Audio decoder for audio channel reconstruction
A method performed by an audio decoder for reconstructing N audio channels from an audio signal containing M audio channels is disclosed. The method includes receiving a bitstream containing an encoded audio signal having M audio channels and a set of spatial parameters, the set of spatial parameters including an inter-channel intensity difference parameter and an inter-channel coherence parameter. The encoded audio bitstream is then decoded to obtain a decoded frequency domain representation of the M audio channels, and at least a portion of the frequency domain representation is decorrelated with an all-pass filter having a fractional delay. The all-pass filter is attenuated at locations of a transient. A matrixed version of the decorrelated signals are summed with a matrixed version of the decoded frequency domain representation to obtain N audio signals that collectively having N audio channels where M is less than N.
1. A method performed by an audio decoder for reconstructing N audio channels from an audio signal containing M audio channels, the method comprising:
receiving a bitstream containing an encoded audio signal having M audio channels and a set of spatial parameters, the set of spatial parameters including an inter-channel intensity difference parameter and an inter-channel coherence parameter;
decoding the encoded audio signal having M audio channels to obtain a decoded representation of the M audio channels;
decorrelating at least a portion of the decoded representation with an all-pass filter to obtain M decorrelated signals, the all-pass filter including a plurality of filter links, wherein a transfer function H(z) in a Z-domain of at least some of the plurality of filter links is at least partially derivable from or based on:
qz
-
m
-
a
1
-
aqz
-
m
where q is a complex valued phase rotation factor, m is a delay length and a is a filter coefficient;
reconstructing N audio channels from the M decorrelated signals and the decoded representation of the M audio channels to obtain N audio signals that collectively having N audio channels, wherein N is two or more, M is one or more, and M is less than N; and
synthesizing the N audio signals with one or more synthesis filterbanks to convert the N audio signals from a frequency domain to a time domain;
wherein the all-pass filter has a fractional delay and the audio decoder is implemented at least in part with hardware.
2. The method of claim 1 , wherein the decorrelating includes reducing the effect of a transient signal.
3. The method of claim 1 , wherein the inter-channel coherence parameter is determined based on a dissimilarity of a first channel and a second channel.
4. The method of claim 1 , wherein the set of spatial parameters further includes an inter-channel time or phase difference parameter.
5. The method of claim 1 , wherein the decorrelating and reconstructing are performed in a frequency domain.
6. The method of claim 1 , wherein the inter-channel intensity difference parameter is a ratio between the energy or level of a first channel and a second channel.
7. The method of claim 6 , wherein the first channel is a left channel, the second channel is a right channel, M=1 and N=2.
8. The method of claim 1 wherein the M audio channels are a linear down mix of the N audio channels.
9. The method of claim 1 wherein the decoding is performed by an MPEG-4 High Efficiency AAC decoder.
10. The method of claim 1 wherein the synthesizing is performed with N synthesis filterbanks.
11. The method of claim 1 wherein the decorrelating is performed with N−1 decorrelators.
12. The method of claim 1 wherein the synthesizing is perform with a QMF synthesis filterbank.
13. A non-transitory, computer readable storage medium containing instructions that when executed by a processor perform the method of claim 1 .
14. An audio decoder for reconstructing N audio channels from an audio signal containing M audio channels, the audio decoder comprising:
an input interface for receiving a bitstream containing an encoded audio signal having M audio channels and a set of spatial parameters, the set of spatial parameters including an inter-channel intensity difference parameter and an inter-channel coherence parameter;
an audio decoder for decoding the encoded audio signal having M audio channels to obtain a decoded representation of the M audio channels;
a decorrelator for decorrelating at least a portion of the decoded representation with an all-pass filter to obtain M decorrelated signals, where the all-pass filter includes a plurality of filter links, wherein a transfer function H(z) in a Z-domain of at least some of the plurality of filter links is at least partially derivable from or based on:
qz
-
m
-
a
1
-
aqz
-
m
where q is a complex valued phase rotation factor, m is a delay length and a is a filter coefficient;
an upmixer to obtain N audio signals from the M decorrelated signals and the decoded representation of the M audio channels, the N audio signals collectively having N audio channels, wherein N is two or more, M is one or more, and M is less than N; and
a synthesis filterbank for synthesizing the N audio signals to convert the N audio signals from a frequency domain to a time domain;
wherein the all-pass filter has a fractional delay.