Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1998-10-08
2001-04-03
Hudspeth, David (Department: 2641)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C704S222000, C704S223000, C704S230000
Reexamination Certificate
active
06212495
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a coding method and coder of the type that compresses an input signal, such as a digital audio signal, by coding the difference between the input signal and a predicted signal, and to a corresponding decoder.
Coders of this type compress digital audio signals by exploiting the strong correlation between nearby samples of the signal. Two well-known examples of this coding method, both of which can be implemented with comparatively simple processing, are differential pulse-code modulation (DPCM) and adaptive differential pulse-code modulation (ADPCM). In these coding methods, the predicted value of each sample is the decoded value of the preceding sample.
DPCM employs a quantizer with a fixed step size. As a result, overload noise is perceived when the input signal level is high, because the coder is lacks sufficient bits to encode the signal, and granular noise is perceived when the signal level is low, because the step size is too large in relation to the signal level. In ADPCM, the step size is varied as the input signal level varies, and the perceived amount of these two types of quantization noise is reduced.
The sensitivity of the human ear to quantization noise is comparatively high at low sound levels, and comparatively low at high sound levels. By taking advantage of this property, ADPCM can also reduce the size of the coded data, as compared with DPCM.
At present, ADPCM is used for coding both voice signals, as in the Japanese personal handy-phone system (PHS), and music signals, e.g. for prevention of skipping in portable compact disc (CD) players. In CD applications, sixteen-bit input sample values are compressed to four-bit coded values. This 4:1 compression ratio is not particularly high, but even so, the decoded signal is noticeably inferior to the original signal, because of the effects of quantization noise on high-frequency components (the CD sampling rate of 44.1 kilohertz permits reproduction of even the highest audible frequency components).
The quality of the decoded signal can be improved by using five bits per sample instead of four, but the size of the coded data is then increased by twenty-five percent. There is a need for a coding method that reduces quantization noise without increasing the data size so much.
SUMMARY OF THE INVENTION
An object of the present invention is to reduce quantization noise by adding less than one extra bit per coded sample value to the coded data.
In the invented coding method, each sample of an input signal is coded by the steps of:
(a) calculating a predicted value;
(b) calculating a difference between the sample value and the predicted value;
(c) quantizing the difference, obtaining a quantized value;
(d) coding the quantized value, obtaining coded data; and
(e) calculating the predicted value of the next sample from the predicted value and quantized value of the current sample.
For at least one sample, steps (a) to (e) are repeated at least once, using a different predicted value in step (a).
In a first aspect of the invention, the samples are grouped into frames. Steps (a) to (e) are repeated at least once per frame, for all of the samples in the frame. A quantization error is calculated for each repetition. The quantization error may be a total quantization error for all samples in the frame, or a maximum individual-sample quantization error in the frame. For each frame, the coded data obtained in the repetition that produced the least quantization error are output.
In a first sub-aspect of the first aspect, the predicted value of the next sample is obtained by multiplying the sum of the predicted value and quantized value of the current sample by a coefficient. The same coefficient is used throughout each repetition of the coding of an entire frame. Different coefficients are used in different repetitions. Information identifying the coefficient yielding the least quantization error is appended to the coded data for each frame.
In a second sub-aspect of the first aspect, the quantized value is obtained by using a step function selected from a group of step functions. The same step function is used throughout each repetition of the coding of an entire frame. Different step functions are used in different repetitions, leading to different predicted sample values. Information identifying the step function yielding the least quantization error is appended to the coded data for each frame.
In a second aspect of the invention, steps (a) to (e) are repeated for an individual sample whenever the coded data obtained in step (d) represent a maximum absolute quantized value. All repetitions for the same sample are preferably carried out with the same quantization step size in step (c). The predicted value used in each repetition of step (a) preferably differs from the preceding predicted value by less than the maximum absolute quantized value, and forces all of the coded data obtained from all of the repetitions to have the same sign bit. The sign bit is preferably removed from the coded data in all but one of the repetitions.
REFERENCES:
patent: 5826226 (1998-10-01), Ozawa
patent: 5963896 (1999-10-01), Ozawa
patent: 5963898 (1999-10-01), Navarro et al.
patent: 5970442 (1999-10-01), Timner
patent: 5999899 (1999-12-01), Robinson
patent: 6009388 (1999-12-01), Ozawa
patent: 6073092 (2000-06-01), Kwon
“Adaptive Quantization With a One-Word Memory”; N.S. Jayant; The Bell System Technical Journal; vol. 52, No. 7, Sep., 1973.
Frank Robert J.
Hudspeth David
OKI Electric Industry Co., Ltd.
Venable
Wieland Susan
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