PURPOSE: To realize the Curnen-Leve transformation according adaptively to a short time characteristic of an input signal by a low information speed by adopting the constitution that a short time autocorelation coefficient of the input signal is formed as a model by a few parameters.

CONSTITUTION: The autocorrelation coefficient of a model having a number of variables fewer than the size of a block by using an input discrete signal series is estimated by a computer 120 at a coder side, a coefficient series computer 140 and a specific value expansion computer 130 apply the orthogonal conversion depending on the estimated autocorrelation coefficient in the form of the input discrete signal series to convert the input discrete signal series into a coefficient series thereby encoding the parameter and the coefficient series of the model. A decoder 320 decodes the code of the parameter and the coefficient series of the model in the decoder side and a recovery signal generator 350 generates a recovered discrete signal series by using the orthogonal conversion by a specific value expansion computer 340 decided by the autocorrelation coefficiend of the decoded model and the decoded coefficient series.