Patent GB-507344-A (GB)
Title: Improvements in or relating to variable attenuator networks
IPC: H04B3/20
Filed: 1937-12-16
Abstract: 507,344. Transmission systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and JEFFERSON, H. Dec. 16, 1937, No. 34922. [Class 40 (iv)] [Also in Group XL] A variable attenuator, for either manual or automatic control, comprises non-linear resistances such as copper-oxide rectifiers 1, 2, arranged as arms of a Wheatstone bridge, whose diagonals are the input and output circuits, and supplied through terminals 11 with control current whereby the bias on the non-linear elements 1, 2 is varied for the purpose of varying the attenuation constant of the network. In the arrangement shown in Fig. 1, the control current is supplied through centre tappings on the input and output transformers 7, 9. In modifications, the resistances 3, 4 shunting the non-linear elements may be omitted, Fig. 3 (not shown) ; or diodes may be used instead of copper-oxide rectifiers, Fig. 4 (not shown); or additional resistances may be connected in series with the input and output terminals of the bridge, Fig. 6 (not shown), or in shunt with them, Fig. 7 (not shown). As the control current applied through terminals 11 is raised, the attenuation rises to a maximum and then decreases. Gain control ; volume-compression, volumeexpansion and current-limitation. The attenuator may be connected in cascade with an amplifier, and either the input or the output of the attenuator, or a combination of them, may be applied to terminals 11 to control the attenuation. The circuit shown in Fig. 5 may be interposed for modifying the control current, the anode-cathode path in valve V3 being connected to terminals 11, Fig. 1, in shunt with resistance R3, while the input or output voltage of the attenuator, or a comhination of them, is applied through a potentiometer P1, Fig. 5, to a valve V1 whose output, modified by the non-linear shunt R5, M3, M4, is rectified and applied to the valve V3 through a network C1, R1, C2, R3. The rates at which the valve V3 responds to an increase and to a decrease in the strength of the signals passing through the attenuator, Fig. 1, may be adjusted by varying the resistances R1, R2, Fig. 5, and may be different in consequence of the action of the rectifiers D1, D2. The valve. V3 may have variable mu or constant mu. In a volume-compressor, the arrangement is such that when signals are strong, so that the valve V3 has a large negative grid-potential, the attenuation of the signals passing through the attenuator, Fig. 1, is a maximum; in the volume-expander the reverse arrangement is adapted. By suitable choice of valves and biassing potentials the system can be made to yield a current-limiting effect. A volume-expansion effect may be produced over part of the volume range and a currentlimiting effect over another part. Eliminating noise. The system above described may be arranged to produce maximum attenuation when noise alone is present, and a reduced attenuation when conversation is superimposed on the noise. For this purpose, frequency-discriminating networks are introduced into the control circuit, Fig. 5. The invention may also be used in fourwire telephone systems to prevent singing, an attenuator in one speech channel being controlled by a current derived from speech in the other channel or by a pilot current.