537,893. Valve amplifying and generating circuits. STANDARD TELEPHONES & CABLES, Ltd. April 19, 1940, No 7102. Convention date, June 21, 1939. [Class 40 (v)] [Also in Group XXXIX] Signals at repeating and receiving stations are maintained at a predetermined intensity relative to the transmitted signals under the joint control of the signals and of an auxiliary wave the intensity of which is varied inversely as the original signal intensity. The control is applicable to audio-frequency, line carrier and wireless transmission systems and compensates only the changes in attenuation due to the transmission medium. Changes in voice level and the number of channels in use in a carrier multiplex system have no effect but the control voltage available without amplification is larger than in the usual auxiliary wave control system. In some arrangements described, the transmitting amplifier is caused to oscillate or a four-wire circuit to sing at the required auxiliary frequency. In the arrangement shown in Fig. 1, the transmitting amplifier TA in a carrier multiplex system is supplied with signals from S and an auxiliary wave from P, its output amplitude being maintained constant by a control voltage applied to the auxiliary wave amplifier PA and derived from the total output of TA by a rectifying and smoothing circuit IC. The output of the repeating amplifier RA is maintained constant by a control voltage applied by a similar rectifying and smoothing circuit IC<SP>1</SP>. The auxiliary frequency may be at the upper edge of the multiplex bands. In a modification, Fig. 2 (not shown) the auxiliary amplifier PA is controlled by a negative feed-back circuit including a non-linear resistance of silver sulphide, fused boron or uranium oxide with a high temperature co-efficient heated by a winding in the output of a 60<SP>N</SP> supply amplifier controlled by the circuit IC. In a further modification, Fig. 7 (not shown), the voltage controlling the amplifier is mainly derived from a circuit including a non-linear resistance depending on output energy of the transmitting amplifier but in part derived from a more quickly acting rectifying circuit. The repeater amplifier may be controlled by a non-linear resistance in a feed-back circuit. In another modification, Figs. 3 and 4 (not shown), compensation is also effected for variations in the shape of the attenuation characteristic by the use of three auxiliary frequencies, one at the upper edge of the band for controlling flat gain as described above, a second at the lower edge for controlling, jointly with neighbouring signal frequencies, the slope of the frequency characteristic, and a third at a suitable intermediate frequency for similarly controlling its curvature. These auxiliary frequencies are used to control networks in a feed-back path of the repeater, variation being effected by non-linear resistances. Transmitting amplifier, Fig. 9, and modifications thereof oscillating at the auxiliary frequency. The amplifier 45 has a negative feed-back including a self-heating non-linear resistance 40 and associated impedances 48-53 so that the amplifier maintains a substantially constant and uniform gain over the transmitted band. The amplifier also has a positive feed-back path comprising a transformer 57 or other phase shifting network, an attenuation pad 56, and a piezo-electric crystal 55 which rejects frequencies other than the desired pilot frequency. As the output is maintained constant by the negative feedback, the auxiliary wave intensity due to positive feed-back varies inversely as signal intensity. The positive feed-back path is associated with the input and output circuits of the amplifier by means of conjugate bridge circuits. In a modification, Fig. 13 (not shown), an amplifier with a uniform gain characteristic is associated by means of hybrid coils with the signal source, the line, and a positive feed-back circuit, and the feed-back circuit includes a narrow band filter passing the required auxiliary frequency and a self-heated non-linear resistance in shunt which aiso receives a portion of the signal waves so that the amount of the feed-back and therefore the intensity of the auxiliary wave varies inversely as signal intensity. Any distortion due to the non-linear resistances is prevented from feeding backwardly by the filter and forwardly by the conjugacy of the output hybrid coil. In other modifications, Figs. 14 to 16 (not shown), using both positive and negative feed-back, the positive feed-back extends over the last two stages of a three valve amplifier and is controlled by a shunt non- linear resistance, the negative feed-back being extended over the three stages. Negative feed-back is restricted by a crystal to frequencies other than the auxiliary frequencies,'or the positive feed-back may be restricted to the auxiliary in a similar manner. Applications to 4-wire circuits, Fig. 10, and modifications thereof. The transmitting amplifiers 105, 106 for the two directions are of fixed gain and the corresponding repeating amplifiers 101-104 have constant outputs controlled by negative feed-back parts having non-linear resistances 121, and equalizers Z for compensating the frequency characteristics of the lines L1-L4. Filters F1 and resistances 109, 110 constitute a singing loop providing an auxiliary frequency of the net gain around the loop and the phase conditions are suitable. When signals are absent, the variable gain amplifiers have their constant outputs entirely constituted by the auxiliary frequency but when signals are present the intensity of the auxiliary is correspondingly reduced. A similar loop F2, and resistances 113, 114 serves to produce another auxiliary frequency for the E-W transmission. It has been assumed above that for a W-E transmission, for example, the E-W amplifiers have no variable effect on the F1 auxiliary intensity. This may be substantially realized by making resistance 109 large and 110 small so that the auxiliary intensity in the E-W path is so small as to have no controlling effect. In modifications however, Figs. 11 and 12 (not shown), each auxiliary wave is also arranged to traverse a greater number of amplifiers in its corresponding transmission path than in the opposite path.