Patent GB-940336-A (GB)
Title: Improvements in or relating to circuit arrangements for adjustable volume and contrast control
IPC: H03G7/02
Filed: 1962-07-06
Abstract: 940,336. Radio receiving circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. July 6, 1962 [July 10, 1961], No. 26035/62. Heading H3Q. In the sound channel of a television receiver a single control, which may be remote, varies both the volume and contrast control. The signal from the sound detector is passed via blocking capacitor 2, Fig. 1, and pre-set amplitude control 3 to a first amplified stage 5 of variable gain, which is R.C. coupled to an output stage 10. This feeds the loud-speaker, via transformer 14 and also, via blocking condenser 16 and variable potentiometer 17, a further amplifier stage 18 for contrast compression. The output of the amplifier 18 is rectified by a diode 22 and after smoothing by capacitor 4, applied to the grid of the valve 5 so as to reduce the gain as the signal strength increases. The degree of contrast compression is adjusted by means of the control 17; to enable it to vary the volume also an auxiliary supersonic signal, i.e. the output of the line timebase, is fed into the anode circuit of the valve 5, over blocking condenser 26. The arrangement is such that when the degree of contrast compression is not large, the sound signal predominates and control 17 varies the degree of contrast control; as, however, control 17 is set towards maximum and the degree of contrast control increases, the sound signal at the valve 18 becomes relatively small and the supersonic signal more predominant. It is rectified by diode 22, causing the valve 5 to be biased back, ultimately reducing the sound volume to zero. The supersonic signal may alternatively be introduced directly into the control 17, via capacitor 27 and a choke 28 used to prevent it reaching the anode circuit of the valve 10. Tone control is applied to the arrangement by means of negative feedback between anode and grid of the output valve 10 via variable resistor 30 and capacitors 31, 32. The auxiliary signal may comprise D.C. instead of supersonic signal, being applied directly to the control 17 : it is prevented from reaching the grid of the valve 18 by means of a blocking capacitor, and is applied from the slider of the control 17 either to the diode 22 or directly to the grid of the valve 5. In a modification, Fig. 2, the signal from the output stage (not shown) is applied via blocking capacitor 16 direct to the grid of the amplifier 18. A voltage divider comprising resistors 41, 32 is connected between H.T.+ and earth, the cathode of the valve 18 being taken to the tap. Grid resistor 30 is returned to an adjustable slider on the resistor 32, movement of which enables the grid bias of the valve to be varied, cut off occurring when the slider is at the earthy end of the resistor. The cathode voltage of valve 18 is applied through a voltage divider comprising resistors 33, 34 to the cathode of the valve 5, giving a substantially fixed bias suitable for volume compression in normal operation. As in the previous embodiment the A.C. output of the valve 18 is rectified by diode 22 and controls the valve 5 to provide contrast control; by moving the slider on the resistor 32 the anode current and gain of valve 18 are varied and hence the degree of volume compression. Due to anode circuit resistor 20 the anode voltage also varies with the position of the control 32, decreasing with increase in volume compression. The anode of valve 18 is connected by resistor 18 to the screen grid of valve 5, so that with increased volume compression the screen grid voltage is reduced, ultimately causing valve 5 to be cut off. Thus, adjustment of control 32 again first produces increased volume compression and finally a reduction of the second volume to zero. The point in the control at which volume control takes effect may be delayed by means of a clamping diode 38 connected between the screen grid of the valve 5 and a tapping point on resistor 41.