Encoding and decoding of digital audio signals using variable alphabet size

1. An encoding system, comprising:

a processor; and

a memory device storing instructions executable by the processor, the instructions being executable by the processor to perform a method for encoding an audio signal, the method comprising:

receiving a digital audio signal;

parsing the digital audio signal into a plurality of frames, each frame including a specified number of audio samples;

performing a transform of the audio samples of each frame to produce a plurality of frequency-domain coefficients for each frame;

partitioning the plurality of frequency-domain coefficients for each frame into a plurality of bands for each frame, each band having a reshaping parameter that represents a time resolution and a frequency resolution,

encoding the digital audio signal to a bit stream that includes each band's reshaping parameter, wherein:

for a first band, the reshaping parameter is encoded using a first alphabet size; and

for a second band different from the first band, the reshaping parameter is encoded using a second alphabet size different from the first alphabet size; and

outputting the bit stream.

2. The encoding system of claim 1 , further comprising:

adjusting a time resolution and a frequency resolution of each band of each frame, the first time resolution and the first frequency resolution being adjusted in a complementary manner by a magnitude described by the reshaping parameter, the reshaping parameter having a value that is an integer selected from one of a plurality of specified ranges of integers, wherein:

the first alphabet size equals a number of integers in a first specified range of integers of the plurality of specified ranges of integers; and

the second alphabet size equals a number of integers in a second specified range of integers of the plurality of specified ranges of integers.

3. The encoding system of claim 2 , wherein the first alphabet size is four, and the second alphabet size is five.

4. The encoding system of claim 1 , wherein prior to the adjusting, the time resolution of the first band equals eight audio samples, and the time resolution of the second band equals one audio sample.

5. The encoding system of claim 2 , wherein:

each band has a size that equals a product of the time resolution of the band and the frequency resolution of the band; and

the time resolution of the band and the frequency resolution of the band are adjusted in a complementary manner without varying the size of the band.

6. The encoding system of claim 5 , wherein the time resolution is adjusted by a factor of 2 c , and the frequency resolution is varied by a factor of 2 −c , where quantity c is the reshaping parameter.

7. The encoding system of claim 2 , further comprising:

forming a reshaping sequence for each frame, the reshaping sequence describing the reshaping parameter for each band; and

normalizing each entry in each reshaping sequence to a range of possible values for the entry, each range of possible values corresponding to the specified range of integers for the band.

8. The encoding system of claim 1 , further comprising:

forming a first sequence for each frame, the first sequence describing the reshaping parameter for the frame as a sequence representing the reshaping parameter for each band, using a unary code;

forming a second sequence for each frame, the second sequence describing the reshaping parameter for the frame as a sequence representing the reshaping parameter for each band, using a quasi-uniform code;

forming a third sequence for each frame, the third sequence describing the reshaping parameter for the frame as a sequence representing the differences in reshaping parameters between adjacent bands, using a unary code;

forming a fourth sequence for each frame, the fourth sequence describing the reshaping parameter for the frame as a sequence representing the differences in reshaping parameters between adjacent bands, using a quasi-uniform code;

selecting the shortest sequence of the first sequence, the second sequence, the third sequence, and the fourth sequence, the shortest sequence being the sequence that includes the fewest number of elements;

embedding data representing the selected shortest sequence into the bit stream, for each frame; and

embedding data representing an indicator into the bit stream for each frame, the indicator indicating which of the four sequences is included in the bit stream.

9. The encoding system of claim 1 , wherein the transform is a modified discrete cosine transform.

10. The encoding system of claim 1 , wherein each frame includes exactly 1024 samples.

11. The encoding system of claim 1 , wherein a number of frequency-domain coefficients in each plurality of frequency-domain coefficients equals the specified number of audio samples in each frame.

12. The encoding system of claim 1 , wherein the plurality of frequency-domain coefficients for each frame includes exactly 1024 frequency-domain coefficients.

13. The encoding system of claim 1 , wherein the plurality of bands for each frame includes exactly 22 bands.

14. The encoding system of claim 1 , wherein the encoding system is included in a codec.

15. A decoding system, comprising:

a processor; and

a memory device storing instructions executable by the processor, the instructions being executable by the processor to perform a method for decoding an encoded audio signal, the method comprising:

receiving a bit stream, the bit stream including a plurality of frames, each frame partitioned into a plurality of bands;

for each band of each frame, extracting a reshaping parameter from the bit stream, the reshaping parameter representing a time resolution and a frequency resolution for the band, wherein:

for a first band, the reshaping parameter is embedded in the bit stream using a first alphabet size; and

for a second band different from the first band, the reshaping parameter is embedded in the bit stream using a second alphabet size different from the first alphabet size; and

decoding the bit stream using the reshaping parameters to generate a decoded digital audio signal.

16. The decoding system of claim 15 , further comprising, for each band of each frame, extracting data indicating:

whether the reshaping parameter in the bit stream is represented as a unary code or a quasi-uniform code, and

whether the reshaping parameter in the bit stream is represented as a sequence representing the reshaping parameter for each band or a sequence representing the differences in reshaping parameters between adjacent bands.

17. The decoding system of claim 15 , wherein the decoding system in included in a codec.

18. An encoding system, comprising:

a receiver circuit to receive a digital audio signal;

a framer circuit to parse the digital audio signal into a plurality of frames, each frame including a specified number of audio samples;

a transformer circuit to perform a transfoiin of the audio samples of each frame to produce a plurality of frequency-domain coefficients for each frame;

a frequency band partitioner circuit to partition the plurality of frequency-domain coefficients for each frame into a plurality of bands for each frame, each band having a reshaping parameter that represents a time resolution and a frequency resolution,

an encoder circuit to encode the digital audio signal to a bit stream that includes each band's reshaping parameter, wherein:

for a first band, the reshaping parameter is encoded using a first alphabet size; and

for a second band different from the first band, the reshaping parameter is encoded using a second alphabet size different from the first alphabet size; and

an output circuit to output the bit stream.

19. The encoding system of claim 18 , further comprising:

a resolution adjustment circuit to adjust a time resolution and a frequency resolution of each band of each frame, the first time resolution and the first frequency resolution being adjusted in a complementary manner by a magnitude described by the reshaping parameter, the reshaping parameter having a value that is an integer selected from one of a plurality of specified ranges of integers, wherein:

the first alphabet size equals a number of integers in a first specified range of integers of the plurality of specified ranges of integers; and

the second alphabet size equals a number of integers in a second specified range of integers of the plurality of specified ranges of integers.

20. The encoding system of claim 19 , wherein the time resolution is adjusted by a factor of 2 c , and the frequency resolution is varied by a factor of 2 −c , where quantity c is the reshaping parameter.