Patent GB-384207-A (GB)
Title: Electrical television apparatus
IPC: H04N7/00
Filed: 1932-06-10
Abstract: 384,207. Television ; valve circuits ; wireless transmitting systems. COMPAGNIE GENERALE DE TELEVISION (PROC•D•S H. DE FRANCE), 22 bis, Rue Jean Mace, Le Havre, Seine, France. June 10, 1932, No. 16418. Convention date, June 13, 1931. [Classes 40 (iii) and 40 (v).] With the object of reducing the sideband width in the transmission of television signals, a definite frequency equal to the number of elementary areas in the picture is imposed by interrupting the scanning or 'transmission of each elementary area, whereby, instead of a continuous gradually fluctuating curve being transmitted for each line, a definite signal is transmitted for each elementary area of the line, the damping in the transmission circuit being sufficient to reduce the signal intensity to zero or to a small constant value during the period of the interruption. In the method illustrated in Figs. 4 and 5, in which the interruption of the modulation is effected during the scanning, the object 1 is illuminated by a light beam derived from source 2 and rendered parallel between lenses 3, 3<1>, and two rotating discs 4, 5 are disposed between the lenses. The disc 5 has a number of adjoining arc-shaped slots 8 arranged in spiral formation, while the disc 4 has similar slots 9, 10 spaced apart by solid sectors 11, the disc 4 making one revolution while the disc 5 moves through an arc corresponding to the length of one slot 8. Thus, as the slots 8 of the disc 5 illuminate successive vertical lines 8<1> on the picture 1, the disc 4 interrupts the illumination once during the scanning of each elementary area. According to alternative methods, the interrupted illumination of the picture may be produced by a lamp supplied with alternating current at the correct frequency, or by a rotary slotted disc. Preferably, however, the interruption of the signals is effected in the transmission circuit, the scanning being accomplished by a Nipkow disc or Weiller wheel. In the circuit shown in Fig. 6, currents derived from the photo-cell 16 and amplified at 17 are combined at -anode of valve 18 with a carrier frequency generated by oscillator 19, the modulated signal being passed to the grid of a valve 20 that is also subjected to impulses produced by a local oscillator 21, the frequency of which is equal to the number of elementary areas in the picture, a signal periodically interrupted at the desired frequency being thus obtained. In a modification, Fig. 7, the photocell currents amplified at 24 are combined at 25 with an alternating current of the desired frequency generated by oscillator 26, and the modulated signal after amplification at 27 is interrupted by valve 28 with strongly biased grid. At the receiver the signals received on aerial 30 pass to a two-grid valve 31 which functions as a frequency changer, thence by amplifier 32, detector 33, and low-frequency amplifier 34 to luminous a gas lamp co-operating with Nipkow disc 37. Synchronism is maintained by connecting in parallel with the lamp 35 an electromagnet 40, Fig. 8, which maintains in phase a two-pole magnet 39 mounted on the shaft of the driving motor 38. In order that the transmitting and receiving circuits may have small time-constants, an essential of the requisite rapid damping, and at the same time sufficient selectivity, coupled circuits are employed between the valves in that part of the transmitter following the scanning operation and in that part of the receiver preceding the detector valve. One form of such circuit is shown in Fig. 9, in which the anode p of one valve and the grid q of the succeeding valve are connected by an oscillating circuit C, L with damping resistance R coupled to a second similar circuit C<1>, L<1>, R<1>, which in turn is coupled to a third circuit C<11>, L<11>, R<11>. When the circuits are to be employed both for radio-telephony or telegraphy and also for television purposes, the damping resistances may be cut out for the former purpose by switches controlled either by hand or automatically by the emission of a special signal.