GB1018715A - Method for controlling the black level of television signals - Google Patents

Abstract

1,018,715. Television black-level clamping systems. FERNSEH G.m.b.H. Feb. 25, 1964 [Feb. 27, 1963], No. 7787/64. Heading H4F. Colour " burst " synchronizing signals F are separated from an N.T.S.C. type of television signal by a gate 23 rectified at 24 to obtain pulses i, and then differentiated and limited at 26, 27 to obtain pulses k occurring shortly after the bursts (i.e. during the back porch intervals) which are applied via adder 43 and pulse former 28 to control a clamp 29 connected to the channel 31, 32 through which the television signal passes. A second series of clamping pulses are derived from the synchronizing pulses g which are delayed at 37 and then differentiated and limited at 38, 39 to obtain pulses m which occur at a suitable fixed interval after the horizontal (and equalizing) pulses such that those which occur during the vertical synchronizing interval T do not ov rlap the negative excursions of the vertical synchronizing pulses. The timing of the pulses however is such that those which occur during the picture interval (i.e. outside interval D) coincide with the colour bursts F. The pulses m are applied via gate 41 which is controlled by pulses i so as to eliminate those which coincide with colour bursts and the resulting pulses n (occurring only during interval D) are then applied to adder 43 whence they are combined with pulses k to produce a composite pulse train which passes through pulse former 28 as pulse train m<SP>1</SP> to control clamp 29. If the original colour television signal comprises colour " bursts " extending throughout the interval D the whole of pulse train m will be eliminated in gate 41. If the original signal changes to a conventional monochrome signal, the absence of " burst " signals will eliminate pulse train k but will allow the whole of pulse train m to pass via gate 41 to adder 43 and thence through pulse former 28 as pulse train 28 to clamp circuit 29. Reference is made to the control of gate 41 by pulses v which coincide with interval D as an alternative to control by pulses i. A detailed transistor circuit for part 162 of Fig. 6 is described with reference to Fig. 7 (not shown). A monitor receiver for use with N.T.S.C. type signals and incorporating the system of Fig. 6 is described with reference to Fig. 8 (not shown). The receiver includes a vectorscope for the colour circuits. A system is described with reference to Fig. 5 (not shown) which includes only elements 23-27 for deriving clamping pulses from the colour synchronizing " bursts."