Patent GB-699905-A (GB)
Title: Improvements in or relating to television apparatus
IPC: H04N5/228
Filed: 1949-10-27
Abstract: 699,905. Television. THEILE, R., and PYE, Ltd. Oct. 27, 1950 [Oct. 27, 1949], No. 27684/49. Class 40 (iii). [Also in Group XL (c)] A video storage tube employed in the generation of television signals is pulse charged by the combined effects of an electron irradiation and a negative field so as to drive the storage surface negative as described in Specification 687,270, [Group XL (a)], and the scanning time in each frame period is reduced in order to allow an adequate interval for the surface to build up the picture charge before scanning recommences. Thus, as shown in Fig. 1, each frame period is divided into two equal intervals of <SP>1</SP>/ 100 th second duration during the first of which scanning is suppressed, the tube pulse charged negative as indicated at a and then allowed to acquire the picture charge as indicated at b, and during the second of which scanning takes place to derive signals as indicated at c. The negative charging period a occupies 10-20 per cent of the first interval. The division of the frame period as above permits signals from two tubes to be transmitted alternately from one transmitter, the generation of video signals in one tube taking place during the negative charging and storing intervals of the other tube. Fig. 5 shows the details of a transmitter for such a system. The pictures transmitted may be those of alternative programmes, or the components of a stereoscopic of two-colour picture. By dividing the frame period into a greater number of equal intervals, e.g. three, it becomes possible to transmit three programmes or three colour components of one picture. In Fig. 5, a master oscillator 3 operates at 82500 c/s., frequency division of 825: 1 providing the 100 c/s. frame time base and frequency division of 4: 1 providing a 20625 c/s. line time base. These frequencies result in a fourfold interlace which is necessary since in any one tube alternate frame scans are blanked. The pulse waveform generators 4, 5 operate at half frame frequency (i.e. 50 c/s.) and generate the light and voltage pulses to effect the negative charging. Blanking pulses are also generated to suppress alternate frame scans. A radio frequency oscillator 25 provides a short marker pulse of oscillation just before the video signal of one picture. The pulse is transmitted within the frame blanking period after the frame synchronizing pulse. In one receiver, Fig. 7, arranged to select either signal for application to cathode-ray tube c, the video amplifier 17 is blanked during reception of the unwanted signal by a gate pulse generator 13 synchronized at frame frequency. The generator may comprise a conventional symmetrical multi-vibrator, Fig. 11 (not shown), providing opositely-phased outputs selectable by a switch 16. To ensure the generator always starts operation in the correct phase, the marker pulse is selected by a gated detector 26 operated by another gate pulse generator 29 fed from the frame synchronizing pulse separator 15. In a modification of this receiver, Fig. 8 (not shown), the blanking of the unwanted signal is effected directly on the gun 18 of the cathode-ray tube. Fig. 9 shows a similar receiver in which both video signals are utilized and distributed to separate tubes C1, C2. The video amplifier 20 feeds both tubes whilst the unwanted signals are blanked out at the tube guns 21, 22 by the oppositely-phased outputs from the gate pulse generator 13. Fig. 10 (not shown) illustrates a modification employing blanking in the video stage. In the case of stereoscopic or colour television, all the received signals may be reproduced on one cathode-ray tube.