wherein the sum of the matrix output signals is a amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

6. An adaptive audio encoding matrix, comprising

4:

2 audio matrix means receiving four audio source signals L, C, R, and S for providing two matrix encoded audio signals L_T and R_T in response thereto, and

means for adaptively changing the matrix encoding characteristics of said 4;

2 audio matrix means such that the matrix means provides as its output two signals L_T and R_T generally in accordance with the relationships

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

when L_T or R_T has the smallest amplitude among L_T, R_T, k(L_T +R_T), and k(L_T -R_T) and provides as its output two signals L_T '"'"' and R_T '"'"' generally in accordance with the relationships ##EQU5## when k(L_T +R_T) or k(L_T -R_T) has the smallest amplitude among L_T, R_T, L_T '"'"' and R_T '"'"'where k is a constant.

7. Apparatus for use in an encoder for a signal transmission or storage and retrieval system in which audio signals in the encoder are represented as frequency components and the frequency components are subject to bit-rate reduction encoding, the encoder having a noise level which varies with signal amplitude level, the encoder receiving the audio output signals of a 4:

2 audio signal matrix, the apparatus adaptively rematrixing frequency component representations of the 4;

2 matrix output signals, comprising

means for determining which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and

means for applying the frequency component representations of the matrix output signals to the bit-rate reduction encoding if one of the matrix output signals has the smallest amplitude and for applying the sum and difference of the matrix output signals to the bit-rate reduction encoding if one of the sum difference of the matrix output signals has the smallest amplitude.

8. The apparatus of claim 7 wherein the sum of the matrix output signals is an amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

9. An encoder for a signal transmission or storage and retrieval system, the encoder receiving the output signals of a 4:

2 audio signal matrix, comprising

means for dividing the matrix output signals into frequency components,

bit-rate reduction encoding means, said bit-rate reduction encoding means having a noise level which varies with signal amplitude level, and

adaptive rematrixing means for determining which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and for applying frequency components representing the matrix output signals to the coding, transmission, or storage and retrieval if one of the matrix output signals has the smallest amplitude and for applying frequency components representing the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude.

10. The apparatus of claim 9 wherein the sum of the matrix output signals is an amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

11. An adaptive 4:

2 audio matrix and encoder for a signal transmission or storage and retrieval system, said matrix and encoder adapted to receive four audio input signals, comprising

4;

2 matrix means receiving said four input signals for providing two matrix output signals,

means for dividing the matrix output signals into frequency components,

bit-rate reduction encoding means, said bit-rate reduction encoding means having a noise level which varies with signal amplitude level, and

adaptive rematrixing means for determining which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and for applying frequency components representing the matrix output signals to the coding, transmission, or storage and retrieval if one of the matrix output signals has the smallest amplitude and for applying frequency components representing the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude.

12. The apparatus of claim 11 wherein the sum of the matrix output signals is an amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

13. The apparatus of claim 9 or 11 wherein said means for dividing the matrix output signals into frequency components includes means for dividing the matrix output signals into time blocks and means for applying a transform to each of said blocks to produce a set of transform frequency coefficients.

14. The apparatus of claim 13 wherein said adaptive rematrixing means operates with respect to each time block and set of transform frequency coefficients.

15. The apparatus of claim 13 wherein said means for applying a transform also groups transform frequency coefficients into frequency bands, and wherein said adaptive rematrixing means operates independently with respect to each or selected ones of frequency band grouped transform coefficients.

16. The apparatus of claim 9 or 11 wherein said means for dividing the matrix output signals into frequency components includes filter bank means.

17. The apparatus of claim 9 or 11 wherein said means for dividing the matrix output signals into frequency components includes quadrature mirror filter means.

18. The apparatus of claim 3 or 11 wherein said 4:

2 audio matrix means provides two output signals in response to four input signals generally in accordance with the relationships

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

where, L is Left channel signal, R is the Right channel signal, C is the Center channel signal and S is the Surround channel signal.

