Patent GB-1108283-A (GB)
Title: Improvements to television circuits
IPC: G06G7/28
Filed: 1965-05-13
Abstract: 1,108,283. Electric analogue calculating. COMPAGNIE GENERALE DE RADIOLOGIE. 13 May, 1965 [13 May, 1964; 16 Feb., 1965], No. 20333/65. Heading G4G. [Also in Division H4] The invention relates to an automatic gain control device which retains for the video frequency amplifier of a television camera, e.g. a radiological television camera, a normal gain for signal values corresponding to brightnesses of the subject televized which are between a zero and a mean value, and then abruptly reduces this gain for signals corresponding to brightnesses between the mean and maximum value. As shown, Fig. 1, the load resistor of the last amplification stage T 1  preceding the output stage T 2  feeding the television receivers comprises a fixed resistor R 1  determining the normal gain and a second resistor R 2  which is connected in parallel with resistor R 1  and in series with a diode D which becomes conductive only when the input signal S 1  exceeds the potential Vbl, thus reducing the effective load resistance and thereby reducing the gain. The inflection point of the curve of the output signal S 2  of the stage T 1  against the input signal S 1  of the same stage, Fig. 2 (not shown), is determined by the value of the direct current potential Vbl which can be regulated by potentiometer P1. The mean luminosity of the image may be controlled by varying with potentiometer P 2  the level of the back level signal S 3  to which the video signal is added and as before for signal values corresponding to the sum of signals S 3  and S 1  higher than Vbl the gain of the amplifier stage T 1  is reduced. The device may be put out of the circuit by application to the diode D potential Vb2 which blocks diode D whatever the value of signal S 1 . In simultaneous cinematography and radiological television the power supplied by the X-ray generator is proportional to and increases with the rate at which the cinematographic pictures are taken and the device described above is arranged as before to reduce in a certain ratio the gain of the signal which exceeds half the maximum standardized level feeding the television receiver so that the image remains readable without dazzling, the standardized level being the level of the video signal which gives the maximum white to the image. In a second embodiment, Fig. 11 (not shown), the device described in Fig. 1 is duplicated, i.e. two resistors (R 2  and R 3 ) are connected in parallel to load resistor (R 1 ) and in series with Two diodes (D 1  and D 2 ) which become conductive when the signals reach two values. Thus the device of Fig. 11 has two gain values for higher signal values, one for mean values of signals received (mean greys) and the other for maximum values of signals received (whites), Fig. 12 (not shown). In order that the device has only one gain when no signal exceeding the standardized level is presented at the output of the camera a diode amplitude selector device (SA), Fig. 13 (not shown), is included in the circuit which operates to unblock the two diodes (D 1  and D 2 ) when the signal exceeds the standardized level. The diodes are unblocked more quickly the higher the value of the signal at the output of the camera resulting in a variation of the inflection points, Fig. 15 (not shown). In a modification, Fig. 16 (not shown), in which the inflection joints are fixed and the slope of one or the other top portion of the curve, or both, are varied, Fig. 17 (not shown), one or more photodiodes, photoresistors or thermistors are associated with the amplitude selector and connected in parallel with the resistor (R 2 ) or resistors (R 2  and R 3 ).