GB676035A - Improvements in or relating to means for deriving frame synchronising signals in television apparatus - Google Patents

Abstract

676,035. Pulse discriminating circuits. GENERAL ELECTRIC CO., Ltd., GLEDHILL, G. A., and GRIFFITH, G. W. S. Jan. 15, 1951 [Jan. 13, 1950], No. 945/50. Class 40 (vi). Frame synchronizing signals are separated from the combined line and frame synchronizing signals in a television signal by charging a capacitor through a resistor during the duration of each synchronizing signal pulse, the time constant of the capacitor resistor network being long compared with the duration of each of the frame synchronizing pulses. The voltage across the capacitor is reduced to a predetermined value at the termination of each pulse and is differentiated to produce a voltage which exceeds a predetermined value and is used to synchronize the frame scan generator only at the termination of an applied frame synchronizing pulse. Fig. 1 shows a frame synchronizing signal separator in which a television signal having positive going synchronizing pulses as shown in Fig. 2 (a) is applied at the input 1 of a valve 3 which is biased so that only negative going synchronizing pulses as shown in Fig. 2 (b) are produced at the anode. Valve 9 is biased so that it is normally conducting but is cut-off for the duration of each negative going synchronizing pulse applied to its control grid so that capacitor 17 is charged through resistors 12 and 13 for the duration of each synchronizing pulse and is rapidly discharged through valve 9 at the termination of each pulse. Thus, a waveform as shown in Fig. 2 (c) is produced at anode of valve 9 and is differentiated by network 18, 19 to produce the waveform as shown in Fig. 2 (d). Diode 21 is biased to conduct only during the negative going peaks 32 (Fig. 2 (d)), which occur at the end of each frame synchronizing pulse, so that a series of pulses, as shown in Fig. 2 (e), are produced at the output 26, the first of which may be used to synchronize the frame scanning generator. The line scanning generator may be synchronized by a waveform produced at terminal 42 by differentiating in network 40, 41, the voltage at the anode of valve 3 or by means of the waveform produced at the screen grid of valve 9 when the decoupling capacitor 39 is omitted.