19. The apparatus of claim 3 or 11 wherein the combined action of said 4:

2 audio matrix means and said adaptive rematrixing means provides as its output two signals L_T and R_T generally in accordance with the relationships

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

when L_T or R_T has the smallest amplitude among L_T, R_T, k(L_T +R_T), and k(L_T -R_T) and provides as its output two signals L_T '"'"' and R_T '"'"' generally in accordance with the relationships ##EQU6## when L_T '"'"' or R_T '"'"' has the smallest amplitude among L_T, R_T, L_T '"'"' and R_T '"'"' where, L is Left channel signal, R is the Right channel signal, C is the Center channel signal, S is the Surround channel signal and k is a constant.

20. In a system for coding, transmission, or storage and retrieval of audio signals received from a 4:

2 audio signal encoding matrix and applied to a complementary 2;

4 audio decoding matrix, the system having a noise level which varies with signal amplitude level, apparatus comprising

means for determining which of the signals among the encoding matrix output signals and the sum and difference of the encoding matrix output signals has the smallest amplitude,

means for applying the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the encoding matrix output signals has the smallest amplitude and for applying the sum and difference of the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the weighted sum and weighted difference of the encoding matrix output signals has the smallest amplitude, said means for applying also applying a control signal to the coding, transmission, or storage and retrieval indicating if the encoding matrix output signals or the sum and difference of the encoding matrix output signals is being applied to the transmission or storage, and

means receiving said matrix output signals or the sum and difference of the matrix output signals, and said control signal from the coding, transmission, or storage and retrieval, said means recovering unaltered, for use by the complementary 2;

4 decoding matrix, the received signals when said means for applying applied the matrix encoder output signals to the coding, transmission, or storage and retrieval and for recovering the sum and difference of the received signals, for use by the complementary 2;

4 decoding matrix, when the means for applying applied the sum and difference of the matrix encoder output signals to the coding, transmission, or storage and retrieval.

21. The apparatus of claim 20 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

22. In a 4:

2;

4 matrix system for coding, transmission, or storage and retrieval of four audio signals on a two-channel medium, the system having a channel noise level which varies with signal amplitude level, apparatus comprising

4;

2 audio encoding matrix means receiving said four audio signals for providing two matrix encoded output signals,

adaptive rematrixing means for determining which of the signals among the encoding matrix output signals and the sum and difference of the encoding matrix output signals has the smallest amplitude, and for applying the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the encoding matrix output signals has the smallest amplitude and for applying the sum and difference of the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude, said adaptive matrix means also applying a control signal to the coding, transmission, or storage and retrieval indicating if the encoding matrix output signals or the sum and difference of the encoding matrix output signals is being applied to the coding, transmission, or storage and retrieval,

decode adaptive rematrixing means receiving said encoding matrix output signals or the sum and difference of the encoding matrix output signals and said control signal from the coding, transmission, or storage and retrieval, said means recovering the received signals unaltered when said adaptive rematrixing means applied the matrix encoder output signals to the coding, transmission, or storage and retrieval and for recovering the sum and difference of the received signals when the adaptive rematrixing means applied the sum and difference of the matrix encoder output signals to the coding, transmission, or storage and retrieval, and

complementary 2;

4 audio decoding matrix means receiving the unaltered received signals or the sum and difference of the received signals for providing four matrix output signals representing the four audio signals applied to the 4;

2 audio matrix encoding means.

23. The apparatus of claim 22 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

24. The apparatus of claim 22 wherein said 4:

2 audio matrix means provides two output signals in response to four input signals generally in accordance with the relationships

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

where, L is Left channel signal, R is the Right channel signal, C is the Center channel signal and S is the Surround channel signal and said complementary 2;

4 audio decoding matrix means provides four output signals in response to two input signals generally in accordance with the relationships ##EQU7##

25. The apparatus of claim 22 wherein the combined action of said 4:

2 audio matrix means and said adaptive rematrixing means provides as its output two signals L_T and R_T generally in accordance with a first set of relationships

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

when L_T or R_T has the smallest amplitude among L_T, R_T, k(L_T +R_T), and k(L_T -R_T) and provides as its output two signals L_T '"'"' and R_T '"'"' generally in accordance with a second set of relationships ##EQU8## when L_T '"'"' or R_T '"'"' has the smallest amplitude among L_T, R_T, L_T '"'"' and R_T '"'"', where L, C, R, and S are the four audio signals received by the encoding matrix means, and

wherein the combined action of said decode adaptive rematrixing means and said complementary 2;

