Patent GB-552117-A (GB)
Title: Frequency modulation facsimile transmission
IPC: H04N1/36
Filed: 1941-04-04
Abstract: 552,117. Copying telegraphy; television. FINCH, W. G. H. April 4, 1941, No. 4509. Convention date, April 5, 1940. [Classes 40 (iii) and 40 (v)] In a picture telegraph system, the carrier current is frequency-modulated by the picture signals, and is modulated in amplitude by the synchronizing signals. Transmitter.-Picture signals from the facsimile scanner 10, Fig. 3, are fed through a correction network 11, 12, 13 (which varies the signal voltage inversely with its frequency in order to produce a change in carrier frequency which is constant over all ranges of the signal frequency) and amplifier 16 to the anode circuits of a balanced modulator 18, 19, the grids of which are connected in phase to a crystal oscillator 21 generating a low-frequency carrier, which is subsequently multiplied at 34 to radio frequency. The anode circuits of valves 18, 19 are connected in opposition to the grid circuit of valve 26, so that when there are no picture signals, the carrier frequency is suppressed in the output of valve 26. Picture signals vary the anode voltages of valves 18, 19 in opposite directions, so that the output of valve 26 is an amplitude-modulated wave from which the carrier has been excluded. The output circuit of each of the valves 18, 19 is made non- reactive by adjusting condensers 24 in series with coils 22; the impedance of the circuits is therefore purely resistive and current in the anode circuit is in phase with the voltage applied to the grids. The voltage in the secondary 25 is therefore 90 degrees out of phase with the voltage of oscillator 21. These phase-displaced side-bands are amplified by valve 26 and applied to a resistance 27 in the anode circuit of a valve 29 fed from the oscillator 21. The voltage on resistance 27 will be the resultant of the constant amplitude carrier from 21, and the side-bands from the balanced modulator, the phase displacement between the two waves being 90 degrees. The resulting frequency-modulated wave is amplified at 31, and passed through a limiter 44, bandpass filter 45, and frequency-multiplier 34 to a power valve 35 and antenna 43. Synchronizing signals derived from the scanner 10 are applied over transformer 38 to the anode circuit of valve 35 and thus modulate the carrier in amplitude independently of the frequency modulation by the picture signals. The synchronizing signal may itself be a direct-current pulse, or a current of a constant frequency which is a multiple of the scanning frequency, and can be used to continuously modulate the amplitude of the carrier. Receiver.-The received waves reduced to intermediate frequency at 49, Fig. 4, pass to the signal and synchronizing channels respectively. The frequency-modulated constant-amplitude picture signals pass through a limiter 60, bandpass filter 61, detector 62, and amplifier 63 to the recorder 58. The variable amplitude synchronizing signals pass through a tuned rectifier 50, 51, 52 to a biased valve 55, which passes only the synchronizing signals, and thence to an amplifier and synchronizing magnet 57. Alternative synchronizing systems.-Instead of using synchronizing pulses, alternating current which is used to drive the transmitting scanner may be used to modulate in amplitude the carrier and be transmitted simultaneously with the picture signals. At the receiver, this alternating current is separated and used to drive or control the receiver, e.g. by maintaining a tuning fork at synchronous speed. The Specification as open to inspection under Sect. 91, also describes (1) the application of the invention to television, and (2) a system of picture telegraphy using frequency modulation for both picture and synchronizing signals. In this case, a synchronizing signal is transmitted by the pulsegenerator through the correcting-network 12, 13, 14 to the balanced modulator &c. at the end of each scanning cycle and at the receiver the signal is " switched " to the synchronizing magnet in known manner. This subject-matter does not appear in the Specification as accepted.