Patent GB-1195388-A (GB)
Title: Synchronizing System for Television Receivers
IPC: H04N7/087
Filed: 1967-09-26
Abstract: 1,195,388. Television; transistor pulse circircuits. R.C.A. CORPORATION. 26 Sept., 1967 [27 Sept., 1966; 30 Jan., 1967], No. 43773/67. Headings H3T and H4F. In response to the equalizing, horizontal synchronizing and serrated vertical synchronizing pulses which occur in the odd and even field vertical blanking intervals of an interlaced television signal, apparatus is provided which generates a continuous pulse train, the pulses of which are in phase with, and of the same duration as, the horizontal synchronizing pulses. The apparatus may comprise two monostable multivibrators connected in series, the first multivibrator having a time constant greater than half a horizontal line period but shorter than an entire horizontal line period, and the second multivibrator having a time constant equal to the duration of an original horizontal sync. pulse. In the preferred embodiment, Fig. 2, the output from a television sync. separator is applied to the base of transistor 101 which under quiescent conditions is conducting but upon the occurrence of a negative (horizontal sync.) pulse it is rendered non-conducting, charging capacitor 111 which produces a voltage pulse with a positive leading edge at the base of transistor 103 driving it into saturation. This produces a negative voltage pulse at its collector which is in synchronism with the leading edge of the input signal. Under quiescent conditions, the voltage developed at the junction of resistors 125 and 127 places a positive charge on capacitor 129. When transistor 103 is driven into saturation, diode 131 is forward biased, and capacitor 129 discharges toward the bias voltage at the emitter electrode of that transistor. The voltage established at the junction of resistors 125 and 127 by the discharge of capacitor 129 is coupled back to the base electrode of transistor 101 and is effective to hold that transistor in its non-conducting state. It will remain in this state until the bias voltage at its base electrode is re-established by virtue of the positive charging of capacitor 129 through resistor 127, at which time it is returned to its conducting state. By making the time-constant of the network formed by resistor 119 and capacitor 111 substantially equal to the duration of a horizontal sync. pulse, transistor 103 is not returned to its non-conducting condition until after this time has passed. A positive-going pulse corresponding to a horizontal sync. pulse is thus developed at output 175. By making the time-constant of the network formed by resistor 127 and capacitor 129 larger than half a horizontal line period, but shorter than an entire horizontal line period, transistor 101 is unaffected by equalizing pulses or vertical serration pulses which may occur at odd multiples of half horizontal line interval after the last horizontal sync. pulse of a field, since the positive charging voltage across capacitor 129 will not yet be of a value to render transistor 101 conductice. The apparatus may be used in the system of Specification 1,132,303 in which line scan signals representative of message images are sequentially multiplexed with the sync. signals of television images at the rate of one line scan signal per message per field time interval of the television images.