Patent GB-650521-A (GB)
Title: Improvements in or relating to television receiver frame time base circuits
IPC: H04N5/10
Filed: 1948-11-24
Abstract: 650,521. Television. MULLARD RADIO VALVE CO., Ltd., BRAYBROOK, C. H., and OVERTON, B. R. Nov. 24, 1948, No. 30517. [Class 40 (iii)] A television receiver time-base arrangement for odd line interlaced scanning comprises means including a thermionic valve for deriving from the received frame and line synchronizing pulses an amplitude-differentiated waveform and a double thermionic valve, one section of which is utilized in a frame pulse separator circuit to which the said waveform is applied, the other section constituting a frame blocking oscillator and the arrangement being such that the frame synchronizing pulses for both odd and even frames which are fed to the blocking oscillator from the frame pulse separator circuit are identical in form and duration. In the arrangement shown in Fig. 1, the amplitude differentiated waveform (Fig. 5) derived from a separator valve 29, to the input circuit of which the combined line and frame synchronizing pulse waveform (Fig. 4), including vision signals, is applied, is fed to one section 10 of a double triode valve 20, the grid of which is positively biassed so that during the occurrence of a frame synchronizing period the anode current is cutoff and a pulse (Fig. 7) is produced. This pulse, which is fed through capacitor 12 to synchronize the operation of section 14 of valve 20, which operates as the frame blocking oscillator, has its waveform shape altered during operation to that shown in Fig. 8. In this arrangement the time constant of the network including capacitor 2 and resistors 5 and 6 is made long relative to the chain of frame pulses so that the waveform (Fig. 6) of the signal which appears at the point 3 is not materially different from the waveform (Fig. 5) at the output of separator valve 29. The saw-tooth scanning wave generated by the section 14 of valve 20 is fed to one section of a further double valve 27 which operates as the frame output valve the other section of which constitutes the line blocking oscillator, the output of which is fed to the line output valve 30, as described in Specification 650,522. In an alternative arrangement (Fig. 2), the grid of the section 10 of valve 20 (Fig. 1) is earthed and a positive bias applied to the cathode such that anode current flows only during the first few frame pulses to give an anode potential waveform as shown in Fig. 11. This waveform is integrated by passage through capacitor 12 and series resistor, 23 to produce a negative pulse which is transferred by the transformer 13 as a positive pulse to the grid of section 14 to synchronize the operation of the frame blocking oscillator. In this arrangement the time constant of capacitor 2 and resistors 5 and 6 is made of the order of two or three linescanning periods. In a modification of the arrangement of Fig. 2 the anode of the triode section 10 is coupled to the grid of the triode section 14 by a small capacitor so that' the operation of the latter section is controlled by the first frame pulse, the leading edge of which is negative-going and prevents conductions, and the trailing edge of which is positive-going and causes conductions (Fig. 3, not shown).