GB553302A - Improvements in television synchronising systems - Google Patents

Abstract

553,302. Television ; thermionic valves. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 10, 1941, No. 11605. Convention date, Sept. 10, 1940. [Classes 39 (i) and 40 (iii)] In a television synchronizing system the receiver is synchronized with a transmitter sending framing pulses which lack a fundamental frequency or fundamental frequencies present in the line pulses by means of a control circuit which responds to the disappearance of said frequency or frequencies: Transmitters for producing such synchronizing signals for non-interlaced and interlaced scanning are also described. These signals, shown respectively at 62, Fig. 3, and 91, 92, Fig. 5, comprise usual linefrequency pulses 53a or 75a, multiple line frequency pulses 73a for equalization in interlacing, and in addition during the field retrace period T or a part t thereof a train of pulses 51a or 71a at a frequency which is a multiple of the frequency of the pulses 53a or 73a respectively. These pulse trains are all derived by known apparatus from a fundamental series of pulses at the frequency of the pulses 51a or 71a. One form of circuit for use at the receiver for detecting, the disappearance of the fundamental frequencies of the pulses 75a and 73a of Fig. 5 is shown in Fig. 8. The pulse trains 91, 92 after separation from the video signal in known manner are supplied by the lines 112 to two diode rectifiers 136, 139 associated with the resistances and condensers shown and with two tuned circuits 130, 131 tuned respectively to the frequency of the line pulses 75a and the double frequency of the pulses 73a. The rectified voltages are applied to grids 141, 144 of a pentode 140 through condensers and resistances 142, 143, and 145, 146. When either pulses 75a or 73a occur, a steady negative bias is applied to the pentode 140 which remains inoperative. When these pulses disappear while the higher multiple frequency pulses 71a occur, the impedence of the tuned circuits 130, 131 being low for the multiple frequency, a pulsation goes through the valve circuits and is transmitted from anode of pentode 140 through a condenser resistance circuit 147, 148, 149 to a line 115 leading to a field synchronizing generator of known kind. The synchronizing train is also supplied direct by a line 114 to a known form of line synchronizing generator. The diodes 136, 139 and the pentode 140 may be combined in a single envelope. In a modification especially intended for the synchronizing train of Fig. 3, a single tuned circuit is used and is connected to the grid of a triode, the anode of which supplies the control pulsation to the field synchronizing generator.