GB654271A - Improvements in or relating to telegraph systems - Google Patents

Abstract

654,271. Multiplex radio signalling. WESTERN ELECTRIC CO., Inc. Feb. 20, 1948, No. 5096. Convention date, June 30, 1943. [Class 40 (v)] [Also in Group XL (b)] In a telegraph signalling system each mark is transmitted as a plurality of frequencies and each space as a plurality of different frequencies and at the receiver a limiter discriminates in favour of the instantaneously stronger current, the output of the limiter being applied to two detectors, one of which detects spacing currents and the other marking currents. Transmission. A master oscillator Oo whose frequency is 85 c.p.s., produces at devices 011 ... 023, called for convenience oscillators, odd harmonics between 425 c.p.s. and 2465 c.p.s. Six channels are provided and in channel 1, for example, oscillators 011 and 012 are keyed by a teleprinter or other suitable transmitting system so that a marking signal causes ocillator 011 to send a 425 c.p.s. current to a common output CB1 while a space sends 595 c.p.s. current. These two frequencies are sent through a low-pass filter F31 and amplifier A2 to a radio transmitter RT where they modulate a carrier. Both mark and space frequencies are also mixed with 5270 c.p.s. current from an oscillator 030 to produce high-frequency currents of 4845 and 4675 c.p.s., respectively, which also modulate the radiofrequency carrier wave via a high-pass filter F32. Each mark and space is thus transmitted as two simultaneous frequencies. Reception. The received carrier signals are demodulated and separated into low or high frequencies by two filters F39, F40. The low frequencies, corresponding to those generated by 011...023 are filtered into separate channels and the marking and spacing frequency for, say, channel 1 is fed to a hybrid network HN. The higher frequencies are again demodulated at DM and reduced to the original frequencies plus 340 c.p.s. (see Specification 654,253), and are also applied to the hybrid networks HN. The hybrid output for channel 1 is amplified and passed via a limiter CL to a pair of mark filters F81, F91 and a pair of space filters F82, F92 designed to accept the four frequencies representing the channel. The mark frequencies are detected at D1 and the space frequencies at D2 to suitably operate a telegraph relay R3 and teleprinter TTY. The function of the limiter CL is to suppress the weaker mark (or space) frequency and noise and to amplify the stronger signal. The effect of fading is this largely eliminated. The system shown comprises also an order-wire channel operating on a tone-to-tone bases to energize a loud-speaker LS.