Patent GB-715435-A (GB)
Title: Means for controlling signal level in radio and line signalling systems
IPC: H04B3/10
Filed: 1952-08-08
Abstract: 715,435. Telephone transmission systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. Aug. 8, 1952 [Sept. 12, 1951], No. 20028/52. Class 40 (4) [Also in Group XL (c)] The signal-to-noise ratio in a radio or line carrier signalling system is maintained substantially constant by the use of a pilot signal wave which, at the receiver, is separated from the message-bearing wave, and rectified to provide a measure of the noise in the system. This noise measure is used retroactively to control a variable amplifying or attenuating element at the receiver in such manner that the noise at the output of the element is kept constant. The control results in the amplitude of the message-bearing wave at the element output varying inversely as the measured noise, and this is counteracted by returning to the transmitter a signal representing the measured noise and utilizing it there to boost the message-bearing wave in proportion to the noise. Fig. 1 shows the system applied to a multiplex carrier system for radio or line communication, a series of message-bearing sub-carriers being combined at 14 and passed through a variable amplifier or attenuator 15 to modulate a carrier transmitter 11. At the receiving end of the radio link or line 12, the carrier is received at 13 and the message-bearing subcarriers are passed through a variable amplifier or attenuator 18 to the utilization circuits 22. A pilot wave from source 17 is mixed with the sub-carriers at the transmitter, and during transmission is modulated by interference &c. so that at the output of the receiver 13 it bears a modulation partly due to noise picked up in. transit, and partly due to thermal noise in the valves. The receiver pilot wave is separated by a filter 19 and after amplification at 20 is rectified at 21 and applied to the element 18 to vary its amplification or attenuation in such sense as to render the noise output from 18 substantially constant. In doing this, the message-bearing wave is varied in amplitude inversely with the noise level. To counteract this variation in message level, the pilot wave is selected by a filter 23 from either the input or output side of the element 18, according to the setting of a switch 24, and is passed back to the transmitter by a line 27 feeding a further filter 28, amplifier 29, and rectifier 30, to vary the amplification or attenuation of the element 15 in the reverse sense to the variation effected at the receiver 18. In this way, a substantially constant signal-to-noise ratio is produced at the input to the utilization circuits 22 despite changes in the noise level. The effect of fading is reduced or overcome by automatic gain control, limiters &c. in the receiver 13. If the pilot wave filters 23, 28 have very narrow frequency limits 20 so as to cut off the noise modulation, a control voltage is still obtained from the pilot wave because its amplitude, owing to the action of the element 18, will vary in inverse proportion to the noise level. In a modified system, Fig. 2 (not shown), the rectified signal from the rectifier 30 is also applied to vary the percentage modulation of the message-bearing wave and/or to'vary the mean amplitude of that wave, and/or to limit the signal strength of the separate primary signals of a multiplex transmitter. At the receiving station, the rectified pilot wave may be used to vary the threshold voltage below which devices placed in each channel of multiplex receiver are inoperative.