Patent GB-1027019-A (GB)
Title: Improvements in television synchronising pulse separators
Inventor: CORNEY JOHN VICTOR JAMES
IPC: H04N5/08
Filed: 1965-01-14
Abstract: 1,027,019. Television circuits. THOM ELECTRONICS Ltd. Jan. 14, 1965, No. 1746/65. Heading H4F. A television sync.-pulse separator designed to overcome the disadvantage of fixed cut-off level separators, wherein the slope of the pulse sides and the amplitude modulation of the pulses by picture content due to D.C. restoration causes the timing of the cut-off pulses to be phase modulated by the picture content, comprises a D.C. differential amplifier I whose output voltage depends on the difference between an input voltage at input X and a reference voltage T, a first feedback path (clamp N and resistor R1) operating only on the front and back porch intervals of the television signal applied at point Q, and tending to equal the voltage of the porch intervals at input X with the reference voltage T, a second feedback path (resistor R 2 ) tending to equal the voltage of the sync. pulse tips with the reference voltage T, whereby the porch and tip voltage levels appear at the amplifier output Y, spaced either side of the reference voltage T, by amounts depending on the relative magnitudes of resistors R 1  and R 2 , a third feedback path (resistor Rf) controlling the gain of the amplifier, and a separator means K coupled to the output Y passing sync. pulse signals greater than a predetermined level V. Thus by making R 1  equal to R 2 , the reference voltage T appears half way up the sync. pulse at output Y and by choice of level V, the separated sync. pulses at output U also start half way up the sync. pulses, thus utilizing the fact that the steepest part of the sync. pulse occurs half way up. The first feedback path is operative only during the porch intervals due to clamp N being closed during said intervals under the control of a clamp pulse generator L, generating clamp pulses, starting at the finish of the sync. pulses and timed by means of the output from the separator K. The voltage level of the sync-pulse tips is determined by sampler J which is rendered operative during the tip intervals by a pulse generator P. Alternatively the circuit J may comprise a peak detector. The voltage V may be obtained from potentiometer R 3 /R 4 . The input impedance and amplification of the circuit may be varied by means of capacitor C 2 , the video frequency input impedance and amplification with the capacitor connected to point B, being R 1 R 2 /R 1  X R 2  and R#(R 1 +R 2 )/R 1 R 2  respectively, and with the capacitor connected to earth R 1  and R#/R 1 . Variation of the video-signal level due to superimposed power frequency ripple is also overcome by the invention. If the picture part of the video signal is at level peak while for several successive lines, the circuit shown can mistake the level peak while signal for the black level, such that sync. separator K produces the blanking pulses at its output and the clamp pulses from generator L are correspondingly incorrect. This can be avoided if a self-biasing sync.-separator (see Fig. 1a and 1b, not shown), is added to the output of amplifier I (see Fig. 5, not shown). This separator clips off the sync. pulses near their tips and causes the production of a second train of clamp pulses which are fed to one input of a coincidence gate, the other input being fed by generator L, and said gate feeding clamp N. A transistorized embodiment of the invention is shown and described in detail (see Fig. 6, not shown).