Scalable compression of audio and other signals

• US 6,947,886 B2
• Filed: 02/21/2003
• Issued: 09/20/2005
• Est. Priority Date: 02/21/2002
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Regunathan, Shankar L., Rose, Kenneth, Aggarwal, Ashish
• Primary Examiner(s): Knepper, David D.
• Application Number:
  US10/372,047
• Publication Number:
  US 20030212551A1
• Time in Patent Office: 942 Days
• Field of Search: 704/200, 704/200.1, 704/230, 704/247, 704500-501
• US Class Current: 704/200.1
• CPC Class Codes: G10L 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding

Abstract

Disclosed are scalable quantizers for audio and other signals characterized by a non-uniform, perception-based distortion metric, that operate in a common companded domain which includes both the base-layer and one or more enhancement-layers. The common companded domain is designed to permit use of the same unweighted MSE metric for optimal quantization parameter selection in multiple layers, exploiting the statistical dependence of the enhancement-layer signal on the quantization parameters used in the preceding layer. One embodiment features an asymptotically optimal entropy coded uniform scalar quantizer. Another embodiment is an improved bit rate scalable multi-layer Advanced Audio Coder (AAC) which extends the scalability of the asymptotically optimal entropy coded uniform scalar quantizer to systems with non-uniform base-layer quantization, selecting the enhancement-layer quantization methodology to be used in a particular band based on the preceding layer quantization coefficients. In the important case that the source is well modeled as Laplacian, the optimal conditional quantizer is implementable by only two distinct switchable quantizers depending on whether or not the previous quantizer identified the band in question as a so-called “zero dead-zone:” Hence, major savings in bit rate are recouped at virtually no additional computational cost. For example, the proposed four layer scalable coder consisting of 16 kbps layers achieves performance close to a 60 kbps non-scalable coder on the standard test database of 44.1 kHz audio.

Claims

1. A bit-rate scalable coder for generating a reduced bit rate representation of a digital signal with an associated distortion metric, the coder comprising:

a first quantizer mechanism operating in at least a base-layer for producing scaled and quantized base-layer coefficients from said coefficients;

a base-layer error mechanism for producing base-layer error signals from the unquantized scaled coefficients and the scaled and quantized coefficients; and

a second quantizer mechanism operating selectively in one or more enhancement-layers quantizer mechanism for producing quantized enhancement-layer signals from said base-layer error signals;

wherein

selection of the second quantizer mechanism is dependent on an outcome of the first quantizer mechanism.

2. The bit-rate scalable coder of claim 1 wherein the enhancement-layer comprises two distinct quantizer mechanisms and a selected said enhancement-layer quantizer mechanism is applied in a particular enhancement-layer to a particular error signal coefficient depending on the outcome of the quantizer mechanism that produced that coefficient in a preceding layer.

3. The bit-rate scalable coder of claim 1 wherein when the first quantizer mechanism produces a value of zero for a particular coefficient in a particular layer, a scaled version of that first quantizer mechanism is used in a subsequent enhancement-layer to quantize error signals for that coefficient.

4. The bit-rate scalable coder of claim 1 wherein when said first quantizer mechanism produces a non-zero quantized signal for a particular coefficient, a uniform quantizer mechanism is used in all the subsequent enhancement-layers to quantize the error signals for that coefficient.

5. The bit-rate scalable coder of claim 1 wherein in at least one enhancement-layer, the quantizer scaling factor associated with said second quantizer mechanism is derived from a quantization interval associated with the first quantizer mechanism.

6. The bit-rate scalable coder of claim 1 wherein the coder is an AAC coder and the reversible compression mechanism implements the function |x|^0.75 [absolute value to the power 3 over 4].

