Patent GB-777674-A (GB)
Title: Improvements in or relating to circuits for monitoring the polarity of television signals
IPC: H04N5/44
Filed: 1955-10-25
Abstract: 777,674. Television. WESTERN ELECTRIC CO., Inc. Oct. 25, 1955 [Oct. 29, 1954], No. 30476/55. Class 40 (3). A circuit 25, 26, 27, 30 connected to a television signal channel a, b-c, d monitors the polarity of the signal and, if incorrect, reverses it by operation of a double-pole change-over switch 22. The circuit comprises an amplifier 25, a clipping stage 26 and a peak-to-peak detector 27 controlling a gate stage 30. The signal at the output of the amplifier corresponds to sync. pulse negative with the correct condition in the channel, and the four signal conditions which may arise are indicated at (a)-(d) in Fig. 5A where the clipping level of stage 26 is indicated by dotted lines. In the case of signals comprising only sync. pulses, the amplitude of the clipping stage output in response to the inverted condition (a) exceeds that produced by the correct condition (b), but there may be no appreciable difference in output, depending on the nature of the video component, where the signal (c) and (d) also includes a video component. The correct signal (d) in this last case is, however, distinguished from the inverted signal (c) in that it falls to zero during each vertical blanking interval. The clipped signal is reversed in amplitude at V4 and then applied to the peak-to-peak detector 27 the output from which appears on capacitor 63. The time constant of capacitor 63 with resistor 64 is chosen so that a D.C. output is obtained for conditions (a), (b) and (c), but for condition (d) the disappearance of the signal during the vertical blanking interval results in an A.C. component as well. In the gate stage 30, V7 is the gate proper, the valve being biased to cut-off level and controlled at its grid from the potential on capacitor 63. The arrangement is such that the valve conducts for conditions (a) and (c) and not (b), but may also possibly conduct for (d). To prevent operation in this last case, the A.C. component which then appears in the output on capacitor 63 is applied through an A.C. amplifier V9 to the suppressor 71 of V7 and by means of diode V8 applies a negative potential to prevent the gate from conducting. The gate thus operates only in response to inverted signals (a) or (c). The gate controls a relay K2 and this in turn operates a relay K1 controlling the double-pole changeover contacts 22. Relay K1 is associated with another relay K3 in a self-locking oscillator circuit whereby it reverses its condition back and forth in response to the operation of relay K2 and finally locks in the condition existing when relay K2 releases. The time constant of the circuit, which is determined by capacitor 81, bridging relay K3, is arranged to equal several television frame intervals. The circuit therefore normally reverses only once before locking. Provision may be made to operate relay K2 manually. In a modified arrangement, the polarity monitoring circuit is connected across the channel at a point preceding the double-pole change-over switch. Specification 749,038, [Group XL (c)], is referred to.