4 audio decoding matrix means provides as its output four signals L'"'"', C'"'"', R'"'"', S'"'"' representing the four audio signals applied to the 4;

2 audio matrix encoding means generally in accordance with the relationships ##EQU9## when the control signal indicates that the adaptive encoding matrixing encoded the L_T and R_T signals in accordance with said first state of relationships, and

wherein the second state of said adaptive 2;

4 audio matrix decoding means provides as its output four signals L'"'"', C'"'"', R'"'"', S'"'"' representing the four audio signals applied to the 4;

2 audio matrix encoding means generally in accordance with the relationships ##EQU10## when the control signal indicates that the adaptive encoding matrix encoded L_T '"'"' and L_T '"'"' in accordance with said second state of relationships, where the subscript D indicates decoded values of the respective signals.

26. An adaptive audio encoding and decoding matrix system for use with signal coding, transmission, or storage and retrieval, comprising

adaptive 4:

2 audio matrix means receiving four audio source signals L, C, R, and S for providing two matrix encoded audio signals L_T and R_T in response thereto for application to signal coding, transmission, or storage, the output signals L_T and R_T having characteristics such that

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

when L_T or R_T has the smallest amplitude among L_T, R_T, k(L_T +R_T), and k(L_T -R_T), where k is a constant, and the output signals L_T and R_T having characteristics such that ##EQU11## when L_T '"'"' or R_T '"'"' has the smallest amplitude among L_T, R_T, L_T '"'"' and R_T '"'"', said means for adaptively changing the matrix encoding characteristics of said 4;

2 audio matrix also producing a control signal indicating which set of relationships define the output signals L_T, R_T, L_T '"'"' and R_T '"'"', and

complementary adaptive 2;

4 audio matrix decoding means receiving said signals L_T and R_T or L_T '"'"' and R_T '"'"' along with said control signal from said coding, transmission, or storage and retrieval for providing four decoded signals L'"'"', C'"'"', R'"'"' and S'"'"' representative of said four audio source signals.

27. Apparatus for use in a signal coding, transmission, or storage and retrieval system in which audio signals are divided into frequency components and the frequency components are subject to bit-rate reduction encoding before application to the coding, transmission, or storage and retrieval, and the encoded signals from the coding, transmission, or storage and retrieval are subject to bit-rate reduction decoding and the decoded frequency components are assembled into representations of the audio signals applied to the system, the system having a noise level which varies with signal amplitude, the system receiving the two audio output signals of a 4:

2 audio signal encoding matrix and the system applying the representations of the audio signals to a 2;

4 audio signal decoding matrix, comprising

adaptive rematrixing means receiving said frequency components for determining which of the signals among the encoding matrix output signals and the sum and difference of the encoding matrix output signals has the smallest amplitude, and for applying frequency components representing the encoding matrix output signals to the bit-rate reduction encoding if one of the encoding matrix output signals has the smallest amplitude and for applying the sum and difference of the encoding matrix output signals to the bit-rate reduction encoding if one of the sum and difference of the matrix output signals has the smallest amplitude, said adaptive matrix means also producing a control signal indicating if frequency components representing the encoding matrix output signals or the sum and difference of the encoding matrix output signals are being applied to the bit-rate reduction encoding, and

decode adaptive rematrixing means receiving said control signal and frequency component representations of said encoding matrix output signals or the sum and difference of the encoding matrix output signals from the bit-rate reduction decoding, said means recovering the received signals unaltered when said adaptive rematrixing means applied frequency representations of the matrix encoder output signals to the bit-rate reduction encoding and recovering frequency component representations of the sum and difference of the received signals when the adaptive rematrixing means applied frequency representations of the sum and difference of the matrix encoder output signals to the coding, transmission, or storage and retrieval.

28. The apparatus of claim 27 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

29. The apparatus of claim 27 wherein the frequency components are grouped into frequency bands, and wherein said adaptive rematrixing means and said decode adaptive rematrixing means operate independently with respect to each or selected ones of frequency band grouped frequency components.

