Sound quality of established low bit-rate audio coding systems without loss of decoder compatibility

A multi-channel audio encoder for coding a digital audio signal sampled at a known sampling rate and having an audio bandwidth, comprising:

• a core encoder having a sampling rate and audio bandwidth less than that of said digital audio signal;
• an extension encoder having a sampling rate and audio bandwidth equal to that of said digital audio signal;
• a decimation low pass filter (LPF) that filters the digital audio signal to remove signal components above the audio bandwidth of the core encoder, said decimation LPF having a transition bandwidth around the audio bandwidth of the core encoder;
• a decimator that down samples the filtered signal to extract a core signal whose sampling rate matches the core encoder, wherein said core encoder codes the core signal into core bits;
• an interpolator that upsamples the core signal to the extension encoder sampling rate to form a reconstructed core signal;
• an interpolation LPF that filters the reconstructed core signal to remove interpolation aliasing;
• and
• a summing node that subtracts said filtered signal from the digital audio signal to form a difference signal, wherein said extension encoder encodes the difference signal into extension bits and allocates bits in said transition bandwidth and above to extend the frequency range of the encoded signal.

A multi-channel audio encoder for coding a digital audio signal sampled at a known sampling rate and having an audio bandwidth, comprising:

• a core encoder that extracts and codes a core signal from the digital audio signal over an audio bandwidth into core bits, said core encoder including an N-band filter bank that decomposes the core signal into N subbands and N subband coders that generate the core bits, N subband decoders that reconstruct the N subband samples to form a reconstructed core signal,
• a summing node that forms a difference signal from the reconstructed core signal and the digital audio signal in a transform or subband domain;
• and
• an extension encoder that encodes the difference signal into extension bits, said extension encoder matching the core encoder over its audio bandwidth and comprising,
• a two band filter bank that splits the digital audio signal into lower and upper bands;
• a N-band filter bank equivalent to the core encoder's that decomposes the digital audio signal in the lower band into N subbands, said summing node existing inside said extension encoder and comprising N subband nodes that subtract the reconstructed N subband samples from the digital audio signal's N subbands, respectively to form N difference subbands;
• N subband coders that code the N difference subbands to form the lower band extension bits;
• a M-band filter bank that decomposes the digital audio signal in the upper band into M subbands;
• and
• M subband coders that code the M subbands to form the upper band extension bits.

A multi-channel audio encoder for coding a digital audio signal sampled at a known sampling rate and having an audio bandwidth, comprising:

• a core encoder that extracts and codes a core signal from the digital audio signal over an audio bandwidth into core bits, said core encoder including an N-band filter bank that decomposes the core signal into N subbands and N subband coders that generate the core bits, N subband decoders that reconstruct the N subband samples to form a reconstructed core signal,
• a summing node that forms a difference signal from the reconstructed core signal and the digital audio signal in a transform or subband domain;
• and
• an extension encoder that encodes the difference signal into extension bits, said extension encoder matching the core encoder over its audio bandwidth and comprising,
• an L-band filter bank that decomposes the digital audio signal into N lower subbands and M upper subbands, the filter characteristics of said L-band filter bank matching those of said N-band filter bank over its N lower subbands, said summing node existing inside said extension coder and comprising N subband nodes that subtract the reconstructed N subband samples from the digital audio signal's N subbands, respectively to form N difference subbands;
• N subband coders that code the N difference subbands to form the lower band extension bits;
• and
• M subband coders that code the M subbands to form the upper band extension bits.

A multi-channel black-box audio decoder for reconstructing multiple audio channels from a bit stream, in which each audio channel was sampled at a known sampling rate and has an audio bandwidth, comprising:

• an unpacker for reading in and storing the bit stream a frame at a time, each of said frames including a core field having core bits and an extension field having a sync word and extension bits, said unpacker extracting said core bits and detecting said sync word to extract and separate the extension bits;
• a core decoder decodes the core bits to form a reconstructed core signal;
• an extension decoder that decodes the extension bits to form a reconstructed difference signal, said extension decoder having a sampling rate and an audio bandwidth greater than said core decoder;
• an interpolator that upsamples the reconstructed core signal to the sampling rate of the extension encoder;
• a low pass filter that filters the upsampled reconstructed core signal to attenuate interpolation aliasing, and
• a summation node that adds the reconstructed difference audio signal to the reconstructed core audio signal to improve the fidelity of the reconstructed core audio signal and extend its audio bandwidth.

