634,501. Television. HASELTINE CORPORATION. Jan. 15, 1947, No. 1397. Convention date, Jan. 25, 1946. [Class 40 (iii)] Two video signals, the black level of the first of which, with respect to a reference level (e.g. peak white), varies in the same manner with variations in signal intensity as the black level of the second signal varies with respect to the peaks of the synchronizing signals, are combined to produce a resultant video signal in which the black level is independent of variations of average amplitude. Fig. 1 shows the application, of one arrangement according to the invention, to a television receiver from the I.F. amplifier 14 of which output signals are applied to a detector unit 15 comprising two separate diodes 25, 30 with their associated load resistors 28, 32, 25 per cent of the output signal of amplifier 14 being fed to the circuit including diode 25 and 75 per cent to the circuit including diode 30 (for a negatively modulated carrier in which 75 per cent of the maximum amplitude is devoted to the transmission of vision signals and 25 per cent to the transmission of synchronizing signals). A further diode 34, together with its associated resistor and condenser 35, 36, respectively, is included in the circuit of diode 30 to stabilize the signals developed by the latter circuit with reference to the peaks of the synchronizing signals. The combined output signals developed by the two detecting circuits, i.e. across resistors 28 and 35 are fed to a video amplifier 45 the output of which, developed across resistor 48, is applied to the intensity control electrode of the picture reproducing tube 17. Fig. 3 shows the variation of the output signals (synchronizing=s ; black=b; white=w) developed across resistors 28 and 35 with variation of input signal from the I.F. amplifier, the signals across resistor 28, which represent an ideal case in which the white level e28w is denoted by modulation of the carrier to zero amplitude, being shown in relation to a fixed reference level a which exists in the circuit of diode 25 due to the bias source 47 and, the signals across resistor 35, in relation to the reference level e35s set by the stabilization circuit including diode 34. The signals developed at terminal 37 which are fed to the video amplifier 45 comprise the combination of these two output signals and represent a signal (e37s ; e37b; e37w) which is stabilized with respect to the black level independently of the variation of the signal input to the detector unit 15. Automatic contrast control (A.C.C.) is provided in the output circuit of the video amplifier by means (16) comprising a diode 49 to the cathode of which the video signals across resistor 48 are applied and to the anode of which is applied an oscillatory voltage produced in generator 52 having a frequency which is high in comparison with the frequency of line synchronizing signals and a peak amplitude equal to the D.C. potential of the anode of valve 45 in the absence of signals applied to its input circuit, i.e. equal to the potential of the anode representative of picture black, so that the diode conducts only during the recurrence of synchronizing signals which are peak rectified, the signals developed across the load circuit 50, 51 being applied to control the gain of the R.F. amplifier 10, the frequency changer 13 and the I.F. amplifier 14. Modifications of the detector 15 of Fig. 1 are described with reference to Figs. 4 to 8 inclusive (not shown). An alternative arrangement for the A.C.C. unit 16 of Fig. 1 is described with reference to Fig. 9 (not shown). Specification 623,431, [Group XL (c)], is referred to.
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