Patent GB-1188200-A (GB)
Title: System for Monitoring Television Video Signals
IPC: H04N17/00
Filed: 1967-11-24
Abstract: 1,188,200. Television; amplitude discrimiriating. COMPAGNIE FRANCAISE THOMSON HOUSTON-HOTCHKISS BRANDT. 24 Nov., 1967 [24 Nov., 1966], No. 53681/67. Headings H3T and H4F. To detect anomalies which may affect the synchronization pulses in television video signals, the signal to be monitored at input 12. Fig. 1, is applied to first and second channels 1 and 2, the first channel including a clamping device 5, a peak clipping device 6, an integrator 7 and a control device 8 such as a D.C. threshold amplifier, and the second channel including a peak-to-peak amplifier 10 and a control device 11 such as a D.C. threshold amplifier, the output signals at terminals 13 and 14 being applied via an OR gate 13 to a switch 4 to control electro-mechanical relays and/or signal lamps at terminals 16 and 17. In device 5 the tips of the sync pulses are established at a potential Va, Fig. 2, which is zero or slightly positive. Device 6 clips this D.C. restored signal above a predetermined value Ve to produce pulses as shown on line 2 of Fig. 2. These pulses are then integrated in device 7 which produces a D.C. voltage, and if this voltage exceeds the critical threshold value of control device 8 a control signal is delivered to terminal 15 by means of OR gate 3 which for example illuminates a lamp. So long as the height of the sync pulse is greater than Ve-Va no control signal is produced, case a Fig. 2. In cases b and c where the sync pulse is inadequate and no longer exists a control signal is produced. In case d, where the luminance signal is small and where the sync pulse is non-existent or very low in amplitude, the integrator 7 is not charged sufficiently to deliver a D.C. voltage higher than the critical threshold value of control device 8. Channel 2 is effective to detect this type of error. In channel 2, device 10 for measuring the peak-to-peak amplitude of the incident video signal delivers a D.C. voltage such that when the peak-to-peak amplitude is lower than a predetermined value the control signal delivered by the control device 11 is present at terminal 14. To avoid the possibility of noise signals causing the system not to detect anomalies which are present, threshold switching device 8 may be replaced by a differential device such that a control signal is delivered to terminal 13 when the D.C. voltage delivered by integrator 7 is higher or lower than a predetermined value corresponding to the normal video signal. The system may be improved by adding a filtering device such as a band pass filter to channel 2, so that only those components close to the repetition frequency of the sync pulses of the video signal i.e. line frequency, are applied to device 10 for measuring the peak-to-peak amplitude. Further, the control device 11 of channel 2 may be arranged to produce a control signal at terminal 14 when the peak-to-peak amplitude exceeds a predetermined value to avoid a high amplitude sinusoidal signal of line frequency causing the system not to detect anomalies. Circuit arrangements Fig. 3; the video signal to be monitored is applied to input 100 of transistor T 1  which is used as an impedance separator, the signal being filtered by LC1 and LC2. In channel 1 (transistors T2-T10) potentiometer P 1  enables the amplitude of the incident signal to be adjusted, the signal being clamped by T 2  and T 3  so that the tips of the sync pulses are at the slightly positive potential of the base of T 2 , adjusted by potentiometer P 2 . T 4  operates as a peak clipper and the resulting pulses are amplified T 5  and T 6  and their base levels clamped by diode D 1  condenser-coupled thereto, integrated R 1  C 1 , and the integrated D.C. voltage is collected on potentiometer P 3  by means of field effect transistor T 8  and applied to D.C. amplifier T 9 , T 10 , the threshold of which is adjusted by potentiometer P 3 . In channel 2, T 11  amplifies the video signal which is clamped by diode D 2  and charges capacitor C 2  by means of diode D 3 . The charge of C 2  depends upon the peak amplitude of the video signal, and an adjustable fraction of the positive D.C. voltage obtained is derived by means of potentiometer P 4  and applied to threshold amplifier T 13 , T 14 . Transistor T 11  may be replaced by two transistors, Fig. 4 (not shown), the collector circuits of which include band pass filters. The D.C. amplifier T 9 , T 10  may be replaced by a differential D.C. amplifier including four transistors, Fig. 5 (not shown).