GB1185095A - Limited Energy Speech Transmission and Recieving System - Google Patents

Abstract

1,185,095. Radio signalling; secret transmission. GENERAL ELECTRIC CO. 5 July, 1967 [6 July, 1966], No. 31070/67. Headings H4L and H4R. In a system for transmitting speech, the audio-frequency signals are converted into a substantially rectangular waveform, the periodicity of which is determined by the zero crossing positions of the audio-frequency oscillations and a radio-frequency carrier is modulated with the waveform. The receiver employs a zero crossing detector whereby intelligible speech signals may be reproduced although the amplitude component is missing. Means are also described whereby the amplitude component is transmitted in addition to the zero crossing pulses. Transmitter, Fig. 3.-The speech signal at 26 is limited, clipped and squared at 27, 28, 29, Fig. 5 and supplied to a zero crossing detector 31, e.g. a differentiating circuit. The resulting zero crossing pulses are supplied to a repetition rate limiter 32 which integrates together pulses which are too closely spaced to be distinguished in the output signal. The output of repetition rate limiter 32 is supplied via a pulse shaper 33, producing pulses of fixed amplitude and duration to modulate a R.F. carrier at 21 and the resulting signal is transmitted. Receiver, Fig. 4.-The output of a conventional receiver 54, 55, 56, consisting of zero crossing pulses is supplied via a squaring circuit 57 to a bi-stable 58 which converts the pulses to a square wave shape signal of constant amplitude but varying periodicity, this signal being supplied via an audio amplifier 59 to a loudspeaker 60. A sinusoidal waveshaping circuit 84 may be included. Modifications.-The transmitter, Fig. 11 (not shown) also includes an audio envelope detector whose output is combined with that of the zero crossing detector so that the zero crossing pulses are amplitude modulated. At the receiver, Fig. 12 (not shown) a speech signal without the amplitude component is recovered as before and this signal is combined in a modulator with the demodulated audio envelope signal. In a further arrangement, Fig. 15 (not shown) the lower speech frequencies are transmitted by conventional modulating means and a separate transmitter, and the higher speech frequencies are transmitted by the use of zero crossing pulses. In a modification of this system, Fig. 16 (not shown) the separate transmitter is dispensed with and the low frequency signals width-modulate the zero crossing pulses. An arrangement is described for transmitting two channels in time division multiplex, Figs. 7, 8 (not shown).