Patent GB-836824-A (GB)
Title: Multiple frequency communication system
Inventor: DOELZ MELVIN L
IPC: H04L27/26
Filed: 1956-10-02
Abstract: 836,824. Printing telegraph systems; electric synchronous movements. COLLINS RADIO CO. Oct. 2, 1956, No. 30015/56. Class 40 (3). [Also in Group XL (c)] Relates to a communication system which allows intelligence to be received when there is a low signal to noise ratio, wherein each character is distinguished by the transmission of a pulse of a particular frequency. At the receiver the incoming pulses are weighted in accordance with their expected shape and since magnetostrictive mechanical resonators are used as detectors, the pulses are given further weighting to compensate for the exponential decay time of the resonators. The transmitter, Fig. 1, comprises a conventional teleprinter 12 feeding a group of gating circuits 13. The thirty-three output leads from the gating circuits are connected to respective gate valves 16, and when a particular valve 16 is conducting the output of a corresponding crystal oscillator 17 is supplied via an amplifier 18 and limiting amplifier 19 to a mixer 23. A crystal oscillator 21, oscillating at a frequency different from the thirty-three other crystals is fed to the limiting amplifier 19 at a lower voltage level than the output of valve 18, so that when the teleprinter is supplying a signal the output from the oscillator 21 is suppressed, but when the teleprinter is not operating the output from the oscillator 21 is passed through the limiting amplifier 19. An oscillator 24 provides a further input to the mixer 23 and the resultant radio frequency output is supplied to a power amplifier 25. In order to eliminate undesirable transients which would occur if the frequency changes were to occur at periods of high output power the power amplifier 25 is controlled by the modulator 26 to provide 100% amplitude modulation of the transmitted signal at, for example, 55 c.p.s., synchronization at this frequency being effected by a crystal oscillator 27. Thus the transmitted pulses are arranged to have a " cosine squared " characteristic. At the receiver, Figs. 2 and 3, the incoming signal is applied to a mixer 32 together with an input from a local oscillator 33. One of the thirtythree frequencies available from the transmitter is used as a reference pulse for ensuring that each received character will be at its correct frequency, and is transmitted periodically and selected by a filter 42, mixed at 43 with the output of a crystal oscillator 44, and via a discriminator 46 utilized to control the frequency of the local oscillator 33. A crystal oscillator 34 provides a synchronizing signal at a frequency of 55 c.p.s. and to ensure synchronism with the transmitter the output of the mixer 32 is fed via a diode detector 36 to a phase detector 37 which also receives an input from the oscillator 34. The phase detector 37 produces an output proportional to the cosine of the phase angle between the oscillations from the oscillator 34 and the modulation envelope of the incoming signal, which is applied via a low-pass filter RC, servo amplifier 38, and motor 39 to control the phase of the oscillator 34. The output of the oscillator 34 is supplied via an amplifier 52 to a synchronous motor 51 which through a shaft 53 drives a brush 54 which engages contacts 56,57 as it rotates. A diode 58 rectifies the output from the filter 42 due to the reference freqeuncy pulse, the output from the diode being supplied via the brush 54 to a servo amplifier 59 which produces an output proportional to the difference in energy supplied to the contacts 56, 57. A motor 61 is controlled by the output of the amplifier 59 and has a shaft which is geared to the mounting of the motor 51 to adjust the phase of the shaft 53. The cosine wave output from the oscillator 34 is applied to the modulator 62 to modulate the incoming signal. This is equivalent to modulating the expected incoming series of " cosine squared " pulses by another similar series of pulses in synchronism with the incoming wave. In this way the incoming signal is weighted according to its expected shape. The output of the modulator 62 is applied to a further modulator 63 where it is modulated by an exponential waveform derived from the oscillator 34 via a wave-shaper 64, thus weighting the incoming signal in accordance with the exponential decay time of the mechanical resonators. The output of the modulator 63 is applied via a terminal 66 to thirty-three amplifiers 67, each provided with a filter tuned to a corresponding frequency and followed by a mixer 69 co-operating with a crystal 71. The outputs from the mixers 69 excite mechanical resonators 72, each resonator comprising metal rods 74 movable in coils 77 and provided with biasing magnets 78. The coils 77 are energized via a commutator 79 co-operating with a brush 81 which is connected to its corresponding mixer 69 and is driven by the shaft 53. A second brush 82 is driven by the shaft 53 and lags the brush 81 by an angle approximately equal to the angle between adjoining commutator segments. The brushes 82 are connected via diode clippers 83, and band-pass filter 84, to thirty-three filters 86, corresponding to the thirty-two characters, the thirty-third corresponding to the " off " frequency. Each filter 86 is connected to a relay 87 actuating a printer.