Adaptive audio decoder system, method and article
A decoder generates decoded signals based on quantized signals. The decoder includes an inverse quantizer and a predictor circuit. The quantized signals are generated in an encoder by low-pass filtering an input signal and encoding the filtered signal using adaptive differential pulse code modulation. The predictor circuit has filter coefficients based on a frequency response of the low-pass filter used to filter the input signal.
1. An apparatus, comprising:
a decoder configured to generate decoded signals based on quantized signals and generating a prediction signal, the decoder including:
a feedback loop including:
an inverse quantizer configured to receive a quantized signal from a variable rate adaptive quantizer; and
a predictor circuit; and
a low-pass filter having a stop band with a bandwidth selected as a function of a sampling rate, wherein the stop band bandwidth is at least 31% of half of the sampling rate
an adder configured to receive an output of the decoder and a modified low-pass filter output;
wherein the low-pass filter and the predictor circuit are coupled and the predictor circuit is configured to provide a predetermined prediction gain for the input signal as a function of the stop band bandwidth of the low-pass filter.
2. The apparatus of claim 1 wherein the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter, wherein the decoder operates on a bit stream that comprises code words and quantized signal words that are not code words.
3. The apparatus of claim 1 , wherein the decoder is utilized by an encoder prior to transmission of encoded signals and further comprising:
an inverse adaptive noise shaping filter coupled between the inverse quantizer and the low-pass filter.
4. The apparatus of claim 3 wherein the inverse adaptive noise shaping filter is configured to receive a signal included in a bit stream received by the decoder and indicative of inverse adaptive noise shaping filter coefficients and is coupled to the adder.
5. The apparatus of claim 1 wherein the decoder includes decoding circuitry directly coupled to a disassembler and configured to generate quantized signal words based on code words in a bit stream received by the decoder.
6. The apparatus of claim 5 wherein the decoding circuitry is configured to respond to:
an escape code indicative of a quantized signal word being included in the bit stream by decoding the quantized signal word 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.
7. The apparatus of claim 5 wherein the decoding circuitry is configured to use Huffman coding to decode code words in the bit stream and is directly coupled to a step size and average bit rate control.
8. The apparatus of claim 1 wherein the inverse quantizer is a variable rate inverse quantizer and is directly coupled to a step size and average bit rate control.
9. The apparatus of claim 1 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 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 inverse quantizer is configured to control a step size according to:
d n+1 =max(β d 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 .
11. A method, comprising:
decoding an encoded signal using a feedback loop, the decoding including:
inverse quantizing a quantized signal using an inverse quantizer that is configured to receive the quantized signal from a variable rate adaptive quantizer;
generating a prediction signal based on the quantized signal using a prediction circuit; and
filtering the decoded signal using a low-pass filter having a stop band having a bandwidth of at least 31% of one half of a sampling rate wherein the predictor circuit has determined control parameters and filter coefficients based on a frequency response of the low-pass filter.
12. The method of claim 11 wherein the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter.
13. The method of claim 12 wherein the filtering includes using an inverse adaptive noise shaping filter coupled between an output of the decoder and an input of the low-pass filter.
14. The method of claim 13 , comprising:
setting filter coefficients of the inverse adaptive noise shaping filter based on a signal included in a bit stream of the encoded signal.
15. The method of claim 11 , comprising:
generating quantized signal words based on code words included in a bit stream of the encoded signal and combining the generated quantized signal words with quantized signal words included in the bit stream that have no associated code words.
16. The method of claim 15 , comprising using escape coding to indicate the quantized signal words that have no associated code words.
17. The method of claim 15 , comprising using Huffman coding to decode code words in the bit stream.
18. The method of claim 11 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 a step size in the log domain to be applied to a next quantized signal word c n+1 .
19. The method of claim 11 wherein the inverse quantizer is configured to control a step size according to:
d n+1 =max(β d 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 .
20. A non-transitory computer-readable medium having contents which configure signal processing circuitry to perform a method, the method comprising:
decoding an encoded signal using feedback, the decoding including:
inverse quantizing a quantized signal using an inverse quantizer that is configured to receive the quantized signal from a variable rate adaptive quantizer; and
generating a prediction signal based on the quantized signal; and
filtering the decoded signal, the filtering including low-pass filtering using a stop band having a bandwidth of at least 31% of one half of a sampling rate, wherein the generating the prediction signal includes using determined control parameters based on a frequency response of the low-pass filtering and the fixed filter coefficients of the low-pass filtering.
