PSEUDO-MULTI-TAP PITCH FILTERS IN A LOW BIT-RATE CELP SPEECH CODER

The pitch filter in a low bit-rate CELP speech coder has a strong impact on the quality of the reconstructed speech. In this paper we propose a pseudo-multi-tap pitch filter with fewer degrees of freedom than the number of prediction coefficients, but which gives a higher pitch prediction gain and a more appropriate frequency response than a conventional one-tap pitch filter. First, we present an analysis model for the pseudo-multi-tap pitch prediction filter. Then, we introduce a pseudo-multi-tap pitch prediction filter with a fractional pitch lag. The prediction gain of the pseudo-multi-tap pitch filter is compared to that of conventional one-tap and three-tap pitch filters with integer and non-integer pitch lags. A switching configuration is also studied. This filter switches modes depending on the prediction gain. The stability of a pseudo-multi-tap pitch synthesis filter in a CELP coder is considered. We proposed a stabilization method with a relaxed stability test. This relaxed test gives better results than a strict stability test. Finally, we have incorporated the pseudo-multi-tap pitch filter into a 4.8 kbit/s CELP speech coder. Both the objective SNR and subjective quality are better than for a conventional one-tap pitch filter.