7. A bit-rate scalable AAC coder for generating a reduced bit rate representation of a digital audio signal having spectral coefficients organized into bands with an associated perceptually weighted distortion metric, the coder comprising:

a reversible compression mechanism for performing a non-linear reversible compression function |x|^0.75 [absolute value to the power 3 over 4] on input signal coefficients from said bands;

a first quantizer mechanism operating in at least a base-layer for producing scaled and quantized base-layer coefficients from said coefficients;

a base-layer error mechanism for producing base-layer error signals from the unquantized scaled coefficients and the scaled and quantized coefficients; and

a second quantizer mechanism operating selectively in one or more enhancement-layers quantizer mechanism for producing quantized enhancement-layer signals from said base-layer error signals;

wherein

selection of the second quantizer mechanism is dependent on an outcome of the first quantizer mechanism;

the enhancement-layer comprises two distinct quantizer mechanisms and a selected said enhancement-layer quantizer mechanism is applied in a particular enhancement-layer to a particular error signal coefficient depending on the outcome of the quantizer mechanism that produced that coefficient in a preceding layer;

when the first quantizer mechanism produces a value of zero for a particular coefficient in a particular layer, a scaled version of that first quantizer mechanism is used in a subsequent enhancement-layer to quantize error signals for that coefficient;

when said first quantizer mechanism produces a non-zero quantized signal for a particular coefficient, a uniform quantizer mechanism is used in all the subsequent enhancement-layers to quantize the error signals for that coefficient; and

in at least one enhancement-layer, the quantizer scaling factor associated with said second quantizer mechanism is derived from a quantization interval associated with the first quantizer mechanism.

8. A bit-rate scalable coder for generating a reduced bit rate representation of a digital signal with an associated weighted distortion metric, the coder comprising:

a compression mechanism for performing a non-linear reversible compression function on input signal coefficients to thereby produce compressed coefficients in an associated companded domain;

a base-layer quantizer mechanism operating in the companded domain and responsive to scaling factors from a distortion metric control circuit for producing quantized companded base-layer signals from said compressed coefficients;

a base-layer error mechanism also operating in the companded domain for producing a companded and scaled base-layer error signal from the unquantized scaled coefficients and the quantized coefficients; and

an enhancement-layer quantizer mechanism operating in the same companded domain as the base-layer quantizer mechanism for producing quantized companded enhancement-layer signals from said companded and scaled base-layer error signals.

9. The bit-rate scalable coder of claim 8 wherein a non-weighted distortion metric is optimized for the said compressed coefficients in said associated companded domain.

10. The bit-rate scalable coder of claim 8 wherein

each said quantizer mechanism comprises a uniform quantizer with dead zone rounding and

said scaling factors represent scaling of an associated said quantizer.

11. The bit-rate scalable coder of claim 8 wherein in at least one enhancement-layer, a scaling factor associated with said enhancement-layer quantizer mechanism is derived from a quantization interval associated with said base-layer quantizer mechanism.

12. The bit-rate scalable coder of claim 8 wherein the coder is an AAC coder and the reversible compression mechanism implements the function |x|^0.75 [absolute value to the power 3 over 4].

13. The bit-rate scalable coder of claim 8 wherein in at least one enhancement-layer, all said scaling factors are the same.

14. The bit-rate scalable coder of claim 8 wherein in at least the base-layer, not all the quantizer scaling factors are the same.

15. The bit-rate scalable coder of claim 8 wherein each of said quantizer mechanisms comprises a nearest integer mechanism.

16. The bit-rate scalable coder of claim 8 wherein each of said quantizer mechanisms is a uniform interval mechanism.

17. A bit-rate scalable AAC coder for generating a reduced bit rate representation of a digital signal having spectral coefficients organized into bands with an associated perceptually weighted distortion metric, the coder comprising:

a compression mechanism for performing the non-linear reversible compression function |x|^0.75 [absolute value to the power 3 over 4] on input signal coefficients to thereby produce compressed coefficients in an associated companded domain;

a base-layer quantizer mechanism operating in the companded domain and responsive to scaling factors from a distortion metric control circuit for producing quantized companded base-layer signals from said compressed coefficients;

