Encoder, decoder, coding method, decoding method, coding program, decoding program and recording medium

1. An encoder comprising:

circuitry configured to:

execute a periodic-combined-envelope generating processing in which the circuitry generates a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to a linear predictive coefficient code obtained from an input time-domain audio signal in a predetermined time segment based on a frequency-domain period corresponding to a period code obtained from the input time-domain audio signal, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates;

execute a variable-length-coding-parameter calculating processing in which the circuitry calculates a variable-length coding parameter dependent on an amplitude value from the periodic combined envelope sequence; and

execute a variable-length coding processing in which the circuitry uses the variable-length coding parameter to encode a frequency-domain sequence derived from the input time-domain audio signal by variable-length coding dependent on the amplitude value and to output a variable-length code to a decoder.

2. An encoder comprising:

circuitry configured to:

execute a periodic-combined-envelope generating processing in which the circuitry generates a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to a linear predictive coefficient code obtained from an input time-domain audio signal in a predetermined time segment based on a frequency-domain period corresponding to a period code obtained from the input time-domain audio signal, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates; and

execute a variable-length coding processing in which the circuitry encodes a frequency-domain sequence derived from the input time-domain audio signal on the assumption that the amplitude value of the frequency-domain sequence is greater for a frequency with a greater value of the periodic-combined envelope sequence and outputs a variable-length code to a decoder.

3. A non-transitory computer-readable recording medium on which the coding program for causing a computer to function as the encoder according to claim 1 or 2 is recorded.

4. A decoder comprising:

circuitry configured to:

execute a periodic-combined-envelope generating processing in which the circuitry generates a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to an inputted linear predictive coefficient code based on a frequency-domain period corresponding to an inputted period code, the inputted linear predictive coefficient code being received from an encoder and obtained from an input time-domain audio signal at the encoder, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates;

execute a variable-length-coding-parameter calculating processing in which the circuitry calculates a variable-length coding parameter dependent on an amplitude value from the periodic combined envelope sequence; and

execute a variable-length decoding processing in which the circuitry uses the variable-length coding parameter to perform decoding an inputted variable-length code.

5. A decoder comprising:

circuitry configured to:

execute a periodic-combined-envelope generating processing in which the circuitry generates a periodic combined envelope sequence W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to an inputted linear predictive coefficient code based on a frequency-domain period corresponding to an inputted period code, the inputted linear predictive coefficient code being received from an encoder and obtained from an input time-domain audio signal at the encoder, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates; and

execute a variable-length decoding processing in which the circuitry decodes an inputted variable-length code to obtain a frequency-domain sequence, on the assumption that the amplitude value of the frequency-domain sequence is greater for a frequency with a greater value of the periodic combined envelope sequence.

6. A non-transitory computer-readable recording medium on which the decoding program for causing a computer to function as the decoder according to claim 4 or 5 is recorded.

7. A coding method executing:

generating a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to a linear predictive coefficient code obtained from an input time-domain audio signal in a predetermined time segment based on a frequency-domain period corresponding to a period code obtained from the input time-domain audio signal, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates;

calculating a variable-length coding parameter dependent on an amplitude value from the periodic combined envelope sequence; and

using the variable-length coding parameter to encode a frequency-domain sequence derived from the input time-domain audio signal by variable-length coding dependent on the amplitude value and to output a variable-length code to a decoder.

8. A coding method executing:

generating a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to a linear predictive coefficient code obtained from an input time-domain audio signal in a predetermined time segment based on a frequency-domain period corresponding to a period code obtained from the input time-domain audio signal, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates; and

encoding a frequency-domain sequence derived from the input time-domain audio signal on the assumption that the amplitude value of the frequency-domain sequence is greater for a frequency with a greater value of the periodic-combined envelope sequence and outputting a variable-length code to a decoder.

9. A decoding method executing:

generating a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to an inputted linear predictive coefficient code based on a frequency-domain period corresponding to an inputted period code, the inputted linear predictive coefficient code being received from an encoder and obtained from an input time-domain audio signal at the encoder, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates;

calculating a variable-length coding parameter dependent on an amplitude value from the periodic combined envelope sequence; and

using the variable-length coding parameter to perform decoding an inputted variable-length code.

10. A decoding method executing:

generating a periodic combined envelope sequence, W M [ 1 ], . . . , W M [N], which is a frequency-domain sequence by changing values of samples of a spectral envelope sequence, W[ 1 ], . . . , W[N], which is a frequency-domain sequence corresponding to an inputted linear predictive coefficient code based on a frequency-domain period corresponding to an inputted period code, the inputted linear predictive coefficient code being received from an encoder and obtained from an input time-domain audio signal at the encoder, wherein values of samples of the spectral envelope sequence are changed as in the following formula:

W M [n]=W[n ]·(1+δ· P[n ]),

wherein P[n] is a periodic envelope sequence P[ 1 ], . . . , P[N], and δ is a predetermined value or a chosen value from candidates; and

decoding an inputted variable-length code to obtain a frequency-domain sequence, on the assumption that the amplitude value of the frequency-domain sequence is greater for a frequency with with a greater value of the periodic combined envelope sequence.