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

2:

• A decoder comprising:
• circuitry configured to:
• when an inputted indicator of a degree of periodicity is within a predetermined range indicating high periodicity, execute a periodic-combined-envelope generating processing in which the circuitry generates a periodic combined envelope sequence, WM[1], . . . , WM[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:
• WM[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
• when the indicator is within the range indicating high periodicity, 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;
• and
• when the indicator is not within the range indicating high periodicity, 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 spectral envelope sequence.

3:

• A decoding method executing:
• decoding an inputted indicator code to obtain an indicator of a degree of periodicity;
• and
• when the indicator is within a predetermined range indicating high periodicity, generating a periodic combined envelope sequence, WM[1], . . . , WM[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:
• WM[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;
• when the indicator is within the range indicating high periodicity, calculating a variable-length coding parameter dependent on an amplitude value from the periodic combined envelope sequence;
• and when the indicator is not within the range indicating high periodicity, calculating a variable-length coding parameter dependent on an amplitude value from the spectral envelope sequence;
• and
• using the variable-length coding parameter to perform decoding an inputted variable-length code.

4:

• A decoding method executing:
• when an inputted indicator of a degree of periodicity is within a predetermined range indicating high periodicity, generating a periodic combined envelope sequence, WM[1], . . . , WM[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:
• WM[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
• when the indicator is within the range indicating high periodicity, 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 a greater value of the periodic combined envelope sequence;
• and
• when the indicator is not within the range indicating high periodicity, 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 a greater value of the spectral envelope sequence.