Patent GB-955697-A (GB)
Title: Improvements in or relating to circuit arrangements for television receivers
IPC: H04N5/50
Filed: 1960-05-03
Abstract: 955,697. Automatic phase control sysfems; television; pulse circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 3, 1960 [May 6, 1959], No. 15556/60. Headings H3A, H3P, H3Q and H4F. In a television receiver a frame sync. pulse amplifier 1 produces on its screen grid circuit at 18 a negative blocking voltage when no television signal or a distorted signal reaches it, which is then used (1) for suppressing direct line sync. pulse control of the line oscillator 24 so that solely a flywheel sync. system of the line oscillator is then in control and/or (2) for suppressing flywheel sync. control of the frame oscillator 42 and permit solely a direct sync. pulse control of the frame oscillator or (3) for blocking the video output stage or (4) the sound output. During normal reception the mean voltage at 18 is zero, being located between HT + and a negatively biased cathode of valve 1, but during no signal, or distorted reception or noise, the self bias developed on the grid of valve 1 disappears or diminishes increasing the cathode current of valve 1, and the screen grid current, to render point 18 negative. As shown, the line oscillator 24 is controlled by a flywheel sync. circuit having a phase detector 25 and line sync. pulses 20 and flyback pulses 27 applied to it, and directly synchronized also by pulses 20 through a gate valve 19 which is blocked by a rectifier 30 when the pulses 20, 27 are in near synchronism. But the negative voltage at 18 during no signal, or a distorted signal, blocks the valve 19 to protect the line output transformer from overload. The frame oscillator 42 may be directly synchronized by sawtooth pulses 39 derived from the pulses 36 on the screen grid, and applied through a capacitor 40 and normally conductive rectifier 41 to the frame oscillator 42. But as synchronism increases between input sync. pulses 3 and pulses 46 on the anode of valve 1 from the oscillator 42 a negative voltage increasingly develops in the anode circuit and is applied through resistors 44, 48 to block the rectifier 41 so that the direct sync. pulses must pass through an attenuating resistance 50 shunting the rectifier 41, and this also reduces interference. Initially, oscillator 42 is low in natural frequency but is approximately synchronized by a flywheel sync circuit including a phase detector valve 35 with pulses 39 and pulses from the oscillator 42, respectively, applied to its cathode and grid to facilitate the final control by weak direct synchronizing pulses. During no signal or a faulty signal such as sound breakthrough in valve 1, the blocking voltage at 18 cuts off the phase detector 35 to prevent its output across capacitor 59 incorrectly de-tuning the oscillator 52 to a point such that direct sync. is unable to synchronize it. Alternatively the negative voltage at 18 may be used for muting of the video output valve. In this case the sync. separator input is taken from a separate amplifier (not the video output amplifier). Or it may be applied to muting the sound output. An integrating capacitor may replace the resistor 17 and capacitor 38, and the lead to the grid of the phase detector valve 35 may be omitted since during no signal or interference the pulses 39 have only a small amplitude and no output is produced from the phase detector valve 35.