A multi-channel open-box audio decoder for reconstructing multiple audio channels from a bit stream, in which each audio channel was sampled at a known sampling rate and has an audio bandwidth, comprising:

• an unpacker for reading in and storing the bit stream a frame at a time, each of said frames including a core field having core bits and an extension fields having a sync word and extension bits, said unpacker extracting said core bits and detecting said sync word to extract and separate the extension bits;
• N core subband decoders that decode the core bits into N core subband signals
• N extension subband decoders that decode the extension bits into a lower N extension subband signals;
• M extension subband decoders that decode the extension bits into an upper M extension subband signals;
• N summation nodes that sum the N core subband signals to the respective N extension subband signals to form N composite subband signals;
• and
• a filter that synthesizes the N composite subband signals and the M extension subband signals to reproduce a multi-channel audio signal.

An article of manufacture for use with an existing base of first generation audio decoders that are capable of reconstructing a core signal up to an audio bandwidth and sample resolution and a developing base of second generation audio decoders having a larger audio bandwidth, comprising:

• a portable machine readable storage medium for use with said first and second generation audio decoders;
• and
• a single digital bit stream representing a multi-channel audio signal written onto said storage medium in a core plus extension format, said bit stream comprising a sequence of synchronized frames, each of said frames including a core field having a core sync word immediately proceeding core bits and an extension fields having an extension sync word immediately proceeding extension bits,
• said sequence of core bits defining a noise floor for the reconstructed core signal across the audio bandwidth of said first generation audio decoders, and
• said sequence of extension bits further refining the noise floor across the core encoder's audio bandwidth and defining a noise floor for the remainder of the audio bandwidth of the second generation audio decoders.

A method of encoding a multi-channel digital audio signal sampled at a known sampling rate and having an audio bandwidth that maintains compatibility with an existing base of first generation audio decoders while providing higher quality sound reproduction with second generation audio decoders, comprising:

• low pass filtering the digital audio signal to remove signal components above a core audio bandwidth;
• down sampling the filtered signal to extract a core signal whose sampling rate matches a core sampling rate;
• in a manner compatible with said first generation audio decoders, encoding the core signal at the core sampling rate and the core audio bandwidth less than that of said digital audio signal into core bits without aliasing fold-back;
• using a first generation audio decoder to decode the core bits to form a reconstructed core signal;
• upsampling the reconstructed core signal to an extension sampling rate;
• low pass filtering the upsampled reconstructed core signal to remove interpolation aliasing;
• subtracting said filtered reconstructed core signal from the digital audio signal to form a difference signal;
• encoding the difference signal at the extension sampling rate and an extension audio bandwidth equal to that of said digital audio signal;
• and
• packing the core bits and the extension bits into a bit stream in a core plus extension format in which the first generation audio decoders can extract and decode the core bits to reproduce an audio signal and the second generation audio decoders can extract the core bits plus the extension bits to reproduce a higher quality audio signal.

A method of encoding a multi-channel digital audio signal sampled at a known sampling rate and having an audio bandwidth that maintains compatibility with an existing base of first generation audio decoders while providing higher quality sound reproduction with second generation audio decoders, comprising:

• low pass filtering the digital audio signal to remove signal components above a core audio bandwidth, said filtering exhibiting a transition bandwidth around the core audio bandwidth;
• down sampling the filtered signal to extract a core signal whose sampling rate matches a core sampling rate;
• in a manner compatible with said first generation audio decoders, encoding the core signal at the core sampling rate and a core audio bandwidth less than that of said digital audio signal into core bits without aliasing fold-back;
• upsampling the core signal to an extension sampling rate to form a reconstructed core signal;
• filtering the reconstructed core signal to remove interpolation aliasing;
• subtracting said filtered reconstructed core signal from the digital audio signal to form a difference signal;
• encoding the difference signal at the extension sampling rate and an extension audio bandwidth equal to that of said digital audio signal into extension bits, said extension bits being allocated in said transition bandwidth and above to extend the frequency range of the encoded audio signal;
• and
• packing the core bits and the extension bits into a bit stream in a core plus extension format in which the first generation audio decoders can extract and decode the core bits to reproduce an audio signal and the second generation audio decoders can extract the core bits plus the extension bits to reproduce a higher quality audio signal.

A method of reconstructing a multi-channel audio signal comprising:

• receiving a sequence of encoded frames, each of said frames including a core field having a core sync word immediately proceeding core bits and an extension fields having an extension sync word immediately proceeding extension bits;
• detecting the core sync word to extract and then decode the core bits into a reconstructed core signal;
• detecting the extension sync word to extract and then decode the extension bits into a reconstructed difference signal at a sampling rate and with an audio bandwidth greater than said core bits;
• upsampling the reconstructed core signal to the sampling rate of the reconstructed difference signal;
• and
• low pass filtering the upsampled reconstructed core signal to attenuate interpolation aliasing;
• and
• summing the filtered and upsampled reconstructed core signal and the reconstructed difference signal to reconstruct the multi-channel audio signal.