21. The non-transitory computer-readable medium of claim 20 wherein the predicting signal is generated using a finite impulse response (FIR) filter and the determined control parameters comprise fixed filter coefficients of the FIR filter.
22. The non-transitory computer-readable medium of claim 20 wherein the filtering includes applying inverse adaptive noise shaping filtering to the decoded signal.
23. The non-transitory computer-readable medium of claim 20 wherein the method comprises:
generating an output comprising quantized signal words in a bit stream of the encoded signal and quantized signal words based on code words included in the bit stream of the encoded signal.
24. A system, comprising:
a decoder configured to generate decoded signals based on quantized signals and generating a prediction signal, the decoder including:
an inverse quantizer configured to receive a quantized signal from a variable rate adaptive quantizer; and
a predictor circuit having fixed filter coefficients; and
an encoder including a low-pass filter having a stop band having a bandwidth of at least 31% of one half of a sampling rate and configured to filter a signal to be encoded by the encoder, the predictor circuit of the decoder having determined control parameters based on a frequency response of the low-pass filter of the encoder, wherein the low-pass filter and the predictor circuit are coupled and the predictor circuit is configured to provide a predetermined prediction gain for the input signal as a function of the stop band bandwidth of the low-pass filter.
25. The system of claim 24 wherein the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter.
26. The system of claim 24 , comprising:
an inverse adaptive noise shaping filter coupled to an output of the inverse quantizer of the decoder.
27. The system of claim 26 wherein the inverse adaptive noise shaping filter is configured to apply filter coefficient based on a synchronization signal included in a bit stream received by the decoder.
28. The system of claim 24 wherein the decoder includes decoding circuitry configured to generate an output of quantized signal words including quantized signal words based on code words in a bit stream received by the decoder from the encoder and quantized signal words in the bit stream that have no corresponding code words in a code book.
29. A system, comprising:
a decoder configured to generate decoded signals based on quantized signals and generating a prediction signal, the decoder including:
an inverse quantizer configured to receive a quantized signal from a variable rate adaptive quantizer; and
a predictor circuit; and
an output filter coupled to the decoder and having determined control parameters to limit a bandwidth of an output of the decoder to less than seventy-five percent of the available bandwidth based on a fixed sampling frequency of the quantized signals that is greater than 48 kHz, wherein the predictor circuit has determined control parameters based on a frequency response of the output filter and a stop band having a bandwidth of at least 31% of one half of a sampling rate of a low-pass filter, wherein low-pass filter and the predictor circuit are coupled and the predictor circuit is configured to provide a predetermined prediction gain for the input signal as a function of the stop band bandwidth of the low-pass filter.
30. The system of claim 29 , comprising an encoder configured to generate encoded signals.
31. The system of claim 29 wherein the output filter is a low-pass filter, the determined control parameters of the low-pass filter are fixed filter coefficients of the low-pass filter, the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter.
32. The system of claim 29 wherein the output filter is a band-pass filter, the determined control parameters of the band-pass filter are fixed filter coefficients of the band-pass filter, the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter.
33. A system, comprising:
a decoder configured to generate decoded signals based on the quantized signals and generating a prediction signal, the decoder including:
an inverse quantizer configured to receive a quantized signal from a variable rate adaptive quantizer; and
a predictor circuit having fixed filter coefficients; and
an output filter configured to filter an output of the decoder, wherein the predictor circuit has determined control parameters based on a frequency response of an encoder low-pass filter having a stop band with a bandwidth of at least 31% of one half of a sampling rate.
34. The system of claim 33 , comprising an encoder including the encoder low-pass filter.
35. The system of claim 33 wherein the predictor circuit comprises a finite impulse response (FIR) filter and the determined control parameters of the predictor circuit comprise fixed filter coefficients of the FIR filter.
36. The system of claim 33 , comprising:
an inverse adaptive noise shaping filter coupled to an output of the inverse quantizer of the decoder.