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

• US 6,226,616 B1
• Filed: 06/21/1999
• Issued: 05/01/2001
• Est. Priority Date: 06/21/1999
• Status: Expired due to Term

• Current Assignee: DTS, Inc. (Xperi Inc.)
• Original Assignee: Digital Theatre Systems Incorporated (Xperi Inc.)
• Inventors: Fejzo, Zoran, Smith, William Paul, Smyth, Stephen, You, Yu-Li
• Primary Examiner(s): Hudspeth, David
• Assistant Examiner(s): ABEBE, DANIEL DEMELASH
• Application Number:
  US09/337,405
• Time in Patent Office: 680 Days
• Field of Search: 704/500, 704/501, 704/502, 704/503, 704/504, 704/201
• US Class Current: 704/500
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 21/038 using band spreading techniques
• CPC Class Codes: G11B 20/10527 Audio or video recording; Data buffering arrangements G11B20/12 - G11B20/18 take precedence
• CPC Class Codes: G11B 20/1217 on discs
• CPC Class Codes: G11B 27/3027 used signal is digitally coded
• CPC Class Codes: H04B 14/046 Systems or methods for reducing noise or bandwidth

Abstract

A multi-channel audio compression technology is presented that extends the range of sampling frequencies compared to existing technologies and/or lowers the noise floor while remaining compatible with those earlier generation technologies. The high-sampling frequency multi-channel audio is decomposed into core audio up to the existing sampling frequencies and a difference signal up to the sampling frequencies of the next generation technologies. The core audio is encoded using a first generation technology such as DTS, Dolby AC-3 or MPEG I or II such that the encoded core bit stream is fully compatible with a comparable decoder in the market. The difference signal is encoded using technologies that extend the sampling frequency and/or improve the quality of the core audio. The compressed difference signal is attached as an extension to the core bit stream. The extension data will be ignored by the first generation decoders but can be decoded by the second generation decoders. By summing the decoded core and extension audio signals together, a second generation decoder can effectively extend the audio signal bandwidth and/or improve the signal to noise ratio beyond that available through the core decoder alone.

Claims

1. 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;

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;

a core decoder that decodes the core bits to form a reconstructed core signal;

an interpolator that upsamples the reconstructed core signal to the extension encoder sampling rate;

an interpolation LPF that filters the upsampled reconstructed core signal to remove interpolation aliasing; and

a summing node that subtracts the filtered signal from the digital audio signal to form a difference signal, wherein said extension encoder encodes the difference signal into extension bits.

2. The multi-channel audio encoder of claim 1, wherein said core bits define a noise floor for the reconstructed core signal across its audio bandwidth, said extension bits further refining the noise floor across the core encoder'"'"'s audio bandwidth and defining a noise floor for the remainder of the extension encoder'"'"'s audio bandwidth.

3. The multi-channel audio encoder of claim 1, wherein said core bits define a noise floor for the reconstructed core signal across its audio bandwidth, said extension bits being allocated at frequencies near a transition bandwidth of said decimation LPF and above to define a noise floor for the remainder of the extension encoder'"'"'s audio bandwidth.

4. The multi-channel audio encoder of claim 1, in which the encoder maintains compatibility with an existing base of first generation audio decoders while providing higher quality sound reproduction with second generation audio decoders, said core decoder comprising one of said first generation audio decoders and said core encoder being compatible with said first generation decoders.

5. The multi-channel audio encoder of claim 4, further comprising a packer that packs 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.

6. 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.

7. The multi-channel audio encoder of claim 6, in which the encoder maintains compatibility with an existing base of first generation audio decoders while providing higher quality sound reproduction with second generation audio decoders, said core decoder comprising one of said first generation audio decoders and said core encoder being compatible with said first generation decoders.

8. The multi-channel audio encoder of claim 7, further comprising a packer that packs 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. The multi-channel audio decoder of claim 12, where the filter is a single M+N band filterbank in which the lower N bands are compatible with the N core subband decoders.

14. The multi-channel audio decoder of claim 12, where the filter comprises:

an N-band filter bank, which is compatible with the N core subband decoders, that synthesizes the N composite subband signals;

an M-band filter bank that synthesizes the M extension subband signals; and

a 2-band filter bank that combines the outputs of the N and M band filter banks to reconstruct the multi-channel audio signal.

15. 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.

16. A digital audio signal embodied in a carrier wave comprising a sequence of synchronized frames, each of said frames comprising a core field having core bits that represent a core signal up to an audio bandwith and sample resolution and an extension field having an extension sync word and extension bits that represent an extension audio signal that extends the audio bandwith and/or increases the sample resolution of the core signal.

17. The digital audio signal of claim 16, wherein said digital audio signal comprises one and only one said sequence of synchronized frames.

18. The digital audio signal of claim 16, wherein each said core field includes a core sync word.

19. 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.

20. 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.

21. 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.