Patent GB-839975-A (GB)
Title: Improvements in and relating to signal switched telecommunication circuits
Inventor: PARKER LESLIE KEARTON
IPC: H04B3/20
Filed: 1955-07-28
Abstract: 839,975. Two-way transmission. TELEPHONE MANUFACTURING CO. Ltd. July 25, 1956 [July 28, 1955], No. 21867/55. Class 40 (4). In a two-way repeater or loud - speaking telephone terminal a voice-switched attenuator is connected in each path at a point where amplification is incomplete. In the two-way repeater shown in Fig. 2, attenuators 12, 13 in the transmit and receive paths 1, 2 are connected at the input sides of the amplifiers 14, 15. The attenuators are controlled by a differential bridge 9 which is fed with D.C. signals derived from points Y1, Y2, as shown, or from points Z1, Z2. If the D.C. signal is derived from point X, the intensity may be sufficient to enable the transmit path to be seized at any time. Pre-amplifiers AP, BP may be used. One differential bridge is designed to respond to signals without delay. Another includes an automatic level control arrangement. Loud-speaking telephone terminal. Fig. 5.- A terminal comprises a line 24 linked through a main hum rejector L1, C9 and sections TR4, TR5 of a hybrid transformer to balanced amplifiers V2, V3 with negative feed-back in the transmit and receive paths. Line relay RA controls the power circuit 26. The microphone M feeds a constant current pre-amplifier V1 which is transformer-coupled to the transmitting amplifier V2. The input to V2 is controlled by an attenuator comprising resistors RM, RN, capacitors CM, CN and rectifiers MR3, MR4 connected across the transformer secondary. The impedance of the rectifiers, and hence the attenuation is determined by the bias applied thereto from control points A, B. The D.C. component of the anode current of the transmitting amplifier V2 is controlled by a D.C. signal derived from the output transformer TR6 and rectifier MR7 in the receive path and transmits speech to line from microphone M and transformer TR3. A delay circuit C14, C15, R30, R31 ensures continuity of control between syllables of speech in the receiving path. The same D.C. signal controls a muting circuit R8, C4 for the pre-amplifier V1 and speeds up the control operation. The attenuator in the input to the receiving amplifier V3 comprises rectifiers MR5, MR6 and a resistor RV2. The amplifier V3 is controlled by a D.C. signal derived from the output of pre-amplifier V1 by means of a rectifier MR1 and a delay circuit R2, C1 and transmits speech from the line to the loudspeaker L. The attenuators are controlled by biasing potentials applied from terminals A, B in the anode circuits of V2 and V3 to the rectifiers in the attenuators. The circuit in which V2, V3 provide potential at terminals A and B in response to D.C. control signals constitutes the differential bridge circuit. In a modification, Fig. 6 (not shown), pentodes are used instead of triodes. The attenuators are connected across auxiliary windings on transformers in the two channels. Modified differential bridges.-A differential bridge with a high-speed response comprises two triodes V10, V20, Fig. 7a, with a common cathode resistor RC. When the two grids GA, GB are at different potentials a potential V2 is applied to the load 2 which may be a variable attenuator network. The anode to grid crossconnections RX, CX increase the speed of response. In another differential amplifier, Fig. 8a, the signals from two input transformers TRA, TRB are limited by rectifiers MRXA, MRXB which conduct if the grid potential of one triode rises above the cathode potential of the other. The two bridges may be used in the two-way arrangement of Fig. 2. Specification 701,607 is referred to.