System and method for partially whitening and quantizing weighting functions of audio signals

• US 6,240,380 B1
• Filed: 06/30/1998
• Issued: 05/29/2001
• Est. Priority Date: 05/27/1998
• Status: Expired due to Term

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Malvar, Henrique S.
• Primary Examiner(s): Dorvil, Richemond
• Application Number:
  US09/108,949
• Time in Patent Office: 1,064 Days
• Field of Search: 704/500, 704/501, 704/230, 704/200, 704/206, 704/201, 704/226, 704/204, 704/203, 704/502, 704/503, 704/504, 704/200.1
• US Class Current: 704/204
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• 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

The coder/decoder (codec) system of the present invention includes a coder and a decoder. The coder includes a multi-resolution transform processor, such as a modulated lapped transform (MLT) transform processor, a weighting processor, a uniform quantizer, a masking threshold spectrum processor, an entropy encoder, and a communication device, such as a multiplexor (MUX) for multiplexing (combining) signals received from the above components for transmission over a single medium. The decoder comprises inverse components of the encoder, such as an inverse multi-resolution transform processor, an inverse weighting processor, an inverse uniform quantizer, an inverse masking threshold spectrum processor, an inverse entropy encoder, and an inverse MUX. With these components, the present invention is capable of performing resolution switching, spectral weighting, digital encoding, and parametric modeling.

Claims

1. In a system for encoding an audio signal, the system having frequency-domain transform coefficients of the audio signal and modified with plural weighting functions, a method for partially whitening the weighting functions, comprising:

flattening each weighting function to produce final weights so that noise spectral peaks are attenuated; and

applying the final weights to the audio signal to mask quantization noise.

2. The method of claim 1, wherein flattening each weighting function comprises raising each weighting function to a power within the range of 0.5 and 0.999.

3. The method of claim 1, further comprising scalar quantizing the weighted transform coefficients and converting the weighted transform coefficients from continuous to discrete values for regulating amounts of side information produced.

4. The method of claim 1, wherein the final weights are used for computing step sizes between the discrete values of the scalar quantized coefficients so that the quantization noise is efficiently masked.

5. A noise whitening system for masking quantization noise during encoding of an audio signal, the noise whitening system having frequency-domain transform coefficients obtained from the audio signal and modified with plural weighting functions and comprising a flatten processor for flattening each weighting function to produce final weights in order to attenuate noise spectral peaks and a mask processor that applies the final weights to the audio signal as a function for masking quantization noise.

6. The noise whitening system of claim 5, wherein the flatten processor is adapted to raise each weighting function to a power within the range of 0.5 and 0.999.

7. The noise whitening system of claim 5, further comprising a scalar quantizer adapted to quantize the weighted transform coefficients and convert the weighted transform coefficients from continuous to discrete values for regulating amounts of side information produced.

8. The noise whitening system of claim 5, wherein the final weights are used for computing step sizes between the discrete values of the scalar quantized coefficients so that the quantization noise is efficiently masked.

9. In a system for encoding an input signal, the system having frequency domain transform coefficients of the input signal, and wherein the coefficients are modified with spectral weighting functions to mask quantization noise, a method for partially whitening the weighting functions, comprising:

flattening each weighting function to produce final weights so that noise spectral peaks are attenuated; and

applying the final weights to the input signal as a function to mask the quantization noise.

10. The method of claim 9 wherein the transform coefficients of the input signal are partially whitened and weighted.

11. The method of claim 10 wherein the weighting function is modeled on auditory masking characteristics of a human ear.

12. The method of claim 10 wherein the weighting function follows an auditory masking threshold curve for a given input spectrum.

13. The method of claim 12 wherein the masking threshold is computed in a quasi-logarithmic scale that approximates critical bands of a human ear.

14. The method of claim 10 wherein the weighting function is partially whitened by raising the weighting function to a power within a range of between about 0 and about 1.

15. The method of claim 13 wherein the masking threshold follows a spread Bark threshold spectrum.

16. The method of claim 15 wherein the Bark threshold spectrum is spread into lower and higher frequencies by convolving all Bark threshold values with a decay into lower and higher frequencies.

17. The method of claim 16 wherein the decay is triangular, and wherein the triangular decay spreads into lower frequencies and higher frequencies.

18. The method of claim 10 wherein the trasnform coefficients are quantized.

19. The method of claim 18 wherein the quantization step size is proportional to the partially whitened weighting function.

20. The method of claim 10 wherein the quantization step size is determined by performing a binary search for an optimum quantization step size.