Predictive code conversion method capable of forcibly putting a feedback loop in an inactive state

In a predictive encoder for encoding an encoder input signal into an encoder output signal and having in a feedback loop an adaptive predictive filter which comprises a plurality of taps assigned with adaptive prediction coefficients and produces a predictive signal predictive of the encoder input signal, an encoder control circuit monitors products between the coefficients and tap signals derived from the taps so as to force each product to zero when an absolute value of each product is not greater than a predetermined value. The feedback loop is temporarily placed in an inactive state when the predictive signal becomes zero as a result of all zeros of the products. A predictive decoder is also operated by the use of a decoder control circuit similar to the encoder control circuit and cooperating with an adaptive predictive filter of the decoder. Each adaptive predictive filter may be either of an FIR type or of an IIR type.