Coding device, decoding device, and method and program thereof

A decoding device comprising:

• circuitry configured to:
• execute first decoding processing in which the circuitry obtains first decoded values by decoding a first code, the first decoded values corresponding to coefficients which are convertible into linear prediction coefficients of more than one order;
• execute second decoding processing in which the circuitry obtains second decoded values of more than one order by decoding a second code if an index Q commensurate with how high a peak-to-valley height of a spectral envelope is, the spectral envelope corresponding to the first decoded values of the coefficients which are convertible into the linear prediction coefficients of more than one order, is larger than or equal to a predetermined threshold value Th1 and/or an index Q′ commensurate with how short the peak-to-valley height of the spectral envelope is, is smaller than or equal to a predetermined threshold value Th1′;
• and
• execute addition processing in which the circuitry obtains third decoded values corresponding to the coefficients which are convertible into the linear prediction coefficients of more than one order by adding the first decoded values and the second decoded values of corresponding orders if the index Q commensurate with how high the peak-to-valley height of the spectral envelope is, the spectral envelope corresponding to the first decoded values of the coefficients which are convertible into the linear prediction coefficients of more than one order, is larger than or equal to the predetermined threshold value Th1 and/or the index Q′ commensurate with how short the peak-to-valley height of the spectral envelope is, is smaller than or equal to the predetermined threshold value Th1′, wherein
• in the second decoding processing the circuitry obtains the second decoded values from a large number of candidates for decoded values by decoding the second code with a bit number depending on a magnitude of the index Q and the index Q′, such that the larger the index Q and/or the smaller the index Q′, the greater the bit number.

A coding method, implemented by a coding device that includes circuitry, comprising:

• a first coding step in which the circuitry obtains a first code by coding coefficients which are convertible into linear prediction coefficients of more than one order;
• and
• a second coding step in which the circuitry obtains a second code by coding at least quantization errors of the first coding step if an index Q commensurate with how high a peak-to-valley height of a spectral envelope is, the spectral envelope corresponding to the coefficients which are convertible into the linear prediction coefficients of more than one order, is larger than or equal to a predetermined threshold value Th1 and/or an index Q′ commensurate with how short the peak-to-valley height of the spectral envelope is, is smaller than or equal to a predetermined threshold value Th1′, wherein
• in the second coding step the circuitry obtains the second code whose bit number becomes greater as the index Q increases and/or the index Q′ decreases.

A decoding method, implemented by a decoding device that includes circuitry, comprising:

• a first decoding step in which the circuitry obtains first decoded values by decoding a first code, the first decoded values corresponding to coefficients which are convertible into linear prediction coefficients of more than one order;
• a second decoding step in which the circuitry obtains second decoded values of more than one order by decoding a second code if an index Q commensurate with how high a peak-to-valley height of a spectral envelope is, the spectral envelope corresponding to the first decoded values of the coefficients which are convertible into the linear prediction coefficients of more than one order, is larger than or equal to a predetermined threshold value Th1 and/or an index Q′ commensurate with how short the peak-to-valley height of the spectral envelope is, is smaller than or equal to a predetermined threshold value Th1′;
• and
• an addition step in which the circuitry obtains third decoded values corresponding to the coefficients which are convertible into the linear prediction coefficients of more than one order by adding the first decoded values and the second decoded values of corresponding orders if the index Q commensurate with how high the peak-to-valley height of the spectral envelope is, the spectral envelope corresponding to the first decoded values of the coefficients which are convertible into the linear prediction coefficients of more than one order, is larger than or equal to the predetermined threshold value Th1 and/or the index Q′ commensurate with how short the peak-to-valley height of the spectral envelope is, is smaller than or equal to the predetermined threshold value Th1′, wherein
• in the second decoding step the circuitry obtains the second decoded values from a large number of candidates for decoded values by decoding the second code with a bit number depending on a magnitude of the index Q and the index Q′, such that the larger the index Q and/or the smaller the index Q′, the greater the bit number.