Patent GB-790489-A (GB)
Title: Improvements in or relating to electrical systems for transmitting speech in two directions
Inventor: FLURY KARL
IPC: H04M9/00
Filed: 1955-02-21
Abstract: 790,489. Two-way transmission. DEFENSOR AKT.-GES. Feb. 21, 1955, No. 5065/55. Class 40 (4). In a loud-speaker communication system echo and acoustic reaction suppression arrangement, the gain of line amplifiers for the two directions is controlled by a single potential, developed at a common point in a control circuit by two opposed potentials which are dependent on the signal intensity in the respective directions, in such a manner that the gain in the speaking direction is enhanced and in the other direction diminished. Gain control of receive amplifier 20, 21 and send amplifier 40- 42, is effected by adjustable potential dividers 24, 25, 29, 28 and 44, 45, 57, 52 respectively in which the internal resistance of valves 28 and 52 is varied in converse manner by the control potential, the control potential which is developed at a point 102 in a control circuit comprising amplifiers 95, 103 and rectifiers 98, 106, being fed direct to the grid of valve 28 in the receive amplifier so that a positive potential such as would arise at point 102 from outgoing speech in microphone 51 reduces the gain and a negative potential increases it and via a phase inverter stage 55 to valve 52 in the send amplifier to achieve the converse effect. Additional valves 28a and 52a may be connected in parallel with valves 28 and 52. Valves 32 and 59 which have their grids biased in dependence on the output of their respective line amplifier are connected so that their internal resistance shunts the signal path and limits the amplifier outputs. Manually adjustable stepped gain controls 67, 68 in the respective amplifiers are mechanically coupled to maintain the loop gain below the level at which singing occurs and a further switch 61 also mechanically coupled to switch 67 disconnects the limiting circuit in the send amplifier when the send amplifier gain is reduced in consequence upon stepping up the receive gain. Switches 75, 78 enable the apparatus shown to be employed over a two-wire or a four-wire circuit and a hybrid arrangement comprising transformers 71-73 is provided for the former. The apparatus includes a dial, and terminals for the connection of a conversation recorder, and a relay 89 disconnects the receive amplifier to prevent clicks and howling during dialling. Amplifier performance-control voltage characteristics are described with reference to Figs. 2 and 3 (not shown). To overcome the effect of ambient noise at the sending end the positive control-potential rectifier 106 is biased by a source C so that the controlpotential is not developed until the " go " signal exceeds a predetermined threshold value which may be pre-set by a potentiometer 111. Potentiometers 50 and 66 are mechanically coupled to potentiometer 111 so that the receive gain is increased and the send gain reduced, and a condenser 112 is shunted across resistor 57 to shunt higher noise frequencies. A two-wire-four-wire push-button switching arrangement, Fig. 4, for a station which is to be normally connected to a four-wire intercommunication system has a beak 125 for locking the switch lever 120 when depressed for connection to a two-wire telephone system and incorporates a synchronous motor 132 driving through magnetic clutches 135, 145, a switch release arrangement wherein contacts 131 energize a release magnet 130 after a predetermined period of no outgoing speech and a call timing arrangement 148 respectively. Clutch magnet 136 is incorporated in the anode circuit of the phase inverter 55. An alternative key release arrangement including a gas discharge tube controlled by the cathode potential of phase inverter 55 and a high time-constant delay circuit is described with reference to Fig. 5 (not shown) and alternative receive and send amplifier gain control arrangements in which the adjustable resistance tubes 28 and 55 of Fig. 1 are controlled jointly by the control potential and the respective amplifier outputs are described with reference to Figs. 6 and 7 (not shown) respectively. The control circuit in a modified suppressor arrangement, Fig. 8, includes preamplifiers 185 and 190 followed by noise filters 187-189 and 195-197, and the preamplifiers are each biased to cut-off when the signal in the other is exercising control. This is achieved in the case of incoming speech over a connection, which includes a limiting rectifier 93, from point 102 to the grid of valve 190, and during outgoing speech valve 185 is biased from the cathode circuit of phase inverter 55. A cathode follower preamplifier stage 175 is included in the grid circuit of valve 28 and change-over is delayed by networks 178, 179 and 181-184. A system including arrangements according to the invention and wherein occupants of flats may hold a loud-speaker conversation with a caller in the hall in response to his operation of the doorbell is described with reference to Fig. 9 (not shown). A further system, Fig. 10, has a push-button switching unit 220 at the main station for selection of a wanted satellite station, which unit includes a buzzer 244 and calling lamps 241-243 whereby a calling satellite is indicated if the main station is free. The calling circuit which is completed by switch 246 at a satellite and a contact on each of switches 236-238 cannot be established if one of the latter is actuated. Volume controls 229, 230,.234, 235 at the satellite station are also mechanically coupled to limit loop gain. A two-wire substation 261, Fig. 11, includes an amplifier 265 energized from supply mains at 269 and is connected to terminals 79, Fig. 1, through a subscriber's unit 250 in which the buzzer and calling lamp circuit is energized over a tuned relay 252. Battery 255 supplies the microphone current. The use of amplifiers incorporating anti-howling arrangements according to the invention as audio-amplifiers in a two-way multiplex radio system is described with reference to Fig. 12 (not shown) and a portable two-way radio station with loudspeaker output in which the microphone and loud-speaker leads and the aerial are provided within the equipment shoulder strap is described with reference to Fig. 13 (not shown).