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 in an audio decoder for reconstructing N audio channels from M audio channels, the method comprising:
receiving an encoded audio bitstream, the encoded audio bitstream including a downmixed audio signal and surround data, the downmixed audio signal having M audio channels and the surround data including a set of spatial parameters, the set of spatial parameters including at least one inter-channel intensity difference parameter and at least one inter-channel coherence parameter;
decoding the surround data to produce decoded surround data;
decoding the downmixed audio signal having M audio channels to obtain a decoded frequency domain representation of the M audio channels, wherein the decoded frequency domain representation of the M audio channels includes a plurality of frequency bands, and each frequency band includes one or more spectral components;
reconstructing a frequency domain representation of the N audio channels from the decoded frequency domain representation of the M audio channels, downmixing information used to generate the downmixed audio signal, and the decoded surround data; and
synthesizing, with one or more synthesis filterbanks, the frequency domain representation of the N audio channels to create a time domain representation of the N audio channels; and
outputting the time domain representation of the N audio channels;
wherein M is one or more, M is less than N;
wherein the inter-channel coherence parameter is difference coded over frequency and the audio decoder is implemented at least in part with hardware.
2. 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.
3. The method of claim 1 , wherein the method further includes an analysis filterbank for decomposing the decoded representation of the M audio channels.
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 inter-channel intensity difference parameter is a ratio between the energy or level of a first channel and a second channel.
6. The method of claim 5 , wherein the first channel is a left channel, the second channel is a right channel, M=1 and N=2.
7. The method of claim 1 , wherein the M audio channels are a linear down mix of the N audio channels.
8. The method of claim 1 , wherein the decoding is performed by an MPEG-4 High Efficiency AAC decoder.
9. The method of claim 1 , wherein the synthesizing is performed with N synthesis filterbanks.
10. The method of claim 1 , wherein the synthesizing is perform with a QMF synthesis filterbank.
11. A non-transitory, computer readable storage medium containing instructions that when executed by a processor perform the method of claim 1 .
12. An audio decoder for reconstructing N audio channels from M audio channels, the audio decoder comprising:
an input interface for receiving an encoded audio bitstream, the encoded audio bitstream including a downmixed audio signal and surround data, the downmixed audio signal having M audio channels and the surround data including a set of spatial parameters, the set of spatial parameters including at least one inter-channel intensity difference parameter and at least one inter-channel coherence parameter;
a first decoder for decoding the surround data to produce decoded surround data;
a second decoder for decoding the downmixed audio signal having M audio channels to obtain a decoded frequency representation of the M audio channels, wherein the decoded frequency representation of the M audio channels includes a plurality of frequency bands, and each frequency band includes one or more spectral components;
a third decoder for reconstructing a frequency domain representation of the N audio channels from the decoded frequency domain representation of the M audio channels, downmixing information used to generate the downmixed audio signal, and the decoded surround data; and
one or more synthesis filterbanks for synthesizing, with one or more synthesis filterbanks, the frequency domain representation of the N audio channels to create a time domain representation of the N audio channels; and
wherein M is one or more, M is less than N;
wherein the inter-channel coherence parameter is difference coded over frequency.