Audio or video encoder, audio or video decoder and related methods for processing multi-channel audio or video signals using a variable prediction direction

Abstract

An encoder/decoder is based on a combination of two audio or video channels to obtain a first combination signal as a mid-signal and a residual signal derivable using a predicted side signal derived from the mid-signal. A decoder uses the prediction residual signal, the first combination signal, a prediction direction indicator and prediction information to derive decoded first channel and second channel signals. A real-to-imaginary transform can be applied for estimating the imaginary part of the spectrum of the first combination signal. The prediction signal used in the derivation of the prediction residual signal, the real-valued first combination signal is multiplied by a real portion of the complex prediction information and the estimated imaginary part of the first combination signal is multiplied by an imaginary portion of the complex prediction information.

1 . An audio decoder for decoding an encoded multi-channel audio signal, the encoded multi-channel audio signal comprising an encoded first combination signal, an encoded prediction residual signal and prediction information, comprising:
 a signal decoder for decoding the encoded first combination signal to acquire a decoded first combination signal, and for decoding the encoded residual signal to acquire a decoded residual signal; and   a decoder calculator for calculating a decoded multi-channel signal comprising a decoded first channel signal, and a decoded second channel signal using the decoded residual signal, the prediction information, the decoded first combination signal and a prediction direction indicator indicating a prediction direction associated with the decoded prediction residual signal, so that the decoded first channel signal and the decoded second channel signal are at least approximations of a first channel signal and a second channel signal of a multi-channel signal,   wherein the decoder calculator is configured for using a first calculation rule for calculating the decoded multi-channel signal in case of a first state of the prediction direction indicator and for using a second different calculation rule for calculating the decoded multi-channel signal in case of a second different state of the prediction direction indicator, and   wherein the encoded audio signal is an encoded stereo audio signal, and wherein the decoded multi-channel signal is a decoded stereo audio signal.   
     
     
         2 . The audio decoder in accordance with  claim 1 ,wherein the encoded first combination signal is a real-valued modified discrete cosine transform (MDCT) representation of a downmix signal generated by mid/side coding as the combination rule, and wherein the encoded prediction residual signal is an MDCT representation. 
     
     
         3 . The audio decoder in accordance with  claim 1 , wherein the prediction direction indicator indicates a prediction direction from a mid signal to a side signal or from a side signal to a mid signal. 
     
     
         4 . The audio decoder in accordance with  claim 1 , in which the decoded first combination signal comprises a mid signal, and in which the first calculation rule comprises the
 calculation of the decoded first channel signal and the calculation of the decoded second channel signal using the mid signal, the prediction information and the decoded residual signal without an explicit calculation of the side signal, or   in which the decoded first combination signal comprises a side signal, and in which the second calculation rule comprises the calculation of the decoded first channel signal and the calculation of the decoded second channel signal using the side signal, the prediction information and the decoded residual signal without an explicit calculation of the mid signal.   
     
     
         5 . The audio decoder in accordance with  claim 1 , in which the decoder calculator is configured for using the prediction information where the prediction information comprises a real-valued portion different from zero and/or an imaginary portion different from zero. 
     
     
         6 . The audio decoder of  claim 1 , wherein the decoded first combination signal is a decoded downmix signal, and wherein the prediction information comprises a complex prediction coefficient, and wherein the decoder calculator is configured for:
 generating a complex downmix signal from the decoded first combination signal,   multiplying the complex downmix signal and the complex prediction coefficient to acquire a result,   adding the result and the decoded residual signal to acquire an inversely predicted signal,   performing an inverse mid/side coding, wherein, in the first state of the prediction direction indicator, the decoded downmix signal represents a mid signal of the inverse mid/side coding and the inversely predicted signal represents a side signal of the inverse mid/side coding, and wherein, in the second state of the prediction direction indicator, the decoded downmix signal represents the side signal of the inverse mid/side coding and the inversely predicted signal represents the mid signal of the inverse mid/side coding.   
     
     
         7 . The audio decoder in accordance with  claim 1 ,
 in which the encoded first combination signal and the encoded residual signal have been generated using an aliasing generating time-spectral conversion,   wherein the decoder further comprises:
 a spectral-time converter for generating a time-domain first channel signal and a time-domain second channel signal using a spectral-time conversion algorithm matched to the time-spectral conversion algorithm; 
 an overlap/add processor for conducting an overlap-add processing for the time-domain first channel signal and for the time-domain second channel signal to acquire an aliasing-free first time-domain signal and an aliasing-free second time-domain signal. 
   
     
     
         8 . The audio decoder in accordance with  claim 1 , wherein the decoder calculator is configured for performing an inverse MDCT to a result of an inverse mid/side coding performed by the decoder calculator to acquire a left channel signal of the decoded stereo audio signal and a right channel signal of the decoded stereo audio signal. 
     
     
         9 . The audio decoder in accordance with  claim 1 , wherein a switching of the prediction direction between the first state and the second state is performed on a per-frame basis. 
     
     
         10 . The audio decoder in accordance with  claim 1 , wherein the encoded audio signal comprises bitstream data, the bitstream data comprising, as the prediction information, a complex prediction coefficient and a prediction direction indicator. 
     
     
         11 . The audio decoder in accordance with  claim 10 , wherein the prediction direction indicator is a bit in the bitstream data. 
     
