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Abstract

An improved vocoder system and method for estimating pitch in a speech waveform which pre-filters speech data with improved efficiency and reduced computational requirements. The vocoder system is preferably a low bit rate speech coder which analyzes a plurality of frames of speech data in parallel. Once the LPC filter coefficients and the pitch for a first frame have been calculated, the vocoder then looks ahead to the next frame to estimate the pitch, i.e., to estimate the pitch of the next frame. In the preferred embodiment of the invention, the vocoder filters speech data in a second frame using a plurality of the coefficients from a first frame as a multi pole analysis filter. These coefficients are used as a 'crude' two pole analysis filter. The vocoder preferably includes a first processor which performs coefficient calculations for the second frame, and a second processor which performs pre-filtering and pitch estimation, wherein the second processor operates substantially simultaneously with the first processor. Thus, the vocoder system uses LPC coefficients for a first frame as a 'crude' multi pole analysis filter for a subsequent frame of data, thereby performing pre-filtering on a frame without requiring previous coefficient calculations for that frame. This allows pre-filtered pitch estimation and LPC coefficient calculations to be performed in parallel. This provides a more efficient pitch estimation, thus enhancing vocoder performance.

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