Patent GB-709468-A (GB)
Title: Television field-identification system
IPC: H04N9/44
Filed: 1952-08-06
Abstract: 709,468. Television. HAZELTINE CORPORATION. Aug. 6, 1952 [Sept. 1, 1951], No. 19810/52. Class 40 (3) In an odd-line interlaced (colour) television system in which the field synchronizing pulses have different time relationships with respect to the line synchronizing pulses during any two successive fields the latter fields are distinguished by means of a signal which is derived by combining signals at line frequency with a signal developed during field synchronizing periods and which has a peak amplitude with a duration of less than the interval between adjacent line synchronizing pulses. In the embodiments described the distinguishing signal is employed to maintain correct colour synchronization in a colour television receiver, particularly in a system of the type described in Specification 702,182 in which the colour phase sequence is periodically changed. Fig. 1 shows one embodiment applied to a colour television receiver operating in a system in which the colour signals modulated on a sub-carrier are applied via a filter network 18, synchronous detectors 19a to 19c and filter networks 20a to 20c to colour responsive cathodes of the picture reproducing tube 17, the general brightness signals being fed to the control grid of the tube from a filter 16. To maintain correct colour phase operation the switching circuit 31, which selectively couples the phase shift circuits 30a, 30b, to different ones of the detectors 19b and 19c on each scanning line (or field), as described in Specification 702,182, is controlled by the field distinguishing signal developed according to the invention in unit 39. The operation of this unit, one embodiment of which is shown in Fig. 2, is as follows: Field synchronizing pulses in the output of the synchronizing signal separator 22 are selected by a circuit 50 and fed to a resonant circuit 51 having a frequency corresponding to one-half the frequency of the line synchronizing pulses and a peak amplitude with a duration of less than the interval between the line synchronizing pulses. The output of 51, curve E, Fig. 2a, is then combined in circuit 52 with pulses at line frequency, (F, Fig. 2a), derived from the line generator 27 and the resulting combined signal, curves G1 and G2, Fig. 2a, depending on whether an odd or even line field is present, is fed to a circuit 53 which supplies an output signal (H, Fig. 2a), during the occurrence of one of the fields, in this case during the occurrence of the field corresponding to the pulses of curve G2. This field is thus distinguished from the field interlaced therewith and the output signal (H, Fig. 2a) from circuit 53 which is fed to the switching circuit 31 may thus be employed to maintain correct colour phase operation of the detectors 19a to 19c. The field pulse selector 33 (Fig. 1), may be omitted and the synchronizing signal separator 22 (Fig. 1), which, as shown in Fig. 3, comprises a synch. signal selector 80, an amplifier 81 and a line/ field separator 82, may be utilized directly. In this embodiment the combined line/field signals are supplied to the resonant circuit 51 which is shock excited by the field pulses above the line signals being ineffective due to the low impedance of the circuit to pulses of line frequency. In a further embodiment the colour phase sequence is controlled on each line (instead of on each field) by including a device in the switching 31 (Fig. 1), which has two stable conditions and which is arranged to be switched from one condition to the other by the control signal from unit 53 (Fig. 1), or by line frequency pulses from generator 27 (Fig. 1), or by both, Fig. 4 (not shown). Fig. 6 shows a detailed modification of this arrangement which includes a trigger circuit (valves 93, 95), having two stable conditions between which the circuit is switched by the application of line frequency pulses of terminals 34. Valve 93 receives these pulses directly and valve 95, via the resonant circuit 51 (shock excited by field pulses from unit 50). With this arrangement proper phase operation of the trigger circuit is ensured since, if the valves happen to be operating 180 degrees out of correct phase, the first or second half-cycle of the signal developed in the resonant circuit 51 will cause the deletion of one effective triggering pulse normally applied to valve 95 so that the circuit remains in the same condition for the duration of two scanning lines which corrects the 180 degrees phase error. To minimize the effects of random noise in shock-exciting the resonant circuit, the control signal circuit (53, Figs. 1 and 2) may be " keyed" on only during field synchronizing periods, Fig. 5 (not shown).