Decoding of multichannel audio encoded bit streams using adaptive hybrid transformation
The processing efficiency of a process used to decode frames of an enhanced AC-3 bit stream is improved by processing each audio block in a frame only once. Audio blocks of encoded data are decoded in block order rather than in channel order. Exemplary decoding processes for enhanced bit stream coding features such as adaptive hybrid transform processing and spectral extension are disclosed.
1. A method for decoding a frame of an encoded digital audio signal, wherein:
the frame comprises frame metadata, a first audio block and one or more subsequent audio blocks; and
each of the first and subsequent audio blocks comprises block metadata and encoded audio data for two or more audio channels, wherein:
the encoded audio data comprises scale factors and scaled values representing spectral content of the two or more audio channels, each scaled value being associated with a respective one of the scale factors; and
the block metadata comprises control information describing coding tools used by an encoding process that produced the encoded audio data, the coding tools including adaptive hybrid transform processing that comprises:
applying an analysis filter bank implemented by a primary transform to the two or more audio channels to generate primary transform coefficients, and
applying a secondary transform to the primary transform coefficients for at least some of the two or more audio channels to generate hybrid transform coefficients;
and wherein the method comprises:
(A) receiving the frame of the encoded digital audio signal; and
(B) examining the encoded digital audio signal of the frame in a single pass to decode the encoded audio data for each audio block in order by block, wherein the decoding of each respective audio block comprises:
(1) determining for each respective channel of the two or more channels whether the encoding process used adaptive hybrid transform processing to encode any of the encoded audio data;
(2) if the encoding process used adaptive hybrid transform processing for the respective channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients of the respective channel for the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients, and
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the respective channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing for the respective channel, obtaining primary transform coefficients for the respective channel by decoding the encoded data in the respective audio block; and
(C) applying an inverse primary transform to the primary transform coefficients to generate an output signal representing the respective channel in the respective audio block.
2. The method of claim 1 , wherein the frame of the encoded digital audio signal complies with enhanced AC-3 bit stream syntax.
3. The method of claim 2 , wherein the coding tools include spectral extension processing and the decoding of each respective audio block further comprises:
determining whether the decoding process should use spectral extension processing to decode any of the encoded audio data; and
if spectral extension processing should be used, synthesizing one or more spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients with an extended bandwidth.
4. The method of claim 3 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
5. The method of claim 3 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.
6. The method of claim 2 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
7. The method of claim 2 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.
8. An apparatus for decoding a frame of an encoded digital audio signal, wherein:
the frame comprises frame metadata, a first audio block and one or more subsequent audio blocks; and
each of the first and subsequent audio blocks comprises block metadata and encoded audio data for two or more audio channels, wherein:
the encoded audio data comprises scale factors and scaled values representing spectral content of the two or more audio channels, each scaled value being associated with a respective one of the scale factors; and
the block metadata comprises control information describing coding tools used by an encoding process that produced the encoded audio data, the coding tools including adaptive hybrid transform processing that comprises:
applying an analysis filter bank implemented by a primary transform to the two or more audio channels to generate primary transform coefficients, and
applying a secondary transform to the primary transform coefficients for at least some of the two or more audio channels to generate hybrid transform coefficients;
and wherein the apparatus comprises:
(A) means for receiving the frame of the encoded digital audio signal; and
(B) means for examining the encoded digital audio signal of the frame in a single pass and for decoding the encoded audio data for each audio block in order by block, wherein for the decoding of each respective audio block the apparatus comprises means for:
(1) determining for each respective channel of the two or more channels whether the encoding process used adaptive hybrid transform processing to encode any of the encoded audio data;
(2) if the encoding process used adaptive hybrid transform processing for the respective channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients of the respective channel for the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients, and
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the respective channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing for the respective channel, obtaining primary transform coefficients for the respective channel by decoding the encoded data in the respective audio block; and
(C) means for applying an inverse primary transform to the primary transform coefficients to generate an output signal representing the respective channel in the respective audio block.
9. The apparatus of claim 8 , wherein the frame of the encoded digital audio signal complies with enhanced AC-3 bit stream syntax.