30. In a system in which the noise level varies with signal amplitude level, apparatus for adaptively rematrixing signals received from coding, transmission, or storage and retrieval in response to a control signal also received from the coding, transmission, or storage and retrieval for applying the adaptively rematrixed signals to a 2:

4 audio decoding matrix, the received signals resulting from encoding by a 4;

2 audio signal encoding matrix and adaptive rematrixing of the encoding matrix output signals such that in one state of the adaptive rematrixing the signals applied to the coding, transmission, or storage and retrieval are the output of the encoding matrix and in another state of the adaptive rematrixing the signals applied to the coding, transmission, or storage and retrieval are the amplitude weighted sum and difference of the output of the encoding matrix, said control signal indicating the state of the adaptive rematrixing, comprising

decode adaptive rematrixing means receiving said matrix output signals or the amplitude weighted sum and difference of the matrix output signals from the coding, transmission, or storage and retrieval for producing audio signals representing the output of said 4;

2 encoding matrix for application to said 2;

4 decoding matrix, said means having a first state for recovering the signals unaltered from the coding, transmission, or storage and retrieval and a second state for recovering the sum and difference of the signals from the coding, transmission, or storage and retrieval, and

means receiving said control signal from said coding, transmission, or storage and retrieval for controlling said decode adaptive rematrixing means in response to said control signal, such that the decode adaptive rematrixing means operates in said first state when the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval and the decode adaptive rematrixing means operates in said second state when the sum and difference of the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval.

31. The apparatus of claim 30 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

32. In a system in which the noise level varies with signal amplitude level, apparatus for adaptively matrix decoding signals received from coding, transmission, or storage and retrieval in response to a control signal also received from the coding, transmission, or storage and retrieval, the received signals resulting from encoding of four audio source signals prior to application to said coding, transmission, or storage and retrieval by adaptive 4:

2 audio signal matrix encoding such that in a second state of the adaptive matrix the matrix outputs are the sum and difference of the outputs of the adaptive matrix in its first state, said control signal indicating the state of the adaptive matrix, comprising

decode adaptive dematrixing means receiving from said coding, transmission, or storage and retrieval the signals from the adaptive 4;

2 audio signal encoding for producing four audio signals representing the four audio source signals, the dematrixing means including 2;

4 matrix decoding means and means for adaptively applying the received signals to said 2;

4 matrix decoding means in a first state of operation and the sum and difference of the received signals to said 2;

4 matrix decoding means in a second state of operation, and

means receiving said control signal from said coding, transmission, or storage and retrieval for controlling said decode adaptive dematrixing means in response to said control signal, such that the decode adaptive dematrixing means operates in the first state when the adaptive matrix encoding is in the first state and operates in the second state when the adaptive matrix encoding is in the second state.

46. The apparatus of claim 32 wherein the sum of the received signals is an amplitude weighted sum and the difference of the received signals is an amplitude weighted difference.

33. In a system in which the noise level varies with signal amplitude level, apparatus for adaptively rematrixing and 2:

4 matrix decoding signals received from coding, transmission, or storage and retrieval in response to a control signal also received from the coding, transmission, or storage and retrieval, the received signals resulting from encoding of four audio source signals prior to application to said coding, transmission, or storage and retrieval by a 4;

2 audio signal encoding matrix and adaptive rematrixing of the encoding matrix output signals such that in one state of the adaptive rematrixing the signals applied to the coding, transmission, or storage and retrieval are the output of the encoding matrix and in another state of the adaptive rematrixing the signals applied to the coder, transmission or storage are the amplitude weighted sum and difference of the output of the encoding matrix, said control signal indicating the state of the adaptive rematrixing, comprising

decode adaptive rematrixing means receiving said encoding matrix output signals or the sum and difference of the encoding matrix output signals and said control signal from the coding, transmission, or storage and retrieval, said means recovering the received signals unaltered when said adaptive rematrixing means applied the matrix encoder output signals to the coding, transmission, or storage and retrieval and for recovering the sum and difference of the received signals when the adaptive rematrixing means applied the sum and difference of the matrix encoder output signals to the coding, transmission, or storage and retrieval, and

complementary 2;

4 audio decoding matrix means receiving the unaltered received signals or the sum and difference of the received signals for providing four matrix output signals representing the four audio signals applied to the 4;

2 audio encoding matrix.

