Encoding method, decoding method, encoder, decoder, program, and recording medium
In encoding, a frequency-domain sample sequence derived from an acoustic signal is divided by a weighted envelope and is then divided by a gain, the result obtained is quantized, and each sample is variable-length encoded. The error between the sample before quantization and the sample after quantization is quantized with information saved in this variable-length encoding. This quantization is performed under a rule that specifies, according to the number of saved bits, samples whose errors are to be quantized. In decoding, variable-length codes in an input sequence of codes are decoded to obtain a frequency-domain sample sequence; an error signal is further decoded under a rule that depends on the number of bits of the variable-length codes; and from the obtained sample sequence, the original sample sequence is obtained according to supplementary information.
1. An encoding method for encoding, with a predetermined number of bits, a frequency-domain sample sequence derived from an acoustic signal in a predetermined time interval, the encoding method comprising:
an encoding step of encoding, by variable-length encoding, an integer value u(n) corresponding to x(n)/g obtained by dividing a value x(n) of each sample n in the frequency-domain sample sequence by a gain g to generate a variable-length code, wherein every integer value u(n) is encoded regardless of whether the obtained integer value u(n) is 0 or not; and
an error encoding step of encoding information indicating whether each of quantization errors r(n)=x(n)/g−u(n) in the encoding step is positive or negative, with a number of surplus bits obtained by subtracting a number of bits of the variable-length code from the predetermined number of bits to generate error codes, the surplus bits being saved by performing the variable-length encoding,
wherein, among said quantization errors r(n), quantization errors r(n) whose corresponding integers are not 0 are encoded with priority in the error encoding step.
2. The encoding method according to claim 1 , wherein
a value determined based on the integer is regarded as an absolute value of a reconstructed value, the absolute value of the reconstructed value is regarded as a reconstructed value corresponding to each of said quantization errors when the each of said quantization errors is positive, and a value obtained by subtracting the absolute value of the reconstructed value from 0 is regarded as a reconstructed value corresponding to each of said quantization errors when the each of said quantization errors is negative, and
when the number of surplus bits is larger than a number of error samples constituting a sequence of quantization errors, information indicating whether a value obtained by subtracting the reconstructed value corresponding to each error sample from the value of the error sample is positive or negative is further encoded with one bit in the error encoding step.
3. The encoding method according to claim 2 , wherein a first absolute value of a first reconstructed value obtained when a first integer is not 0 is larger than a second absolute value of a second reconstructed value obtained when a second integer is 0.
4. An encoder configured to encode, with a predetermined number of bits, a frequency-domain sample sequence derived from an acoustic signal in a predetermined time interval, the encoder comprising:
processing circuitry configured to
perform an encoding step of encoding, by variable-length encoding, an integer value u(n) corresponding to x(n)/g obtained by dividing a value x(n) of each sample n in the frequency-domain sample sequence by a gain g to generate a variable-length code, wherein every integer value u(n) is encoded regardless of whether the obtained integer value u(n) is 0 or not; and
perform an error encoding step of encoding information indicating whether each of quantization errors r(n)=x(n)/g−u(n) in the encoding step is positive or negative, with a number of surplus bits obtained by subtracting a number of bits of the variable-length code from the predetermined number of bits to generate error codes, the surplus bits being saved by performing the variable-length encoding,
wherein, among said quantization errors r(n), quantization errors r(n) whose corresponding integers are not 0 are encoded with priority in the error encoding step.
5. The encoder according to claim 4 , wherein
a value determined based on the integer is regarded as an absolute value of a reconstructed value, the absolute value of the reconstructed value is regarded as a reconstructed value corresponding to each of said quantization errors when the each of said quantization errors is positive, and a value obtained by subtracting the absolute value of the reconstructed value from 0 is regarded as a reconstructed value corresponding to each of said quantization errors when the each of said quantization errors is negative, and
when the number of surplus bits is larger than a number of error samples constituting a sequence of quantization errors, information indicating whether a value obtained by subtracting the reconstructed value corresponding to each error sample from the value of the error sample is positive or negative is further encoded with one bit in the error encoding step.
6. The encoder according to claim 5 , wherein a first absolute value of a first reconstructed value obtained when a first integer is not 0 is larger than a second absolute value of a second reconstructed value obtained when a second integer is 0.
7. A non-transitory computer-readable recording medium having stored thereon a program for causing a computer to execute the steps of the method according to any one of claims 1 to 3 .