GB713931A - Improvements in or relating to television transmitters - Google Patents

Abstract

713,931. Television. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Oct. 16, 1952 [Oct. 25, 1951], No. 26015/52. Class 40 (3) The black level at the output of a television transmitter is maintained constant by means of a negative feedback path between an output circuit of the transmitter and a preceding video stage The feedback path comprises the series combination of a detector, low-pass filter, first electronic switch, integrator and record electronic switch the pass-band of the L.P. filter being sufficiently inside to transmit video and synchronizing signals and the time-constant of the inegrator circuit being such that self-oscillation is prevented. In Fig. 3, which shows the general arrangement the detector comprises a double-diode D connected, via transformer L, to an output circuit of the transmitter and, via the L.P. filter L1, C2, to a clipper stage E which clips the input signal at point C (curve C, Fig. 4) on both sides of the black level to produce a signal of about 20 per cent of the input signal amplitude. The clipped output (curve D, Fig. 4) is then fed, via a cathodefollower stage V, to a first electronic switch Cr, (which is "opened" by pulses synchronized with the synchronizing signals and fed thereto via condensers C3, C4) the output circuit of which (terminal f) is connected to a condenser C7 which with the internal resistance of the diodes of switch Cr, forms an integrating circuit with a timeconstant such that variations of black level occurring at line frequency (10 to 20 Kc/s) are faithfully transmitted whilst signals of higher frequency (e.g. above 100 Kc/s approx.), are not transmitted. The signal developed across condenser C7 (curve F, Fig. 4) is then supplied via a second electronic switch Ce (controlled in the same manner as switch Cr) to the grid of a video stage (valve T) which may be the first stage of a D.C. coupled amplifier immediately preceding the modulated H.F. stages of the transmitter.