ADAPTIVE AUDIO CODEC SYSTEM, METHOD AND ARTICLE
An adaptive noise shaping filter flattens signal components below a threshold frequency range in a filtered signal to be encoded. An encoder generates quantized signals based on a difference signal and includes an adaptive quantizer and a decoder. The decoder generates feedback signals and has an inverse quantizer and a predictor. The predictor has determined control parameters based on the threshold frequency range.
1 . An apparatus, comprising:
an input filter configured to filter input signals and having an upper-edge frequency;
an adaptive noise shaping filter configured to flatten filtered signals below a threshold frequency range based on the upper-edge frequency;
an encoder coupled to the adaptive noise shaping filter, wherein the encoder is configured to generate quantized signals based on a difference signal and includes:
an adaptive quantizer; and
a decoder configured to generate feedback signals and having an inverse quantizer and a predictor circuit, the predictor circuit having determined control parameters based on the threshold frequency range.
2 . The apparatus of claim 1 wherein the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters comprise fixed filter coefficients of the FIR filter.
3 . The apparatus of claim 1 wherein the adaptive noise shaping filter is configured to generate a signal indicative of filter coefficients of the adaptive noise shaping filter.
4 . The apparatus of claim 1 wherein the encoder includes coding circuitry configured to generate code words based on quantized signal words generated by the adaptive quantizer.
5 . The apparatus of claim 4 wherein the coding circuitry is configured to generate an escape code in response to at least one of:
a quantized signal word not being associated with a corresponding coding code word;
an end of a signal channel; and
an end of a signal to be encoded.
6 . The apparatus of claim 4 wherein the coding circuitry is configured to use Huffman coding to generate the code words.
7 . The apparatus of claim 1 wherein the adaptive quantizer is a variable rate quantizer.
8 . The apparatus of claim 7 wherein the adaptive quantizer is configured to control a quantization step size according to:
d n+1 =βd n +m ( c n /L factor ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
9 . The apparatus of claim 7 wherein the adaptive quantizer is configured to control a quantization step size according to:
d n+1 =max( βd n +m ( c n /L factor ), d min ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, d min is a threshold step size in the log domain, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
10 . The apparatus of claim 1 wherein the input filter comprises one of:
a low-pass filter; and
a band-pass filter.
11 . A method, comprising:
filtering an input signal to remove components above a cut-off frequency;
applying adaptive noise shaping to the filtered input signal to flatten signal components below a threshold frequency range in the filtered input signal; and
encoding the noise-shaped signal, the encoding including:
generating quantized signals based on a difference signal; and
generating a feedback signal using a predictor circuit, the predictor circuit having determined control parameters based on the threshold frequency range.
12 . The method of claim 11 , comprising:
generating a signal indicative of filter coefficients used to apply the adaptive noise shaping.
13 . The method of claim 11 , comprising:
generating code words based on quantized signal words.
14 . The method of claim 13 , comprising:
using escape coding.
15 . The method of claim 11 , comprising controlling a quantization step size according to:
d n+1 =βd n +m ( c n /L factor ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
16 . The method of claim 11 , comprising controlling a quantization step size according to:
d n+1 =max( βd n +m ( c n /L factor ), d min ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, d min is a threshold step size in the log domain, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
17 . The method of claim 11 wherein the encoding includes:
generating the difference signal based on the feedback signal and the noise-shaped signal.
18 . The method of claim 11 wherein the filtering the input signal comprises one of:
low-pass filtering; and
band-pass filter.
19 . A non-transitory computer-readable medium having contents which configure signal processing circuitry to perform a method, the method comprising:
filtering an input signal to remove components above a cut-off frequency;
applying adaptive noise shaping to the filtered input signal to flatten signal components below a threshold frequency range in the input signal; and
encoding the noise-shaped signal, the encoding including:
generating quantized signals based on a difference signal; and
generating a prediction signal using determined control parameters based on the threshold frequency range.
20 . The non-transitory computer-readable medium of claim 19 wherein the method comprises:
generating a signal indicative of filter coefficients used to apply the adaptive noise shaping.
21 . The non-transitory computer-readable medium of claim 19 wherein the method comprises:
generating code words based on quantized signal words.
22 . The non-transitory computer-readable medium of claim 21 wherein the method comprises:
using escape coding.
23 . The non-transitory computer-readable medium of claim 22 wherein the method comprises controlling a quantization step size according to:
d n+1 =βd n +m ( c n /L factor ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
24 . The non-transitory computer-readable medium of claim 22 , wherein the method comprises controlling a quantization step size according to:
d n+1 =max( βd n +m ( c n /L factor ), d min ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, d min is a threshold step size in the log domain, and d n+1 corresponds to a step size in the log domain to be applied to a next quantized signal word c n+1 .
25 . The non-transitory computer-readable medium of claim 18 wherein the filtering the input signal comprises one of:
low-pass filtering; and
band-pass filtering.
