A method, system and program for encoding and decoding speech according to a source-filter model whereby speech is modelled to comprise a source signal filtered by a time-varying filter. The method comprises: receiving a speech signal comprising successive frames. For each of a plurality of frames of the speech signal: adding a predetermined noise signal generated by a quantization gain multiplied by 0.5 times an inverse of a pitch correlation to the speech signal to generate a simulated signal, determining linear predictive coding coefficients based on the simulated signal frame, and determining a linear predictive coding residual signal based on the linear predictive coding coefficients and one of the speech signal and the simulated signal. Then forming an encoded signal representing said speech signal, based on the linear predictive coding coefficients and the linear predictive coding residual signal.