Patent GB-903659-A (GB)
Title: Improvements in and relating to the conversion of television scanning standards
Inventor: DEVEREUX VICTOR GERALD
IPC: H04N7/01
Filed: 1960-02-03
Abstract: 903,659. Television. BRITISH BROADCASTING CORPORATION. Feb. 2, 1961 [Feb. 3, 1960], No. 3875/60. Class 40 (3). [Also in Group XL (c)] In a television scanning standard conversion system in which pulses of constant amplitude are inserted (4, Fig. 1), during the back-porch of television signals displayed (as a picture) with a first field scanning frequency on tube 5 so that from the output of camera 6, which operates with a different field scanning frequency, signals may be derived via detector 9 corresponding to the scanning of the edge of the display on tube 5 on which the inserted pulses are reproduced as a stripe and employed as to produce a gain control voltage for amplifier 7 to eliminate the flicker resulting from " beats " between the two field frequencies, a residual flicker, due to the inherent delay in the circuits producing the gain control voltage, is compensated by arranging for the scanning of each line of the stripe (by camera 6) to occur before the scanning of the corresponding lines of the picture. Fig. 2 (a) shows the waveform of the main flicker (full line) and that of the gain control voltage from detector 9, after smoothing (broken line), the result of the inherent delay in the smoothing circuits giving rise to a residual flicker as shown in Fig. 2 (b). To eliminate this latter flicker the signals derived by camera 6 when scanning a given line of the stripe are arranged to be advanced with respect to the signals produced when the corresponding lines of the picture are scanned. One means for effecting this consists in applying a pulse to the (or to auxiliary) field scanning means of camera 6 during the scanning of the stripe so as to displace the beam in the appropriate direction and by the appropriate amount. Assuming field scanning in the vertical direction a downwardly deflection is necessary when the field scanning rate of camera 6 is greater than that of display tube 5 and an upwardly deflection when the tube 5 operates with the greater field frequency, the amount of the deflection (determined by the delay introduced by the smoothing circuit) being made such that the steepest part of the two waveforms of Fig. 2 (a) coincide. In an alternative method of producing the required gain control voltage, a separate camera " receiving " the same display (or a separate like display) and having its vertical scan suitably " staggered " with respect to that of the other camera may be employed. Figs. 3 and 4 show a further method in which a narrow prism 10 is positioned between the camera and the stripe 12 on the display tube 5, the effect of this being to displace the scan of a given line of the stripe relatively to the corresponding lines of the remainder of the display. In another optical method the prism may be replaced by a flat strip of glass inclined with respect to the optical axis and in a further optical method a mirror having a narrow edge strip which is inclined with respect to the remainder of the mirror is employed to reflect the whole of the display on to the camera. Specification 801,140 is referred to.