Patent GB-450675-A (GB)
Title: Improvements in or relating to television and the like transmitting systems
IPC: H04N5/16
Filed: 1934-11-19
Abstract: 450,675. Television transmitters. BROWNE, C. O., 29, Monk's Drive, West Acton, London, and HARDWICK, J., 21, Drayton Park Avenue, West Drayton, BLYTHEN, F., Heathcote, Kerth Road, Hayes, and WHITE, E. L. C., 32, The Rise, Hillingdon, all in Middlesex. Nov. 19, 1934, Nos. 33295/34, 10127/35, and 10393/35. [Class 40 (iii)] According to one aspect of the invention, spurious signals, produced by the switching on and off of the scanning cathode ray or during the return scanning stroke, are removed and replaced by signals fixedly related to picture black, the direct and low-frequency components of the picture signal being present at the point of removal. According to another aspect of the invention, the picture signal amplifier has its amplification reduced during the return scanning stroke, the direct and low-frequency components being present. The original signal comprises picture impulses AB, Fig. 1 (33295 /34), an impulse BC produced by a black edge to the picture being scanned, and spurious signals CD occurring during the return scanning stroke. Impulses VIJKLW, Fig. 3, are generated, e.g. by an apertured disc, and divided by suitably biassed limiting valves into impulses VILW, Fig. 4, and IJKL, Fig. 5. The impulses VILW, Fig. 4, are used to decrease the amplification of the signal amplifier and are also superposed on the signal of Fig. 1 (33295/34), giving the signal of Fig. 6, the direct and low-frequency components being reinserted relatively to the black signal BC, either immediately after or at the same time as the superposition. Preferably, both sets of impulses are fed to the grid of a valve whose grid circuit has a suitable series condenser and shunt leak, the signals NO being ineffective as the impulses of Fig. 4 serve to keep the grid highly negative during the period MP. The output is fed to a limiting valve producing the signal train of Fig. 7, which are then mixed with impulses of the kind shown in Fig. 4, but of opposite sense to produce the signal train of Fig. 8, and finally the synchronizing signals of Fig. 2 are superposed giving the final signal train of Fig. 9. The impulses of Fig. 5 are used to cut off the cathode ray beam. The valve mixing the trains of Figs. 4, 6 may be biassed to act as a limiter and produce the train of Fig. 7. The signal BC, Fig. 1 (33295/34), may be " whiter than white." The impulses of Fig. 2 may be produced by apertures in the apertured disc producing the train of Fig. 3. The direct and low-frequency components may be inserted with respect to black peaks in the picture signal without providing a special signal. If the duration of the impulses used to drive the spurious signals " whiter than white " differs from the duration of the impulses used to reduce MRSP, Fig. 7, to MP, the difference is made to result in a " blacker than black " peak which is removed by a limiting valve. These two impulses may be derived by dividing a triangular impulse into two by suitably biassed limiting valves, or the latter may be produced from the former by applying them directly and through a delay network to the grids of a hexode valve so that the latter are of longer duration than the former. Their centre lines may be made to coincide bv use of a delay network. The signal train of Fig. 6 may be fed to a grid of a hexode and the impulses VILW, Fig. 4 fed to the other grid to produce the signal train of Fig. 8. In a modification, the signal train of Fig. 2a is fed to a valve V1, Fig. 1 (10393/35), whose output includes a reinserting device V2, C1, R1 in the grid circuit of valve V3. To prevent the spurious impulses S, Fig. 2a, affecting the reinsertion, impulses, Fig. 2b, are applied to the grid of valve V2, so that the signal at the grid of valve V3 is as shown in Fig. 2e. The anode of valve V3 is connected to the anode of valve V4 which is fed with impulses as shown in Fig. 2d so that the grid of valve V5 receives the signal train of Fig. 2e, in which the spurious signals are " blacker than black." The valve V5 acts as a limiter to produce the signal train of Fig. 2f and valve V7 serves to introduce the synchronizing signals of Fig. 2g to produce the final signal train of Fig. 2h. The impulses of Figs. 2b. 2d may be obtained from a single source. Specifications 422,658 and 422,906 are referred to.