34. The apparatus of claim 33 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

35. Apparatus for adaptively matrix decoding signals received from coding, transmission, or storage and retrieval in response to a control signal also received from the coding, transmission, or storage and retrieval, the received signals resulting from the adaptive audio 4:

2 matrix encoding of four audio source signals L, C, R, and S such that the adaptive matrix encoding operates in a first state providing two matrix encoded audio signals L_T and R_T having characteristics such that

space="preserve" listing-type="equation">L.sub.T =L+0.707C+0.707S,

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">R.sub.T =R+0.707C-0.707S

when L_T or R_T had the smallest amplitude among L_T, R_T, k(L_T +R_T), and k(L_T -R_T), where k is a constant and the adaptive matrix encoding operates in a second state providing two matrix encoded audio signals L_T '"'"' and R_T '"'"' having characteristics such that ##EQU12## when L_T '"'"' or R_T '"'"' had the smallest amplitude among L_T, R_T, L_T '"'"' and R_T '"'"', the adaptive audio matrix encoding also producing a control signal indicating which set of relationships defined the output signals L_T and R_T or L_T '"'"' and R_T '"'"', comprising

decode adaptive 2;

4 audio matrix decoding means receiving said L_T and R_T or L_T '"'"' and R_T '"'"' signals from said coding, transmission, or storage and retrieval for providing four decoded signals L'"'"', C'"'"', R'"'"' and S'"'"' representative of the corresponding four audio source signals, the decode adaptive 2;

4 audio matrix decoding means including 2;

4 matrix decoding means and means for adaptively applying the received signals to said 2;

4 matrix decoding means in a first state of operation and the sum and difference of the received signals to said 2;

4 matrix decoding means in a second state of operation, and

means receiving said control signal from said coding, transmission, or storage and retrieval for controlling said decode adaptive matrix decoding means in response to said control signal, such that the decode adaptive matrix decoding means operates in the first state when the adaptive matrix encoding is in the first state and operates in the second state when the adaptive matrix encoding is in the second state.

36. The apparatus of claim 35 wherein said adaptive 2:

4 audio matrix decoding means provides as its output four signals L'"'"', C'"'"', R'"'"', S'"'"' representing the four audio signals applied to the 4;

2 adaptive audio matrixing generally in accordance with the relationships ##EQU13##

47. The apparatus of claim 35 wherein the sum of the received signals is an amplitude weighted sum and the difference of the received signals is an amplitude weighted difference.

37. In a system in which the noise level varies with signal amplitude level, apparatus for use in a decoder complementary to an encoder in which audio signals are divided into frequency components and the frequency components are subject to bit-rate reduction encoding, the decoder receiving the output of the encoder via transmission or storage and retrieval, wherein the decoder bit-rate-reduction decodes and assembles decoded frequency components into representations of the audio signals applied to the encoder, the encoder receiving the two audio output signals of a 4:

2 audio signal encoding matrix and the decoder applying decoded representations of the audio signals to a 2;

4 audio signal decoding matrix, the encoder adaptively rematrixing frequency component representations of the 4;

2 encoding matrix output signals such that in one state of the adaptive rematrixing the signals applied to the bit-rate reduction for transmission or storage are frequency components representations of the output of the encoding matrix and in another state of the adaptive rematrixing the signals applied to the bit-rate reduction encoding for transmission or storage are frequency component representations of the sum and difference of the output of the encoding matrix, said adaptive matrixing producing a control signal indicating the state of the adaptive rematrixing, comprising

decode adaptive rematrixing means receiving from the decoder bit-rate reduction decoded frequency component representations of said 4;

2 encoder matrix output signals unaltered or the sum and difference thereof for producing frequency components which are assembled by the decoder into representations of the audio signals applied to the encoder by the 4;

2 encoding matrix, the decode adaptive rematrixing means having a first state with characteristics substantially the same as the first state of the adaptive matrix encoding and a second state with characteristics substantially the same as the second state of the adaptive matrix encoding, and

means receiving said control signal from said transmission or storage and retrieval for controlling said decode adaptive rematrixing means in response to said control signal, such that the decode adaptive rematrixing means operates in said first state when the matrix encoder output signals are applied to the transmission or storage and retrieval and the decode adaptive rematrixing means operates in said second state when the sum and difference of the matrix encoder output signals are applied to the transmission or storage and retrieval.

