GB919207A - Improvements in or relating to gain control circuits - Google Patents

Abstract

919,207. Automatic gain control arrangements. ELECTRIC & MUSICAL INDUSTRIES Ltd. June 12, 1959 [March 27, 1958], No. 9838/58. Classes 40 (5) and 40 (6). [Also in Group XL (b)] In an arrangement comprising a plurality of amplifying channels, e.g. a simultaneous colour television system in which channels 1, 2, 3, Fig. 1, deal respectively with red, green and blue signals, means 8 are provided for applying at intervals to the said channels monitoring signals of substantially equal amplitude and the said signals are compared after amplification in each channel with a reference potential from a common reference source 13. the outputs of the comparison means 10-12 being utilized to control the gain of the respective channels. The arrangement is utilized to maintain the gain in all three channels equal: it may be varied simultaneously in all either by variation of the potential derived from the comparison source 13 or by variation by means of potentiometer 9 of the amplitude of the monitoring signal. The monitoring signal comprises rectangular pulses which are fed to the amplifying channels during line or frame return periods; the comparison means 10-12 are switched into an observing condition when the pulses appear in the outputs of the amplifying channels, pulses from the source 8 being fed to switches which connect the comparison means to the outputs of the amplifiers at the appropriate times. Each channel comprises a gain control stage 17, Fig. 2, and an amplifier 18 from which signals are passed via coupling condenser 19 and a D.C. reinserting clamp circuit 23 operating at line frequency to a cathode follower 20. This is coupled to a second cathode follower 22 via condenser 21 and D.C. reinserting clamp circuit 24 operating during frame return periods, which corrects for any errors associated with the line clamping circuits 23. The output of the cathode follower 22 is fed to a suppression circuit comprising diodes 25, to the anodes of which suppression pulses 26 are applied so as to remove from the signals the monitoring pulses. The signals are finally amplified in a further amplifier 27. Monitoring pulses are derived from generator 8 and are fed via potentiometer 9 to a cathode follower 28 and thence via preset resistor 9a to the input of the amplifier; they are also fed via similar resistors to the inputs of the other channels. The pulses are fed to the clamp circuits 23, 24 to render them inoperative when the monitoring pulses are present in the channel: they are also fed to a switch 29 connected to the output of the valve 22 so that when the pulses are present in the channel the switch is operated and condenser 30 is charged in dependence on the amplitude of the monitoring pulses appearing at the output of the valve 22. The charge on the condenser 30 is compared with a reference voltage, applied to line 43, in a cathodecoupled pair 31, 33, the difference being fed via lead 34 to the gain control stage 17. The reference potential may be derived from a manually adjustable potentiometer 13, or alternatively from a rectifier 35 connected via a low-pass filter 37 to the output of the channel, so that the rectified signals provide the reference potential. The reference potential is preferably obtained from all the channels, for which purpose a switch 42 which may be electronic, is provided, being operated at one-third line frequency or at frame repetition rate to connect the output of each channel in turn to the rectifier 35. Instead of deriving the reference potential from a rectifier fed with the three channels in sequence, it may be derived from that channel which passes the signal of greatest amplitude. Three diodes 64, 65, 66, Fig. 6, are fed from the respective channels, the reference potential being derived from the diode to which the signal of greatest amplitude is applied. The output of the diodes is fed to one valve 67 of a cathode-coupled pair to charge a condenser 68 which with resistance 69 provides a timeconstant of one field period. The other valve 70 is fed from line 71 with an overall gain control voltage derived from a manually variable potentiometer and the combined output from its anode is fed via potentiometer 72 and valve 73 to a switch 74 operated once every field so as to charge condenser 75 once every field. Condenser 75 feeds further valve 76, the output of which is fed via potentiometer 77 to the grid of one valve 78 of a cathode-coupled pair. The other 79 is fed with monitoring pulses via a switch 29 similar to that in Fig. 2, the valves 78, 79 comparing the reference potential and the amplitude of the monitoring pulses, an A.G.C. voltage proportional to the difference being derived from the anode of the valve 79 and fed to the stage 17 of Fig. 2. The potentiometer 77 also feeds similar cathode-coupled pairs in the other channels. Gain control stage 17. This may comprise a cathode-coupled pair (Fig. 3, not shown) with the signal applied to the grid of one valve and taken from the anode of the other, the gain control voltage being applied as variable bias to the grids of both valves. An alternative arrangement takes the form of a potentiometer with resistor 52, Fig. 4, as a fixed series arm and with valve 51, which is provided with 100% A.C. negative feedback via condenser 56, forming a variable shunt arm the effective impedance of which is varied by variation of the grid bias. The working point is stabilized by means of D.C. negative feedback provided by anode to grid resistance 57. In a modification (Fig. 5, not shown) the shunt arm of the potentiometer comprises the cathode circuit of a valve provided with variable grid bias.