GB540454A - Automatic gain control system - Google Patents

Abstract

540,454. Automatic volume control. PHILCO RADIO & TELEVISION CORPORATION. April 16, 1940, No. 6850. Convention date, May 24, 1939. [Class 40 (v)] In a composite signal system employing a signal having components established on opposite sides of a predetermined amplitude level, a signal channel means for deriving a signal from said channel, a gain control valve operable by said described signal, means for causing the effective anode current of said valve to decrease rapidly and abruptly from a substantial value to cut off, means whereby the operating conditions of said valve can be adjusted so that said amplitude level is normally slightly above the cut-off point of the valve, to render the valve fully responsive to one of the said signal components and but slightly responsive to the other component, thereby to obtain an output signal which is substantially independent of variations of the other signal component, and means for applying the output signal to the signal channel to control the gain thereof. Fig. 1 shows the invention applied to a television receiver. The control signal is taken from the LF. output and is applied to the grid of a triode 9 across a leak resistor 10 connected to a source of negative bias 11. The cathode is slightly less negative, being connected to the centre tap of a voltage divider 12, 13. The plate load resistor 16 is connected by a smoothing filter 18, 19 to the anode of a diode 20 from where the A.V.C. voltage is fed over a line 21 to the I.F. amplifier or earlier stage. Until the signal applied to the grid of valve 9 is sufficiently large to cause the plate to go negative, no negative control voltage is developed across condenser 19 but when the plate goes negative diode 20 ceases to conduct, and condenser 19 is charged through resistor 18. When the signal declines condenser 19 discharges until another signal peak occurs to drive the plate of valve 9 negative. Curves are shown to demonstrate that this arrangement gives a control voltage which is large relative to the change in signal amplitude and as applied to a standard television signal serves to maintain the blanking level and the amplitudes of the synchronizing pulses constant. The use of the diode 20 introduces a delay action which may be varied by altering the bias of the diode. Specification 537,856 is referred to.