Robust closed-loop pitch estimation for harmonic coders by time scale modification

Harmonic coders that synthesize speech without transmitting phase information abandon the benefits of closed-loop parameter estimation via waveform matching. In this paper, we show that effective closed loop parameter estimation can be achieved when a suitable time-scale modification is applied to the speech LP residual in harmonic coders. The concept is demonstrated here specifically for pitch estimation, but is more broadly applicable. For each of a set of pitch candidates generated by a time-domain pitch estimator, the residual is modified to match the pitch contour derived from that candidate. The best candidate is selected by evaluating for each candidate the match between the modified residual and the synthesized residual. The new pitch estimation algorithm significantly reduces gross pitch errors compared to a conventional time-domain pitch estimator and enhances the perceptual performance of a 4 kbps harmonic coder.