PURPOSE: To provide a sound signal band compander in which signals are processed in an analog waveform state in spite of using system parameter for band compression and band compression transmission is attained through an analog signal transmission system by performing A/D and D/A conversions.

CONSTITUTION: A line predictive analyzer 103 obtains a prediction coefficient ai from a digital sound signal y(nΔt), an inverse filtering circuit 104 obtains a prediction residual signal x(nΔt) using the coefficient ai, a down sampling circuit 109 decreases the sampling rate to convert it to fundamental signal x'(nΔT), supplies the result to an autoregression system linear predictive synthesizer 110, obtains a narrow band time sequential signal w(nΔT) and transmits it to a receiver side. On the receiver side, the signal w(nΔT) is decoded to a sound signal by reversal processing. Since the signal y(nΔt) and the signal w(nΔT) have the same coefficient ai, spectrum information having the original sound signal y(nΔt) is faithfully transmitted only by sending the output signal w(nΔT) consisting of a narrow band analog waveform.