GB1334177A - Vocoder excitation system - Google Patents

Abstract

1334177 Vocoder systems STANDARD TELEPHONES & CABLES Ltd 10 June 1971 19957/71 Heading H4R In a channel vocoder analysis synthesis telephone system having analysis channels of different widths the excitation function signal is derived by summing in parallel the outputs of some or all of the analyser channel rectifiers and applying the resulting signal direct, and via a smoothing circuit 31, Fig. 3, to the two inputs of a differential amplifier 30. The differential amplifier output is squared, at 32, and drives a monostable, 40, Fig. 5, feeding a low pass filter 41 the output of which is a DC signal dependent on the input speech pitch frequency. As described, at the vocoder analyser, the level of the signal in a group of analysis channels for a higher frequency region of speech is compared with that in a group of channels for a lower frequency region in order to provide a voiced/ unvoiced switching signal. At the synthesizer the voiced/unvoiced signal controls a gate 50, Fig. 6, through which the D.C. excitation frequency signal is applied to a multivibrator 43, and also controls a switching circuit 55, 56 to switch either the output of the multivibrator or the output of a noise generator 51 to the input of an excitation pulse generator, Fig. 8, via a squaring circuit 60, Fig. 6. At the excitation pulse generator, Fig. 8, positive going edges of the input signal trigger monostable 81, the timing capacitor of which has its charge and discharge paths separated by diode 84 so that the charge on the capacitor is dependent on the elapsed time since the previous pulse, which tends to equalize the power of the pulse train irrespective of its frequency. The system can be used as a helium speech processor.