Patent GB-474428-A (GB)
Title: Improvements in or relating to alternating current signalling systems
IPC: H04M1/58
Filed: 1936-04-30
Abstract: 474,428. Anti -- side tone circuits. RYALL, L. E. April 30, 1936, No. 12325. [Class 40 (iv)] In a substation or operator's set employing an amplifier and loud-speaker, an automatic volume control device controlled by directcurrent signals derived from speech prevents excessive side-tone for outgoing signals and enables incoming signals to be reproduced at a satisfactory level. An alternating device may be included in the transmitter circuit to block weak noise currents, the device being cut out when speech takes place. In the arrangement shown in Fig. 1 (Prov.), the amplifying valve PV and the primary winding of the output transformer T4 form one arm of a bridge of which the other arms are resistances R3.. R5. Control potential taken off from one diagonal of the bridge is passed through an automatic volume control device AVC comprising opposed rectifiers shunting a third winding of input transformer T3 with a resistance R2 between the junction of the rectifiers and the mid-point of the winding. The control circuit also passes through a further rectifier network N connected in the transmitter circuit. Local speech, but not weak noise, is able to pass through network N to the line L, and some of the speech also passes to valve PV producing a direct-current voltage across points 1, 7 which controls network AVC to prevent excessive side-tone in the loud-speaker. The control voltage also reduces the attenuation of the network N to a low level. Speech incoming over the line L if weak is unable to influence the network AVC and is reproduced with maximum amplification. Speech of greater intensity, however, influences network AVC to reduce the amplification factor and also reduces the attenuation of network N thus increasing the shunting effect across the input circuit T3. In a second arrangement, Fig. 2 (Prov.) (not shown), the valves are connected in two arms of the bridge, the second valve which feeds the loud-speaker having its grid-bias varied in accordance with the output of the first valve. In a modification of this arrangement, Fig. 3 (Prov.) (not shown), the control voltage from a diagonal of the bridge is applied only to network AVC and the network N is separately controlled by rectified signals taken from the output of the second valve. In a further arrangement shown in Fig. 1 (Comp.) the line L is associated with the transmitting and receiving circuits through a differential transformer T2 with a balancing line R1, and the control potential for both networks is obtained from the output circuit of a valve V2 normally biassed to give no anode current and controlled by rectified signals from the main output transformer T4. A potentiometer R2 is adjusted to give maximum signals without oscillation. Preferably the loss produced by network AVC becomes effective before the reduction of loss in the network N. The Provisional Specification states that the invention is also applicable to public address systems and deaf-aid sets. In public address systems, the network AVC may be omitted and the network N prevents howling conditions when the system is not in use. If the network AVC is provided, its loss is inserted after the loss N is removed.