a base-layer error mechanism also operating in the companded domain for producing a companded and scaled base-layer error signal from the unquantized scaled coefficients and the quantized coefficients; and

an enhancement-layer quantizer mechanism operating in the same companded domain as the base-layer quantizer mechanism for producing quantized companded enhancement-layer signals from said companded and scaled base-layer error signals;

wherein

a non-weighted distortion metric is optimized for the said compressed coefficients in said associated companded domain;

each said quantizer mechanism comprises a uniform quantizer with dead zone rounding;

said scaling factors represent scaling of an associated said quantizer;

in at least one enhancement-layer, a scaling factor associated with said enhancement-layer quantizer mechanism is derived from a quantization interval associated with said base-layer quantizer mechanism; and

each of said quantizer mechanisms is a uniform interval mechanism.

18. The bit-rate scalable coder of claim 17 wherein in at least one enhancement-layer, all said scaling factors are the same.

19. The bit-rate scalable coder of claim 17 wherein in at least the base-layer, not all the quantizer scaling factors are the same.

20. The bit-rate scalable coder of claim 17 wherein each of said quantizer mechanisms comprises a nearest integer mechanism.

21. The bit-rate scalable coder of claim 17 wherein each said quantizer mechanism comprises a uniform quantizer with dead zone rounding and each said scaling factors represents scaling of the quantizer mechanism in a respective coefficient band.

22. The bit-rate scalable coder of claim 17 wherein the coder is an AAC coder and the reversible compression mechanism implements the function |x|^0.75 [absolute value to the power 3 over 4].

23. The bit-rate scalable coder of claim 17 wherein in at least one enhancement-layer, said quantizer scaling in at least some of said coefficients are directly derived from the quantizer scaling of the corresponding coefficients at the base-layer.

24. The bit-rate scalable coder of claim 17 wherein in at least the base-layer, not all the scaling factors are the same.

25. The bit-rate scalable coder of claim 17 wherein the quantizer mechanism comprises a nearest integer mechanism.

26. A bit-rate scalable coder for generating a reduced bit rate representation of a digital signal with an associated weighted distortion metric, the coder comprising:

a base-layer quantizer mechanism responsive to scaling factors from a distortion metric control circuit for producing unquantized scaled coefficients and quantized base-layer coefficients in a scaled domain;

a base-layer error mechanism also operating in the scaled domain for producing base-layer error signals from the unquantized scaled coefficients and the quantized coefficients; and

an enhancement-layer quantizer mechanism operating in the same scaled domain as the base-layer quantizer mechanism for producing quantized enhancement-layer signals from said base-layer error signals.

27. A bit-rate scalable AAC coder for generating a reduced bit rate representation of a digital signal having spectral coefficients organized into bands with an associated perceptually weighted distortion metric, the coder comprising:

a compression mechanism for performing a non-linear reversible compression function |x|^0.75 [absolute value to the power 3 over 4] on input signal coefficients from said bands;

a base-layer quantizer mechanism responsive to scaling factors from a distortion metric control circuit for producing unquantized scaled coefficients and quantized base-layer coefficients in a scaled domain;

a base-layer error mechanism also operating in the scaled domain for producing base-layer error signals from the unquantized scaled coefficients and the quantized coefficients; and

an enhancement-layer quantizer mechanism operating in the same scaled domain as the base-layer quantizer mechanism for producing quantized enhancement-layer signals from said base-layer error signals.

wherein

each said quantizer mechanism comprises a uniform quantizer with dead zone rounding and each said scaling factors represents scaling of the quantizer mechanism in a respective coefficient band;

in at least one enhancement-layer, the quantizer scaling factors for at least some of said coefficients are directly derived from respective quantizer scaling factors of corresponding coefficients at the base-layer;

in at least the base-layer, not all the scaling factors are the same;

at least some of the quantizer mechanisms comprises a uniform interval mechanism; and

in at least one enhancement-layer, the quantizer scaling factors are the same for at least some of said bands.