Patent GB-528192-A (GB)
Title: Improvements relating to discriminating circuits for television and the like
IPC: H04N5/08
Filed: 1939-04-22
Abstract: 528,192. Television. KOLSTER-BRANDES, Ltd., and BEATTY, W. A. April 22, 1939, No. 12127. [Class 40 (iii)] A synchronizing pulse is obtained from a television train of rectangular signal pulses of mixed duration by causing the leading and/or trailing edges of the rectangular signal pulses to produce short marking pulses and feeding these two sets of marking pulses or one of these sets and an original train through a circuit such as described in Specification 491,728 which delays one set relatively to the other set or train, whereby one particular marking pulse in a set is made to coincide with a pulse in the other set or train to produce a larger amplitude pulse or to cancel each other, the output of the circuit of Specification 491,728 then yielding the desired synchronizing pulse. In one such system, starting with a train of interlaced television picture, line, and frame signals, Fig. 1, (1) the line and frame pulses 21, 22 are first extracted, then (2) this extracted train of pulses, Fig. 2, is made to charge and discharge a condenser in a circuit having a very short time constant (.00001 second) whereby the leading and trailing edges 23, 24 and 25, 26 of these signal pulses produce marking pulses 27, 28 and 29, 30, Fig. 3, (3) the positive and negative marking pulses 27, 29 and 28, 30 are separated from each other and the phase of the negative pulses changed thereby yielding two separate trains 31, 32, Fig. 4a, and 33, 34, Fig: 4b, and (4) the train of pulses of Fig. 4a is fed to the terminal 2 of the circuit of Fig. 5, and the train of pulses of Fig. 4b is fed to the terminal 2a of the circuit, the delay network 7 of the circuit being set for a delay equal to .1 of a scanning line period which is the width of the line pulses 21. The circuit, Fig. 5, operates in the manner described in Specification 491,728. The first additive combination of signals occurs at the leading edge of the second frame pulse 22 and this causes an output signal from the circuit serving as an accurate synchronizing pulse. This synchronizing method may also be used for changing the interlacing factor. For example, the trailing edge of the last frame pulse of a transmission system in which the interlacing factor is two may be gradually advanced in the transmitted signals in a cycle of three frames, and marking pulses may be derived from these trailing edges at the receiver and added to the next black-out pulse, preceding pulses having been suppressed, thereby producing synchronizing signals of gradually decreasing time periods converting the interlacing factor from two to six at the receiver by a slight displacement of the picture frames in groups of three. When the required delay in the method above is comparatively large, the delay network 7 of Fig. 5 is replaced by more suitable delay devices, for example, a photo-electric device controlled by waves in a light control cell of the kind described in Specification 439,236, an adjustable slit being provided to determine the instant of time at which the light beam will affect the photo-electric cell. In a modification of this an adjustable rochelle salt or like crystal is used to receive at the required time the wave impulse in the aforesaid controlling cell.