38. The apparatus of claim 37 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

39. The apparatus of claim 37 wherein the frequency components are grouped into frequency bands, and wherein said decode adaptive rematrixing means operates independently with respect to each or selected ones of frequency band grouped frequency components.

40. A method for adaptively rematrixing the audio output signals of a 4:

2 audio signal matrix for coding, transmission, or storage and retrieval in a system in which the noise level varies with signal amplitude level, comprising

determining which of the signals among the matrix output signals and the sum and difference of the matrix output signals has the smallest amplitude, and

applying the matrix output signals to the coding, transmission, or storage and retrieval if one of the matrix output signals has the smallest amplitude and for applying the sum and difference of the matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the matrix output signals has the smallest amplitude.

41. The method of claim 40 wherein the sum of the matrix output signals is an amplitude weighted sum and the difference of the matrix output signals is an amplitude weighted difference.

42. In a system for coding, transmission, or storage and retrieval of audio signals received from a 4:

2 audio signal encoding matrix and applied to a complementary 2;

4 audio decoding matrix, the system having a noise level which varies with signal amplitude level, a method comprising

determining which of the signals among the encoding matrix output signals and the sum and difference of the encoding matrix output signals has the smallest amplitude,

applying the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the encoding matrix output signals has the smallest amplitude and applying the sum and difference of the encoding matrix output signals to the coding, transmission, or storage and retrieval if one of the sum and difference of the encoding matrix output signals has the smallest amplitude, and also applying a control signal to the coding, transmission, or storage and retrieval indicating if the encoding matrix output signals or the sum and difference of the encoding matrix output signals is being applied to the transmission or storage, and

receiving said matrix output signals or the sum and difference of the matrix output signals, and said control signal from the coding, transmission, or storage and retrieval, and recovering unaltered, for use by the complementary 2;

4 decoding matrix, the received signals when the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval and recovering the sum and difference of the received signals, for use by the complementary 2;

4 decoding matrix, when the sum and difference of the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval.

43. The apparatus of claim 42 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.

44. In a system in which the noise level varies with signal amplitude level, a method for adaptively rematrixing signals received from coding, transmission, or storage and retrieval in response to a control signal also received from the coding, transmission, or storage and retrieval for applying the adaptively rematrixed signals to a 2:

4 audio decoding matrix, the received signals resulting from encoding by a 4;

2 audio signal encoding matrix and adaptive rematrixing of the encoding matrix output signals such that in one state of the adaptive rematrixing the signals applied to the coding, transmission, or storage and retrieval are the output of the encoding matrix and in another state of the adaptive rematrixing the signals applied to the coding, transmission, or storage and retrieval are the sum and difference of the output of the encoding matrix, said control signal indicating the state of the adaptive rematrixing, comprising

receiving said matrix output signals or the sum and difference of the matrix output signals from the coding, transmission, or storage and retrieval and producing audio signals representing the output of said 4;

2 encoding matrix for application to said 2;

4 decoding matrix, recovering unaltered the matrix output signals from the coding, transmission, or storage and retrieval in a first state of operation and recovering the sum and difference of the matrix output signals from the coding, transmission, or storage and retrieval in a second state of operation, and

receiving said control signal from said coding, transmission, or storage and retrieval and controlling the state of operation in response thereto such that when the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval, the operation is in the first state and when the sum and difference of the matrix encoder output signals are applied to the coding, transmission, or storage and retrieval, the operation is in the second state.

45. The apparatus of claim 44 wherein the sum of the encoding matrix output signals is an amplitude weighted sum and the difference of the encoding matrix output signals is an amplitude weighted difference.