Patent GB-1010295-A (GB)
Title: An arrangement for the stepwise pilot-controlled level control of communication transmission systems
IPC: H04B3/10
Filed: 1962-07-27
Abstract: 1,010,295. Pilot controlled automatic gain control. TELEFUNKEN PATENTVERWERTUNGS, G.m.b.H. July 27, 1962 [Aug. 1, 1961; Aug. 10, 1961], No. 28933/62. Heading H4R. In a pilot controlled automatic gain control system in which the level of the pilot voltage is monitored by a voltage discriminator which responds on the occurrence of a change in pilot level by adjusting the stepping direction of a gain adjusting device, e.g. of Specification 992,663, which supplies stepwise adjusted current to a regulating member, e.g. a thermistor, on receipt of the pulses from a pulse generator, the pulse generator is coupled to the discriminator so that it starts generating pulses a predetermined short time after the discriminator is energized by a change in level of the pilot. Fig. 3 shows one embodiment in which the pilot signal voltage is applied via terminals 1 and 2 to Schmitt Triggers ST1 and ST2. With normal level of pilot signal, transistors TS1 and TS4 are conducting and the astable multivibrators TG1 and TG2 are prevented from operating since the Schmitt Triggers, via diodes D1 and D2, hold transistors TS5 and TS7 cut off. An increase in the pilot signal voltage causes transistor TS3 to conduct and Schmitt Trigger ST2 to operate cutting off transistor TS4 and allowing mono-stable multivibrator TG2 to oscillate, the first pulse being applied, via differentiating capacitor C2 and isolating diode D4, to the gain adjusting device EV after a period of time dependent on the charging time of capacitor C4 via resistor R6. Similarly should the pilot voltage fall, Schmitt Trigger ST2 is operated and " enables " the pulse generator TG2, the differentiated pulses of which operate the adjusting device EV. In a modification, Fig. 5, the backlash of the Schmitt triggers ST1 and ST2 is overcome by resetting the operated trigger to its normal condition after each pulse has been transmitted to the adjusting device EV. In this embodiment the Schmitt Triggers ST1 and ST2 control pulse generators each comprising two mono-stable multivibrators, e.g. M1 and M2 or M3 and M4. The mode of operation is that when the trigger, e.g. ST1, operates as a result of a deviation from normal of the pilot voltage level the monostable multivibrator M1 is triggered via C5. At the end of the pulse duration, which is comparatively short, multivibrator M2 is triggered over capacitor C6, and a stepping pulse is immediately transmitted to the adjusting device EV. At the end of this pulse duration, which is comparatively long, the multivibrator M2 returns to normal and delivers a differentiated pulse, via phase inverting transistor TS16, diode D4, and capacitor C4, to the base of transistor TS2 to reset trigger ST1. At the end of the reset pulse, if the pilot voltage is still higher than normal, the trigger ST1 again operates and the process repeats until the pilot voltage is within required limits. In a similar fashion trigger ST2 operates, generates a stepping pulse, and is reset, on the occurrence of a fall in pilot signal voltage; in this case, however, the phase inverting transistor, i.e. TS16, is not required since the output of transistor TS14 is of the correct polarity to reset trigger TS2 directly. In addition, the two channels are interconnected via diodes D5 and D6 which, during the generation of the pulse in one channel hold the Schmitt trigger in the other channel in its normal condition.