Patent GB-246274-A (GB)
Title: Improvements in or relating to electric signal transmission systems
IPC: H04B3/10
Filed: 1924-12-04
Abstract: 246,274. Western Electric Co., Ltd., Western Electric Co., Inc.). Dec. 4, 1924. Systems for transmitting music etc.; amplifying systems. -In electric line or radio signalling systems, the initial signal current is amplified at the sending end and reduced at the receiving end (or vice versa), whenever the volume of the signal becomes less (or greater) than the average. In this way the signal amplitude in the transmitting medium is maintained substantially constant, thus preventing overloading of line repeaters and radio transmitters and receivers by strong initial signals, and preserving a favourable ratio of signal strength to line noise or atmospherics, when the initial signals are weak. In order to ensure simultaneous control in opposite sense of the amplifiers at the sending and receiving ends, a control current of a frequency that will not interfere with the signal, is supplied to the sending-amplifier, and variations in the received strength of the control current are utilized either automatically, or by hand control by a listening operator, to regulate the amplification at the receiver. Variations in the efficiency of the, transmission line or medium are reflected in the received strength of the control current, and enable the amplifier at the receiving end to be adjusted automatically or otherwise to bring the over all efficiency back to normal. In Fig. 1 the system is shown applied to line telephony with automatic correlation of the sending and receiving amplifiers. Signal current is supplied through a filter F and potentiometer 10 to the sending amplifier A, and thence over the. line L to a potentiometer 12, receiving amplifier A, and filter F, to the signal receiver. The potentiometer 10 is controlled by a linkage 11 operated bv a solenoid traversed by rectified signal current, in such a way that loud signals cause a reduction of amplification. Control current from an alternator G is supplied through a filter F<1> to the potentiometer 10. and at the. receiving end this control current is selected by a filter F<1>, and passed to a rectifier D1. The current in the output coil R, is proportional to the received strength of the control current, and whenever this is above or below normal it moves an armature into engagement with an upper or lower contact, thereby closing the circuit of one of the stepping-magnets 13, 14 in series with an interrupter 18. A shaft 16 rotated by the stepping- device 15 carries an arm 17 linked to the potentiometer 12, thus altering the amplification of A1 in inverse proportion to that of A. Signals passing to the signal receiver are thus maintained in correct intensity gradation. An alteration in line efficiency, due to change of temperature &c., also affects the received strength of the control current and causes a compensating change in amplification at the receiver. Owing to the delaying-action of the filter chain F on the signal currents, the potentiometer 10 is adjusted before the arrival of the signal currents at the amplifier A. By placing separate amplifiers and separate linked potentiometers beyond the filters F1, F1<SP>1> the same delayed action may be effected at the receiver. In a modification applied to wireless systems, amplification at the receiving end is controlled by hand by an operator, who listens to the received control current and keeps its strength equal to that of a standard current generated locally.