     
         12 . An audio encoder for encoding a multi-channel audio signal comprising two or more channel signals, comprising:
 an encoder calculator for calculating a first combination signal and a prediction residual signal using a first channel signal and a second channel signal and prediction information and a prediction direction indicator indicating a prediction direction associated with the prediction residual signal, so that a prediction residual signal, when combined with a prediction signal derived from the first combination signal or a signal derived from the first combination signal and the prediction information results in a second combination signal,   wherein the encoder calculator comprises a combiner for combining the first channel signal and the second channel signal in two different ways to acquire the first combination signal and the second combination signal;   an optimizer for calculating the prediction information so that the prediction residual signal fulfills an optimization target;   a prediction direction calculator for calculating the prediction direction indicator indicating the prediction direction associated with the prediction residual signal;   a signal encoder for encoding the first combination signal and the prediction residual signal to acquire an encoded first combination signal and an encoded prediction residual signal; and   an output interface for combining the encoded first combination signal, the encoded prediction residual signal and the prediction information to acquire an encoded multi-channel audio signal,   wherein the multichannel audio signal is a stereo audio signal comprising the two channel signals, and wherein the encoded multi-channel audio signal is an encoded stereo audio signal.   
     
     
         13 . A method of decoding an encoded multi-channel audio signal, the encoded multi-channel audio signal comprising an encoded first combination signal, an encoded prediction residual signal and prediction information, comprising:
 decoding the encoded first combination signal to acquire a decoded first combination signal, and decoding the encoded residual signal to acquire a decoded residual signal; and   calculating a decoded multi-channel signal comprising a decoded first channel signal, and a decoded second channel signal using the decoded residual signal, the prediction information, the decoded first combination signal and a prediction direction indicator indicating a prediction direction associated with the decoded prediction residual signal, so that the decoded first channel signal and the decoded second channel signal are at least approximations of a first channel signal and a second channel signal of a multi-channel signal,   wherein calculating the decoded multi-channel signal comprises using a first calculation rule for calculating the decoded multi-channel signal in case of a first state of the prediction direction indicator and using a second different calculation rule for calculating the decoded multi-channel signal in case of a second different state of the prediction direction indicator, and   wherein the encoded audio signal is an encoded stereo audio signal, and wherein the decoded multi-channel signal is a decoded stereo audio signal.   
     
     
         14 . A method of encoding a multi-channel audio signal comprising two or more channel signals, comprising:
 calculating a first combination signal and a prediction residual signal using a first channel signal and a second channel signal, prediction information and a prediction direction indicator indicating a prediction direction associated with the prediction residual signal, so that a prediction residual signal, when combined with a prediction signal derived from the first combination signal or a signal derived from the first combination signal and the prediction information results in a second combination signal;   combining the first channel signal and the second channel signal in two different ways to acquire the first combination signal and the second combination signal;   calculating the prediction information so that the prediction residual signal fulfills an optimization target;   calculating the prediction direction indicator indicating the prediction direction associated with the prediction residual signal;   encoding the first combination signal and the prediction residual signal to acquire an encoded first combination signal and an encoded residual signal; and   combining the encoded first combination signal, the encoded prediction residual signal and the prediction information to acquire an encoded multi-channel audio signal,   wherein the multichannel audio signal is a stereo audio signal comprising the two channel signals, and wherein the encoded multi-channel audio signal is an encoded stereo audio signal.   
     
     
         15 . A non-transitory digital storage medium having a computer program stored thereon to perform the method of decoding an encoded multi-channel audio signal, the encoded multi-channel audio signal comprising an encoded first combination signal, an encoded prediction residual signal and prediction information, said method comprising:
 decoding the encoded first combination signal to acquire a decoded first combination signal, and decoding the encoded residual signal to acquire a decoded residual signal; and   calculating a decoded multi-channel signal comprising a decoded first channel signal, and a decoded second channel signal using the decoded residual signal, the prediction information, the decoded first combination signal and a prediction direction indicator indicating a prediction direction associated with the decoded prediction residual signal, so that the decoded first channel signal and the decoded second channel signal are at least approximations of the first channel signal and the second channel signal of the multi-channel signal,   wherein calculating the decoded multi-channel signal comprises using a first calculation rule for calculating the decoded multi-channel signal in case of a first state of the prediction direction indicator and using a second different calculation rule for calculating the decoded multi-channel signal in case of a second different state of the prediction direction indicator, and   wherein the encoded audio signal is an encoded stereo audio signal, and wherein the decoded multi-channel signal is a decoded stereo audio signal,   when said computer program is run by a computer.   
     
     
         16 . A non-transitory digital storage medium having a computer program stored thereon to perform the method of encoding a multi-channel audio signal comprising two or more channel signals, said method comprising:
 calculating a first combination signal and a prediction residual signal using a first channel signal and a second channel signal, prediction information and a prediction direction indicator indicating a prediction direction associated with the decoded prediction residual signal, so that a prediction residual signal, when combined with a prediction signal derived from the first combination signal or a signal derived from the first combination signal and the prediction information results in a second combination signal;   combining the first channel signal and the second channel signal in two different ways to acquire the first combination signal and the second combination signal;   calculating the prediction information so that the prediction residual signal fulfills an optimization target;   calculating the prediction direction indicator indicating the prediction direction associated with the prediction residual signal;   encoding the first combination signal and the prediction residual signal to acquire an encoded first combination signal and an encoded residual signal; and   combining the encoded first combination signal, the encoded prediction residual signal and the prediction information to acquire an encoded multi-channel audio signal,   wherein the multichannel audio signal is a stereo audio signal comprising the two channel signals and wherein the encoded multi-channel audio signal is an encoded stereo audio signal,   when said computer program is run by a computer.