Patent GB-1265825-A (GB)
IPC: H04N5/52
Filed: 1969-04-02
Abstract: 1,265,825. Automatic gain control; television. BALL BROS. RESEARCH CORP. 2 April, 1969, No. 17133/69. Headings H4F and H4R. In an automatic gain control system a television signal is applied to two parallel channels, one of which has unity gain while the other has the gain controlled in accordance with the output signal after the two channels have been recombined by a switching device. Preferably, only the synchronizing signals are derived from the unity gain channel and the picture video signal from the gain controlled channel with the gain controlled in accordance with measurements made on the peak white to peak black variations in the controlled signal. As described, the complete video signal is fed from terminal 21, Fig. 2, to a differential amplifier feeding an emitter follower with negative feedback from the emitter follower to the differential amplifier, and the differential amplifier having a constant current feed to its two halves, to provide a very low impedance feed of the signal to the two channels A and B. Channel A provides the unity gain channel and includes a delay 23, to equalize the timing of the signal with that in the other channel, and a circuit 72 to maintain the D.C. level of the signal at a reference level. Channel B includes the automatic gain controlling variable attenuator 48 and a D.C. level controlling circuit 88 maintaining the signal at a predetermined reference voltage level. The outputs from the two channels are applied to a diode switch circuit 27 which selects the synchronizing signal from channel A and the video picture signal from channel B. The switching is derived from a local oscillator 152 operating at 31À5 kHz via a divider 156 and a monostable multivibrator 157 which produces inverted line blanking pulses which are combined with field blanking pulses derived from the same oscillator, via dividers 174 to 183, before feeding the diode switching circuit 27. The oscillator 152 is phase locked to the synch. pulses of the incoming video signal via synch. separator and discriminator circuits 165, 162 &c., so that the switching of the diode gate is synchronized with the incoming video signal. A detector circuit 61 to 64 is provided to detect fades of the picture signal to black level, and on detection of such a fade, operates to switch the a.g.c. circuit to a unity gain condition by operation of switch 33. The a.g.c. control signal and the D.C. reference levels are derived from the output signal of the unit by the sampling detectors 123, 124, and 125, which are gated to detect respectively the peak white, and peak black level of the picture signal and the video blanking D.C. level. These signals are combined to provide, from the peak white and peak black detectors, the automatic gain control signal, and from the peak black and video blanking detectors, voltages to control the D.C. level of the signals applied to the diode switch in order that the " set up " of the output signal may be adjusted and maintained. In a colour system the monochrome and chrominance signals may be treated separately, being split by a circuit as shown in Fig. 20 (not shown) which is the subject-matter of Specification 1,265,826 and which replaces the low impedance feed circuit formed by 51 and 83 in Fig. 2. Automatic gain control circuit may use a light sensitive resistor in a potentiometer in the signal path illuminated by a source whose intensity is proportional to the gain controlling signal, the resistor and source being a unitary component. A varistor is provided in a shunt circuit over the photo sensitive resistor to compensate non-linearity of the gain control system. Alternatively, Fig. 18, the video signal may be fed to the common emitters 1123, 1133 of two pairs of transistors of complementary type with the current division between the transistors of each pair being controlled by the gain controlling voltage and the output being obtained by mixing the signals from one transistor of each pair. The resulting output signal varying linearly with control voltage and being of very low distortion due to the use of the complementary pairs.