26 . A system, comprising:
means for removing frequency components in an input signal above a cutoff frequency;
means for applying adaptive noise shaping to an output of the means for removing to flatten signal components below a threshold frequency range;
means for generating quantized signals based on a difference signal; and
means for generating a prediction signal using determined control parameters based on the threshold frequency range.
27 . The system of claim 26 , comprising:
means for transmitting a signal indicative of filter coefficients of the means for applying adaptive noise shaping.
28 . The system of claim 26 , comprising:
means for generating code words based on quantized signal words.
29 . The system of claim 28 wherein the means for removing frequency components comprises a low-pass filter.
30 . The system of claim 26 , comprising:
means for decoding encoded signals.
31 . An apparatus, comprising:
a decoder configured to generate decoded signals based on quantized signals representing a coded signal, the decoder including:
an inverse quantizer; and
a finite impulse response (FIR) filter;
an inverse adaptive noise shaping filter configured to receive a control signal included in a bit stream including the coded signal, the control signal being indicative of adaptive noise shaping applied to flatten signal components below a threshold frequency range in the coded signal; and
an output filter configured to filter inverse noise-shaped signals and having an upper-edge frequency.
32 . The apparatus of claim 31 wherein the decoder includes decoding circuitry configured to generate quantized signal words based on code words in the bit stream.
33 . The apparatus of claim 32 wherein the decoding circuitry is configured to respond to at least one of:
an escape code indicative of a quantized signal word being included in the bit stream;
an escape code indicative of an end of a signal channel; and
an escape code indicative of an end of a signal to be encoded.
34 . The apparatus of claim 32 wherein the decoding circuitry is configured to use Huffman coding to decode code words in the bit stream.
35 . The apparatus of claim 32 wherein the inverse quantizer is a variable rate inverse quantizer.
36 . The apparatus of claim 35 wherein the inverse quantizer is configured to control a step size according to:
d n+1 =βd n +m ( c n /L factor ),
where c n is a current quantized signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, and d n+1 corresponds to step size in the log domain to be applied to a next quantized signal word c n+1 .
37 . The apparatus of claim 35 wherein the inverse quantizer is configured to control a step size according to:
d n+1 =max( βd n +m ( c n /L factor ), d min ),
where c n is a current quantization signal word, d n corresponds to a current step size in a log domain, L factor is a loading factor, m(c n /L factor ) is a log multiplier selected based on the current quantized signal c n and the loading factor L factor , β is a leakage coefficient, d min is a threshold step size in the log domain, and d n+1 corresponds to step size in the log domain to be applied to a next quantization signal word c n+1 .
38 . A method, comprising:
decoding quantized signals representing a coded signal, the decoding including:
inverse quantizing the quantized signals using an inverse quantizer; and
generating a prediction signal using a prediction circuit;
applying inverse adaptive noise shaping to the decoded quantized signals based on a control signal indicative of adaptive noise shaping applied to flatten signal components below a threshold frequency range in the coded signal; and
filtering inverse noise shaped signals to remove components above a cut-off frequency.
39 . The method of claim 38 , comprising:
generating quantized signal words based on code words in a bit stream representing the coded signal.
40 . The method of claim 39 , comprising:
using escape coding to decode the code words.
41 . The method of claim 39 wherein the filtering the inverse noise shaped signals comprises low-pass filtering the inverse noise shaped signals.
42 . A non-transitory computer-readable medium having contents which configure signal processing circuitry to perform a method, the method comprising:
decoding quantized signals representing a coded signal, the decoding including:
inverse quantizing the quantized signals; and
generating a prediction signal;
applying inverse adaptive noise shaping to the decoded quantized signals based on a control signal indicative of adaptive noise shaping applied to flatten signal components below a threshold frequency range in the coded signal; and
filtering inverse noise shaped signals to remove components above a cut-off frequency.
43 . The non-transitory computer-readable medium of claim 42 wherein the method comprises:
generating quantized signal words based on code words in a bit stream representing the coded signal.
44 . The non-transitory computer-readable medium of claim 43 wherein the method comprises:
using escape coding to decode the code words.
45 . The non-transitory computer-readable medium of claim 43 wherein the filtering inverse noise shaped signals comprises:
low-pass filtering the inverse noise shaped signals.
46 . A system, comprising:
means for inverse quantizing a quantized signal representing a coded signal;
means for generating a prediction signal;
means for generating a decoded signal based on the inverse quantized signal and the prediction signal;
means for applying inverse adaptive noise shaping to the decoded signals based on a control signal indicative of adaptive noise shaping applied to flatten signal components below a threshold frequency range in the coded signal; and
means for removing components above a cut-off frequency in inverse noise-shaped signals.
47 . The system of claim 46 , comprising:
means for generating quantized signal words based on code words in a bit stream representing the coded signal.
48 . The system of claim 46 wherein the means for removing comprises:
a low-pass filter.