Patent GB-601060-A (GB)
Title: Improvements in or relating to television receiving systems
IPC: H04N5/12
Filed: 1945-03-14
Abstract: 601,060. Television receivers. ELECTRIC & MUSICAL INDUSTRIES, Ltd., and WHITE, E.L.C. March 14, 1945, No. 6445. Addition to 517,170. [Class 40 (iii)] [Also in Group XL (c)] In a television receiver wherein, as in the parent Specification, the synchronizing pulses are suppressed and regenerated pulses are added to the picture signal so as to eliminate interference from the synchronizing signals, the regenerated pulses are derived from a freerunning oscillator, the regenerated and received synchronizing pulses being compared to develop a control signal dependent on their phase difference which is then used to effectively lock the regenerated pulses to the received pulses in the desired phase relationship. The output of the receiver shown at A, Fig. 2, comprises line and frame synchronizing signals, picture signals and interference X. A picture separator selects the picture signals as shown at B and a synchronizing separator selects the synchronizing signals as shown at C, both operations being performed by suitable limiting. The output of a free-running pulse generator is applied to a phase discriminator to which is also applied the line synchronizing pulses from the synchronizing separator. The discriminator develops a control signal, which may be smoothed, and this is fed back to the pulse generator to maintain it in a fixed phase relationship with the incoming synchronizing pulses. These pulses are then mixed with the picture signals to produce a signal D which is fed to a number of viewing units in parallel. The frame synchronizing pulses are also compared with the output of a second generator in a similar manner, the output E of the generator being applied to the viewing unit direct. A further embodiment, Fig. 3 (not shown), describes a relay link in an odd line interlaced scanning system. A single oscillator operating at twice the line frequency is used, the output being divided by two for the line synchronizing signals and in the case described, by 405 in stops of 5 x 9 x 9 for the frame signals. The line and frame signals are selected separately from the receiver output, the former by a highpass filter. These are applied together with the frequency dividers' output to their respective phase discriminators. The control signal from the line frequency discriminator controls the oscillator frequency and the frame frequency control signal alters the first division ratio to either 4 or 6 should the phase of the frame signal require to be corrected. The regenerated synchronizing pulses are combined to produce the standard synchronizing signal and are then inserted into the picture signal from which the original pulses have been suppressed. This suppression is achieved by adding the regenerated signal at a large amplitude to the received signal so that interference is depressed below the black level and then limiting at the black level. The signal then passes to a transmitter for re-transmission and to a local viewer for monitoring. The frame frequency discriminator and associated circuits are illustrated in Fig. 5. The received signal is applied to condenser 26 so that on the receipt of a pulse the condenser 29 commences charging. The condenser 29 is able to charge sufficiently to overcome the bias on diode 30 and thus apply the signal to triode 31 only on the application of a comparativelylong frame synchronizing signal. The locallygenerated signals are applied to condenser 26a and thus to triode 31a which together with triode 31 forms a differential amplifier which delivers no output when the pulses occur in phase. Should the pulses occur out of phase, a signal is applied to cathode follower 39 which controls the cathode potential of blocking oscillator 43. The output of the free-running oscillator is applied through condenser 45 to the circuit which acts in the manner described in Specification 471,731 as the first-stage frequency divider. The division ratio is controlled by the variations in the cathode potential of the oscillator 43.