GB883694A - Improvements in and relating to clamping circuits for television video signals - Google Patents

Abstract

883,694. Television. BRITISH BROADCASTING CORPORATION. Feb. 26, 1960 [March 19, 1959], No. 9616/59. Class 40(3). The D.C. component of a video signal which has been transmitted through a circuit having imperfect D.C. couplings or which introduces low frequency interference is restored by means of a circuit in which a sampling stage 4 examines the level of recurring elements (the back porch in the arrangement described) to produce pulses representing the deviations of the level from a reference potential. The pulses are applied through an amplifier and inventor 6 to a further sampling stage 7 which controls the application of the peaks of the pulses to a capacitor storage circuit having a time constant (100 microseconds) of the order of the video signal recurrence interval. The resulting varying potential is then combined with the original video signal at the input of a cathode follower 2 which feeds the sampling stage and is effective to restore the D.C. component. The sampling circuits in stages 4 and 7 comprise four-diode gates, Figs. 2 and 3 (not shown), which are operated by pulses from associated phase splitters 5 and 8 respectively. Pulses to operate the phase splitters are derived from the synchronizing pulses with the original signals which are separated by a stage 11 and applied to a 2 microsecond delay line made up of sections 12 and 13. Section 13 is short-circuited whereby the reflected pulse is reversed in polarity. In response to negative synchronizing pulses of 10 microsecond duration there is produced for phase splitter 5, 4 microsecond positive pulses commencing with the trailing edges of the synchronizing pulses, and for phase splitter 8, 3.4 microsecond pulses commencing 0.3 microseconds after and terminating 0.3 microseconds before the pulses for phase splitter 5. Where the invention is employed with an N.T.S.C. type colour signal sub-carrier, trap 10 and a further trap at the input of sampling stage 4 are included. Pulse amplifier 6 is A.C. coupled.