Linear prediction obtained by encoding the LSP parameter corresponding to the quantized LSP parameter of the previous frame used in the time-domain encoding with the encoding distortion of the frequency-domain encoding being smaller than the conventional one. Obtained from a coefficient equivalent to the coefficient. The LSP linear conversion unit (300) sets p to an integer of 1 or more, and a [1], a [2],..., A [p] are linear obtained by linear prediction analysis of sound signals in a predetermined time interval. Let ω [1], ω [2], ..., ω [p] be a frequency domain parameter sequence derived from the linear prediction coefficient sequence a [1], a [2], ..., a [p] , Frequency domain parameter sequence ω [1], ω [2],..., Ω [p] as input and post-conversion frequency domain parameter sequence ~ ω [1], ~ ω [2], ..., ~ ω [p] Each of ω [i] (i = 1, 2,..., P) in is transformed by linear transformation based on the value relationship between ω [i] and one or more frequency domain parameters adjacent to ω [i]. The value of the frequency domain parameter ~ ω [i] after conversion is obtained.
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