Patent GB-605681-A (GB)
Title: Improvements relating to railway train communication systems
IPC: H04M7/16
Filed: 1946-01-03
Abstract: 605,681. Radio signalling. WESTING- HOUSE BRAKE & SIGNAL CO., Ltd. Jan. 3, 1946, No. 254. Convention date, Jan. 30, 1945. [Class 40 (v)] In a railway communication system having a plurality of channels, the transmitter at each operating point can be conditioned to provide frequency modulated carriers of different frequencies and receiving equipment is provided responsive to all the channels and normally conditioned to receive calling signals on any of the channels. A two-channel system is described. The transmitter comprises a thermionic reactance valve V1 modulating an oscillator V2, followed by an amplifier V3 and output valves V4, V5 feeding a sending loop, the leads 138, 139 of which are connected to the wheels 47 and 48 of the vehicle on which the system is installed. The transmitter H.T. supply, from terminal B400 of a motor generator MG, is controlled by relay 25. A wave-change relay 50 is adapted, upon energization to add capacities to, or change the tappings on, the tuned circuits of the transmitter to tune them to the second channel frequency. The circuit of the microphone transformer T1 feeding modulating voltages to V1 includes a " preemphasis " network 67, 68. The microphone output is also fed to an amplifier comprising valves V6, V7, the output from which is fed over lead 13 to a glow tube 77 in the control unit CU. The operator is instructed to speak loudly enough into his microphone 63 to cause tube 77 to flash. The effect of background noise is thus reduced as the transmitter is fully modulated only by an input which energizes tube 77. For calling on the carrier frequency to which the transmitter is set, key K1 is depressed to energize relay 25 to switch on the transmitter, and to energize a relay 19 which connects valve V6 as an audio-frequency oscillator. The tuned circuit thus associated with V6 comprises the condenser 86 and the secondary of transformer T1, the primary of which is open-circuited by relay contact 70. Contact 91 is provided on the wave-change relay 50 to connect condenser 90 in parallel with condenser 86 to provide a different characteristic audio-frequency calling note when the second radio frequency is transmitted. An emergency calling key K2 is provided which energizes the H.T. relay 25, the audio oscillator relay 19 and also a self-pulsing slow-acting relay 76 which periodically energizes and deenergizes the wave-change relay 50, thus sending out calling tone on both radio-frequency channels. Normal control of the wave-change relay 50 is by the switch S2, the relay being un-energized in the position shown and energized in the other switch position. Lamps 110, 112 indicate which radio frequency is in use. A key 5 on the hand-set HS is depressed to energize the H.T. relay 25 when speaking.. Two receivers F1, F2 are provided, one for each channel, and fed from pick-up coils 92, 93 respectively, their outputs being fed to the switch S2. When the hand-set HS is on its hook 51, the outputs of both receivers are fed in parallel to valve V9, feeding a loud-speaker LS which reproduces calling tones. When the switch S1 is released by lifting the hand-set, switch S2 directs the appropriate receiver output to the hand-set earpiece 107 and the other receiver output to the loud-speaker LS. A contact 122 on the H.T. relay 25 connects resistance 119 or 123 across the R.F. amplifiers of receiver F1 or F2, according to the setting of relay 50, so that when the transmitter is in operation the receiver tuned to the same frequency delivers enough side-tone to the earpiece 107 to enable monitoring to take place.