Techniques for measurement of perceptual audio quality
An audio processing tool measures the quality of reconstructed audio data. For example, an audio encoder measures the quality of a block of reconstructed frequency coefficient data in a quantization loop. The invention includes several techniques and tools, which can be used in combination or separately. First, before measuring quality, the tool normalizes the block to account for variation in block sizes. Second, for the quality measurement, the tool processes the reconstructed data by critical bands, which can differ from the quantization bands used to compress the data. Third, the tool accounts for the masking effect of the reconstructed data, not just the masking effect of the original data. Fourth, the tool band weights the quality measurement, which can be used to account for noise substitution or band truncation. Finally, the tool changes quality measurement techniques depending on the channel coding mode.
1. In an audio encoder with a quantization loop, a method comprising:
encoding audio, including:
reconstructing a block of spectral data for the audio, the block of spectral data quantized as plural quantization bands;
processing the reconstructed block of spectral data as plural critical bands according to an auditory model, wherein the plural critical bands are variable relative to the plural quantization bands with respect to one or more of number and position; and
measuring quality of the reconstructed block of spectral data; and
outputting the encoded audio in a bitstream.
2. The method of claim 1 wherein the encoding further includes:
before the quantization loop, applying a quantization matrix to the block of spectral data, thereby quantizing the block as the plural quantization bands;
quantizing the block of spectral data with a uniform, scalar quantization step size that is adjustable in response to one or more feedback criteria.
3. The method of claim 2 wherein the reconstructing comprises:
inverse quantizing the block of spectral data with the quantization step size; and
inverse weighting the block of spectral data by the quantization matrix.
4. The method of claim 2 wherein the one or more feedback criteria are based at least in part upon the measured quality and a bitrate criterion.
5. The method of claim 1 wherein the measured quality is a ratio between a noise pattern and an effective excitation pattern for the block.
6. The method of claim 1 wherein the reconstructing comprises an inverse multi-channel transformation.
7. The method of claim 1 wherein the measuring comprises applying a set of band weights for the block.
8. The method of claim 1 wherein the measuring comprises measuring quality for each of the plural critical bands of the reconstructed block of spectral data.
9. The method of claim 1 wherein the measuring comprises measuring quality for each of the plural quantization bands of the reconstructed block of spectral data.
10. A computer-readable medium having encoded therein computer-executable instructions for causing a computer programmed thereby to perform a method comprising:
encoding audio, including, in an iteration of a quantization loop,
reconstructing a block of frequency coefficients for the audio, the block of frequency coefficients quantized as plural quantization bands;
processing the reconstructed block as plural critical bands according to an auditory model, wherein the plural critical bands are variable relative to the plural quantization bands; and
measuring quality of the reconstructed block; and
outputting the encoded audio in a bitstream.
11. The computer-readable medium of claim 10 wherein the encoding further includes, in the iteration of the quantization loop:
adjusting quantization level as necessary to satisfy one or more loop criteria.
12. The computer-readable medium of claim 11 wherein the quantization level is a quantization step size, and wherein a quantization matrix and quantization band boundary matrix establishes the plural quantization bands before the iteration.
13. The computer-readable medium of claim 10 wherein the plural critical bands differ from the plural quantization bands in one or more of number and position.
14. The computer-readable medium of claim 10 wherein the reconstructing comprises an inverse multi-channel transformation.
15. The computer-readable medium of claim 10 wherein the measured quality is a ratio between a noise pattern and an effective excitation pattern for the block.
16. The computer-readable medium of claim 10 wherein the block has a variable size, further comprising normalizing the block before the processing.
17. An audio encoder with a quantization loop, the encoder comprising:
means for encoding audio, wherein the encoding includes:
reconstructing a block of spectral data for the audio, the block of spectral data quantized as plural quantization bands;
processing the reconstructed block of spectral data as plural critical bands according to an auditory model, wherein the plural critical bands are variable relative to the plural quantization bands with respect to one or more of number and position; and
measuring quality of the reconstructed block of spectral data; and
a multiplexer for outputting the encoded audio in a bitstream.
18. The encoder of claim 17 wherein the encoding further includes:
before the quantization loop, applying a quantization matrix to the block of spectral data, thereby quantizing the block as the plural quantization bands;
quantizing the block of spectral data with a uniform, scalar quantization step size that is adjustable in response to one or more feedback criteria.
19. The encoder of claim 18 wherein the reconstructing comprises:
inverse quantizing the block of spectral data with the quantization step size; and
inverse weighting the block of spectral data by the quantization matrix.
20. The encoder of claim 18 wherein the one or more feedback criteria are based at least in part upon the measured quality and a bitrate criterion.
21. The encoder of claim 17 wherein the measured quality is a ratio between a noise pattern and an effective excitation pattern for the block.
22. The encoder of claim 17 wherein the reconstructing comprises an inverse multi-channel transformation.
23. The encoder of claim 17 wherein the measuring comprises applying a set of band weights for the block.
24. The encoder of claim 17 wherein the measuring comprises measuring quality for each of the plural critical bands of the reconstructed block of spectral data.
25. The encoder of claim 17 wherein the measuring comprises measuring quality for each of the plural quantization bands of the reconstructed block of spectral data.