Patent GB-537689-A (GB)
Title: Improvements in wave signalling systems, particularly applicable to facsimile telegraphy
IPC: H04N1/00
Filed: 1939-12-27
Abstract: 537,689. Wireless signalling; valve amplifying circuits ; copying telegraphy. PRESS WIRELESS, Inc. Dec. 27, 1939, No. 32890. Convention date, Oct. 27. [Classes 40 (iii) and 40 (v)] [Also in Group XXXIX] In a system in which signals of varying amplitude (e.g. picture or speech signals) are converted into signals of variable frequency below the radiofrequency range, the frequency band of the variable frequency signals is less than one octave, the limitation of frequency being effected at the transmitter or receiver or both. The limitation may be effected by filter circuits or by adjusting the frequency modulator so that its frequency range corresponding to the upper and lower limits of the amplitude signals is less than an octave. Transmitter, Figs. 1 and 2. Picture or other amplitude-modulated signals on a sub-carrier received from T pass through amplifier 8, double-triode valve 28 providing a balanced input to the push-pull amplifier 41, rectifier 44 and filter 45 which remove the sub-carrier, to the valve 50. Frequency modulator. This comprises two valve oscillators 65, 86, one 86 generating a fixed frequency of, say, 600,000 whilst the frequency of the other 65 may be varied from say, 601,800 to 603,000 by the amplitude-modulated signals, which vary the impedance of valve 50 connected across the tuned circuit 59, 60, 61 determining the frequency generated by valve 65. Each valve 65, 86 comprises two triodes; the anode of each being connected by a condenser to the grid of the other. The outputs of each valve 65, 86 are applied to the grids of a heptode mixer 90, the output frequency of which varies from 1800 to 3000 and is applied through amplifier 103 and low-pass filter 107 (which passes only frequencies of this range) to a radio-transmitter RT. In a modified system Fig. 6 (not shown) the amplitude-modulated signals vary the frequency of each of the oscillators 65, 86, increasing one when the other is decreased, so that for a given amplitude variation, the frequency shift is doubled. Monitoring and adjustment of modulator. To facilitate the adjustment of the valves 65, 86 so that they produce a frequency of 1800 corresponding to one limit of the amplitude-modulated signals, e.g. black, part of the output of amplifier 103 is taken by lead H to the lower grid of a double triode 20, the upper grid of which is connected to the output of amplifier 8 handling the original amplitude-modulated signals, the sub-carrier frequency of which may be 1800 to facilitate comparison with the signals on lead H. The anodes of the double triode are connected to the contacts of a switch 23 by which either can be connected through jack 24 to an operator's telephone. Receiver, Figs. 3, 4. The received carrier is demodulated at RR and the frequency-modulated signals pass though a filter 206, passing from, 1500-3300 cycles, to a double-triode amplifier 217, from which part of the signals passes to a monitoring meter 226, facilitating the adjustment of the average level of the signal, and the other part passes through pentode amplifiers, 236, 251 to another double-triode amplifier 264. The left-hand triode applies signals to a rectifier 252 from which A.V.C. potentials are applied to the grids of the pentodes 236, 251 thereby maintaining the frequency-modulated signals at a constant amplitude. The right-hand triode 264 applies the signals via a balanced circuit to a pushpull limiting-amplifier 271, having a resistance 270 and zero bias in its grid circuits and a resistance 273 in its anode circuits. This limiting amplifier 271 amplifies signals up to a predetermined value and then, as the signal increases, the output remains constant over a wide range, thus supplementing the operation of the A.V.C. which may not respond to sudden or very large changes in volume. The output of amplifier 271 is applied through amplifier 276 to a series resonant circuit 281, 282, 283, tuned to a frequency below 1800, whereby the frequency variations are converted into amplitude variations. The impedance of the combination 282, 283 should be low compared to that of resistance 281 at the highest frequency passed. Modifications of the resonant circuit are described, Figs. 4a and 4b (not shown). The converted output is applied through a double-triode amplifier 287 in balanced relation to a push-pull amplifier 301, the output from which is rectified at 224 and passed through a filter 304 to the recorder. If however, the recorder responds to a modulated audiofrequency of say 1800, the output from the filter 304, and an audio-frequency of 1800 generated by oscillator 307, are applied to a pushpull modulator 318 the output from which passes through.an amplifier 322 and filter 326 to the recorder R. Specification 537,699 is referred to.