475,310. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. Dec. 11‹, 1936, No. 34064. Convention date, Dec. 11, 1935. [Class 40 (iv)] [Also in Group XL] In a system using volume range compression and subsequent expansion, gain or loss is controlled by non-linear impedance elements, such as elements comprising silicon carbide, included in a negative feed-back path of an amplifier the impedance of the elements being varied by passing through them a control current, preferably alternating current of high frequency, of amplitude determined by the signal level. In the system shown in Fig. 1, volume range is compressed in an amplifier 4 having its gain controlled by loss elements in a negative feed-back path. The output of the amplifier is applied over a similar control amplifier 5 to a control circuit 7 which applies high frequency control current to the impedance elements in the feed-back paths of both the amplifiers 4 and 5. The expander comprises a variable loss device 8 controlled by a feed back control amplifier 10 and control circuit 12 similar to those used at the compresser. Volume compression, Fig. 2. The line amplifier comprises resistance-capacity coupled valves 13-15 and a " series type " feed-back path extends from resistance 17 common to the anode and grid circuits of valve 15, over a bridge 16 of non-linear elements to a similar resistance 18 in the circuit of valve 13. The D.C. potential connected to the horizontal diagonal of the bridge is adjusted to zero by a resistance network 24, associated with plate battery 26, and the variable frequency characteristic of the bridge 16 is compensated by network 20. The output of the amplifier is tapped off at 6 and fed to a similar feed-back amplifier 13<1>-15<1> the output of which is fed to a control circuit comprising a pair of push-pull valves 31, 32 normally biassed beyond cut-off. When the inputto the control circuit exceeds a limiting value, valves 31, 32 pass current, condenser 36 is charged rapidly over diode 38 and condenser 35 is charged over resistance 37 equivalent to the conducting resistance of the diode. These charges reduce the normally negative grid bias on valve 41 which thereupon transmits high-frequency of proportionate amplitude from oscillator 44 over transformers 42, 43 to the vertical diagonals of the bridges in the feed-back paths of the line and control amplifiers thereby reducing the impedance of the non-linear elements, increasing the feed back and reducing gain. When the output of the line amplifier falls, condensers 36, 35 discharge relatively slowly over resistances 40 and 39. The reduction in gain due to an increase in the input to the line amplifier is shared equally by the line amplifier and control amplifier so that the increase in level in the output of the line amplifier is halved. A modified compresser, Fig. 4 (not shown) utilizes a " shunt type " feed-back connection between the anode circuit of valve 15 and the grid circuit of valve 13, the variable frequency characteristic of the bridge 16 being compensated by a phase opposed feed back over another non-linear element. Volume expansion, Fig. 3. Expansion is effected by a bridge of non-linear impedances 50 connected in one leg of the line, the frequency characteristic of the bridge being equalized by a network 56 connected in the other leg of the line. The control amplifier 13<11>-15<11> and control circuit are the same as the corresponding portions of Fig. 2, and an increase in the input to the expander causes the subjection of the bridges to control high frequency, but in the present case the resultant reduction of the impedance of the bridge at the expander, increases the output therefrom. The network 56 has the same frequency characteristic as the impedance bridge 50 but feeds forward currents in phase opposition to those fed forward by the bridge.