Pitch-tracked CELP speech coding with transparent DTMF signaling

A diagnostic analysis was conducted on code-excited linear prediction (CELP)-coded speech at 4.8 kbit/s for mobile radio communications. The occasional roughness in CELP-coded speech was traced to pitch contour infidelity, as well as the discontinuity of speech spectra. The use of a first-order integer-valued pitch filter also degraded the coded speech. A pitch-tracked CELP (PT-CELP) coding scheme was developed. Frame-to-frame variation of speech spectra was controlled in a smooth manner. An open-loop multiframe pitch tracker was employed, followed by closed-loop, third-order pitch filter analysis. PT-CELP demonstrated higher quality speech for enhanced mobile radio services. A simple algorithm was also developed for transparent dual-tone multifrequency (DFMT) signaling through the vocoder.<
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