GB1261552A - Formant vocoder utilizing resonator damping - Google Patents

Abstract

1,261,552. Formant vocoder. PHILCOFORD CORP. 23 Jan., 1969 [25 Jan., 1968], No. 3859/69. Heading H4R. In the speech synthesizer of a formant vocoder the formant resonators are heavily damped for a short time prior to the application of each excitation pulse. Fig. 1 shows a formant vocoder in which the analyser derives signals representing the frequency, F1 &c., and amplitude, A1 &c., of the first three formants of an incoming speech signal, together with a signal P, indicating the pitch frequency and a signal V, indicating a voiced or unvoiced condition. At the receiver synthesizer the pitch signal controls the frequency of pulses which are applied, during voiced sounds, via a gating circuit 10 to modulators 14, 16, and 18 where the pitch pulses are modulated by the respective formant amplitude signals, A1, A2, and A3, filtered by variable frequency resonators whose resonant frequency is adjusted by the respective formant frequency signals, F1, F2, F3, before the signals are further filtered and combined to form the synthesized speech. The variable frequency resonators are arranged to be heavily damped by a pulse derived from the excitation signal immediately before the excitation pulse is applied to excite the filter, so as to eliminate any effect on the output from the filter during one pulse, of the energy still remaining from the previous pulse. Fig. 3 shows a circuit for the synthesis of one formant comprising a resonator 20 including a differential amplifier 44 connected in a loop with variable frequency high pass, 46, and low pass, 48, filters, each of which is tunable by the signal F1 to provide a circuit resonant at F1 formant frequency. The damping circuit, 32, comprises a fet connected from one input of the differential amplifier 44 to earth, and which is switched on just before a pitch pulse is generated via differential amplifier 38 fed with a signal derived from the pitch pulse generator 8. The pitch pulse generator comprises a capacitor 35 charged via the emitter collector path of transistor 33, the impedance of which is controlled by the pitch frequency signal. Just before the charging potential reaches a value to cause a pitch pulse to be generated by pulse generator 41, differential amplifier 38 is switched from negative to positive saturation, as the charging potential reaches the reference potential set by potentiometer 36, causing fet 40 to be switched on and the damping condition to be established. Reference has been directed by the Comptroller to Specification 1,146,184.