10. The apparatus of claim 9 , wherein the coding tools include spectral extension processing and for the decoding of each respective audio block the apparatus further comprises means for:
determining whether the decoding process should use spectral extension processing to decode any of the encoded audio data; and
if spectral extension processing should be used, synthesizing one or more spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients with an extended bandwidth.
11. The apparatus of claim 10 , wherein the coding tools include channel coupling and for the decoding of each respective audio block the apparatus further comprises means for:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
12. The apparatus of claim 10 , wherein the coding tools include channel coupling and for the decoding of each respective audio block the apparatus further comprises means for:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.
13. The apparatus of claim 9 , wherein the coding tools include channel coupling and for the decoding of each respective audio block the apparatus further comprises means for:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
14. The apparatus of claim 9 , wherein the coding tools include channel coupling and for the decoding of each respective audio block the apparatus further comprises means for:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.
15. A non-transitory medium that records a program of instructions executable by a device to perform a method for decoding a frame of an encoded digital audio signal, wherein:
the frame comprises frame metadata, a first audio block and one or more subsequent audio blocks; and
each of the first and subsequent audio blocks comprises block metadata and encoded audio data for two or more audio channels, wherein:
the encoded audio data comprises scale factors and scaled values representing spectral content of the two or more audio channels, each scaled value being associated with a respective one of the scale factors; and
the block metadata comprises control information describing coding tools used by an encoding process that produced the encoded audio data, the coding tools including adaptive hybrid transform processing that comprises:
applying an analysis filter bank implemented by a primary transform to the two or more audio channels to generate primary transform coefficients, and
applying a secondary transform to the primary transform coefficients for at least some of the two or more audio channels to generate hybrid transform coefficients;
and wherein the method comprises:
(A) receiving the frame of the encoded digital audio signal; and
(B) examining the encoded digital audio signal of the frame in a single pass to decode the encoded audio data for each audio block in order by block, wherein the decoding of each respective audio block comprises:
(1) determining for each respective channel of the two or more channels whether the encoding process used adaptive hybrid transform processing to encode any of the encoded audio data;
(2) if the encoding process used adaptive hybrid transform processing for the respective channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients of the respective channel for the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients, and
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the respective channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing for the respective channel, obtaining primary transform coefficients for the respective channel by decoding the encoded data in the respective audio block; and
(C) applying an inverse primary transform to the primary transform coefficients to generate an output signal representing the respective channel in the respective audio block.
16. The medium of claim 15 , wherein the frame of the encoded digital audio signal complies with enhanced AC-3 bit stream syntax.
17. The medium of claim 16 , wherein the coding tools include spectral extension processing and the decoding of each respective audio block further comprises:
determining whether the decoding process should use spectral extension processing to decode any of the encoded audio data; and
if spectral extension processing should be used, synthesizing one or more spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients with an extended bandwidth.
18. The medium of claim 17 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
19. The medium of claim 17 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.
20. The medium of claim 16 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling, deriving spectral components from the inverse secondary transform coefficients to obtain primary transform coefficients for coupled channels.
21. The medium of claim 16 , wherein the coding tools include channel coupling and the decoding of each respective audio block further comprises:
determining whether the encoding process used channel coupling to encode any of the encoded audio data; and
if the encoding process used channel coupling:
(A) if the respective channel is a first channel to use coupling in the frame:
(1) determining whether the encoding process used adaptive hybrid transform processing to encode the coupling channel,
(2) if the encoding process used adaptive hybrid transform processing to encode the coupling channel:
(a) if the respective audio block is the first audio block in the frame:
(i) obtaining all hybrid transform coefficients for the coupling channel in the frame from the encoded audio data in the first audio block, and
(ii) applying an inverse secondary transform to the hybrid transform coefficients to obtain inverse secondary transform coefficients,
(b) obtaining primary transform coefficients from the inverse secondary transform coefficients for the coupling channel in the respective audio block;
(3) if the encoding process did not use adaptive hybrid transform processing to encode the coupling channel, obtaining spectral components for the coupling channel by decoding the encoded data in the respective audio block; and
(B) obtaining primary transform coefficients for the respective channel by decoupling the spectral components for the coupling channel.