Supplementing the Formoder with Fixed Vocoder-Like Bands

Abstract

The Formoder is a speech compression system using parametric signals. The most important parameters are those used to control the average pitch frequency; the central frequencies and intensities of (1) two damped oscillations for nonturbulent sounds tunable in the first (F1) and second (F2) formant region, and (2) two noise spectra for turbulent sounds, one tunable in the second formant region (C2), and the other in the higher frequency region (C3) from 2500 to 6000 cps. Assuming time sharing of channels for parameters of (1) and (2) there are five parametric channels. To improve the performance of the Formoder a scheme is proposed in which the original Formoder parameters describing the average pitch, F1, F2, and C2 are retained while vocoderlike fixed bands (one parameter per band} are used in the upper frequency range 2500–6000 cps. Four such bands are used for turbulent sounds to replace C3 and two bands are added for nonturbulent sounds with central frequencies set at 2500 and 3500 cps. The total number of parametric channels is thus increased from five to seven. Preliminary experiments show some promise and the detailed results will be reported. [This research is sponsored by the Air Force Cambridge Research Center under Contract No. AF19 (604)-3465.]