Patent GB-425220-A (GB)
Title: Improvements in and relating to television and the like transmission systems
IPC: H04N5/073
Filed: 1933-08-08
Abstract: 425,220. Television. PERCIVAL, W. S., 6, York Mansions, York Avenue, Hanwell, Middlesex, BROWNE, C. O., 29, Monk's Drive, West Acton, London, WHITE, E. L. C., 32, The Rise, Hillingdon, and ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, both in Middlesex. Aug. 8, 1933, No. 22191. [Class 40 (iii)] In television, synchronizing signals comprise a series of impulses having the initial portions of the wave forms steep and of the same shape, the initial portions recurring at the line frequency, while groups of one or more of the impulses are of longer duration than the remainder, these groups recurring at the image frequency. At the receiver, the synchronizing signals are separated as described in Specification 423,685, and all the impulses are used to control the line scanning while the groups of impulses control the image scanning. A film 1, Fig. 1, is drawn past a stationary slit 2, Fig. 2, through which light passes to a rotating mirror drum 6, each mirror of which traverses an image of the slit 2 over an aperture 9, Fig. 3, in front of a cell 10. At the end of each line, the drum 6 traverses an image of a slit 15, Fig. 2, over a second slit 16 in the screen 7 to illuminate a second cell 57 and so produce the line synchronizing frequency. A shutter 14, Fig. 2, having two slots 24, 25, rotates to disclose a portion 12 of the slit 15 to elongate the image traversed over the aperture 9 at the end of each complete scanning to produce the image synchronizing frequency. Preferably three such elongated impulses are produced at the end of each complete scanning. Arms 20, 21, cover the slit 2 during the movement from one frame to the next. The synchronizing impulses are transmitted as "blacker than black" so that during the return scanning strokes the screen is not illuminated. Additional " black-out " impulses may be produced by two notches 22, 23, Fig. 2, uncovering a slit 27 to illuminate either of the cells 10, 57 at the end of each picture. A shutter 31 is adapted to cover the slit 2, through which the film is illuminated, on the failure of the film driving mechanism, to prevent burning of the film by light source 3. This shutter does not cover slit 12, so that synchronizing impulses continue to be transmitted and the cathode ray at the receiver does not remain stationary to damage the fluorescent screen. The arm 32 of the shutter may cover the slit 12 leaving the portion 15 uncovered so that the line synchronizing signals only are transmitted ; arm 33 covers the "black-out" slit 27. When starting the transmitter, the lamp 13 illuminating the slit 12 is lit and relay 38, in series with it, is energized to actuate a friction clutch between the shutters 14, 3, so that rotation of the shutter 14 by the film driving mechanism rotates the shutter 31 against a spring control to stop 35 allowing light to pass through the slit 2 to the film. The slit 12 may be replaced by two separate slits for producing line and image synchronizing impulses. At the receiver, all the impulses control the line scanning generator 69 and a low pass filter 70 integrates the groups of longer impulses to control the image frequency generator 72. In a modification, the synchronizing signals are produced electrically. Line impulses a Fig. 5, and image impulses b, generated in known manner, e.g. by commutators, are mixed to produce impulses c, which are fed to a limiting valve to produce impulses d which are decapitated in a further limiting valve to give impulses e. Impulses a and e are then mixed (f) and applied to a third limiting valve to produce the final train g. If desired, the final train may contain " black-out signals as h by combining the outputs of the first or second limiting valves with the impulses b. Suitable circuits for producing the final trains are described, Figs. 4, 4a, (not shown). Specification 395,499 also is referred to.