GB1396704A - Noise suppression system for hf ssb radio telephones - Google Patents

Abstract

1396704 Radio signalling HATFIELD INSTRUMENTS Ltd 22 Sept 1972 [23 Sept 1971] 44369/71 Heading H4L In a suppressed carrier communication system means are provided for transmitting a tone signal of predetermined duration at the start of the intelligence transmission and thereafter further interrupted tone signals of short duration at regular intervals during the continuation of the intelligence transmission. This enables the receiver in the system to be muted in the absence of a transmitted signal. Transmitter.-When a press-to-talk switch is operated, signals from microphone 10, Fig. 1, are supplied via contact SIA and adding circuit 11 to a single sideband modulator 12. fed by oscillator 13. Closure of contacts SIB, via an integrator-differentiator 14 (which eliminates switch contact noise) starts a monostable 15 which controls a tone generator 16 whereby a pulse of tone of 0À5 secs. duration is fed to the adding circuit 11 for transmission. Switch SIB also starts a multivibrator 17 whereby generator 16 produces short pulses of tone of about 120 m.secs. duration which are transmitted via adder 11. Receiver.-Receiving and demodulating means 20, Fig. 2, are in continuous operation and the received audio signal is supplied via muting means 22 to the transducer 23. The signal from 20 is also limited at 24, the tone pulses being selected at 25, rectified at 26, integrated at 27 and supplied to a circuit 28 which when a predetermined threshold is exceeded, operates a monostable 29 which triggers a further monostable 30 with a period of 7À5 sees. Operation of monostable 30 unmutes the circuit 22 and the monostable 30 will remain operated for 7À5 sees. if no further short tone bursts are received, but will be re-triggered each time a burst is received. Selective calling.-Tones of differing frequency may be transmitted, each receiver responding only to one of the tones.