GB680544A - Improvements relating to circuits for establishing a d.c. component in electrical signals - Google Patents

Abstract

680,544. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. July 19, 1950 [July 27, 1949], No. 19707/49. Class 40 (iii). A circuit for levelling electrical signals in a channel at a point where the peak-to-peak level of the signals is inadequate for direct levelling, the signals including recurrent datum portions which in the absence of distortion should have a uniform level, comprises a subsidiary amplifier fed with signals tapped from the channel and means for developing a signal proportional to the difference between the level of the datum portions and the mean level of the signals at the output of the amplifier, and means for applying a fraction of the developed signal to the channel to bring the datum portions of the signals therein to a uniform level. As illustrated, the invention is applied to a television system where it is desired to apply signals from a point 2 in the main channel 1 to a monitoring channel 3, the peakto-peak voltage of the signals at point 2 being of the order of only 1 volt. Connected to point 2 is an amplifier stage 6 having a gain of the order of 12, the amplifier feeding a rectifier stage which develops a current in the loop 13- 16 proportional to the difference between the levels of the extremities of the synchronising pulses and the mean level of the signals at the anode of stage 6. This current is representative of the D.C. component of the signals and that fraction of the potential developed across resistors 14, 15, 16 which appears across resistors 15 and 16 is applied in series with resistor 5 in the coupling circuit 4, 5 between point 2 and the monitoring channel. The fraction is made adjustable by resistor 15, the necessary value being of the order of the reciprocal of the product of the gain of the amplifier stage and the efficiency of the rectifier stage. Point 17 returns to a bias potential. Condenser 19 smooths the potential developed across resistors 15 and 16. In an alternative arrangement a fraction of the whole of the amplified signals after the D.C. component has been established therein in a conventional manner may be fed to resistor 5.