Patent GB-550376-A (GB)
Title: Improvements in and relating to the transmission of picture
IPC: H04N3/32
Filed: 1941-11-08
Abstract: 550,376. Facsimile picture transmission television; relaxation oscillators. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 31, 1941, No. 14048. Convention date,'Nov. 1, 1940. [Classes 40 (iii) and 40 (v)] Relates to facsimile and picture transmission and television in which a line-by-line scanning operation is performed to illuminate a photoelectric cell, and in which the instantaneous speed of scanning is a function of the depth of tone of the point being scanned. According to the invention, the illumination of the cell by the scanning operation is arranged to produce an alternating current the frequency of which varies between predetermined limits, the instantaneous rate of change of the frequency is a function of the instantaneous illumination, and the alternating current is rectified to control the scanning operation in such a way that in the scanning of any line the position of a point is a function of the instantaneous frequency whilst the rate of change of frequency is a function of the depth of tone of the point. In the transmitter, Fig. 1, an image 1 is scanned by the light spot produced a cathode ray 2, and the light from the image is directed to a photo-electric cell 10 by an optical device 9. The photo-electric cell 10 is in the circuit of a valve 12 adapted to produce an alternating current gradually varying in frequency between two limits as follows. When contacts 16, 17 close, a condenser 11 is charged through resist- 'ance 14; the anode current of valve 12 through coil 15 is then strong enough to open the contacts 16. 17, and a condenser 21 is gradually charged until discharged by the break down of a neontube 19 after which condenser 21 again charges un thereby producing an alternating current the frequency of which depends on the charging rate of the condenser 21. This rate depends on the mutual conductance of the valve 12 and therefore on the voltage on the condenser 11, which initially is fully charged through the contacts 16, 17 and thereafter gradually discharged at a rate depending at any instant on the illumination of the cell 10. When the frequency has fallen to a given low limit, the current in coil 15 is insufficient to hold contacts 16, 17 open, these contacts close again, the frequency is restored to its higher limit and the operation repeated. The alternating current is transmitted through a transformer 20, a limiting device 23, a transformer 24, a high-pass filter 25 passing all the frequencies between' the two limits, an amplifier 27, and a transformer 28 to a full-wave rectifier 29, the output of which controls the line deflecting coils 32. 33 of the cathode-ray tube 2. The secondary of the transformer 24 is also: connected through a low-pass filter 26 passing only the lower limit frequency, a transformer 34, an amplifier, 35, and a transformer 36 to a full-wave rectifier 37. A tapping on the output resistance 38 of this rectifier is connected to a coil 40 controlling contacts 41 connected through an adjustable resistance 42 to a condenser 43 and the grid of a valve 44. The anode circuit of this valve includes the frame scanning coils 45, 46 of the cathode-ray tube 2 and a coil 47 controlling contacts. 48 in a charging circuit for the condenser 43 including a resistance 49. The contacts 48 are open while a frame is being scanned. The line scanning circuit is such that when the frequency of the alternating current is at its higher limit, the scanning spot is at the beginning of line, and when the frequency is at the lower limit the scanning, spot is at the end of the line, each spot of the line corresponding to a given intermediate instantaneous frequency. At the end of the scanning of a line a pulse of the lowest frequency is passed by the filter .26 causing a temporary closure of contacts 41 and thereby a small discharge of the condenser 43 which reduces the current in the frame scanning coils to shift the scanning spot to the next line. When all the lines have been scanned, the condenser 43 is discharged to such an extent that the anode current of valve 44 through coil 47 is unable to hold contacts 48 open. These contacts close, condenser 43 is again fully charged, and the scanning spot returned to the first line in readiness to scan the next frame. The alternating current from the secondary of the transformer 20 is also fed to a receiver by .means of a radio-transmitter 51 or by a cable link. At the receiver, the alternating frequency is passed by similar devices to similar full-wave rectifiers controlling a similar cathode-ray apparatus, the scanning spot of which produces an impression on a photographic plate. For television, the several contacts and their operating coils are replaced by quicker acting ionic or electronic switching devices. If the image 1 is negative, the photo-electric circuit is modified as shown in Fig. 3.