Patent GB-1207340-A (GB)
Title: Loud speaking telephone circuits
Inventor: BAKER ARTHUR HERBERT PATRICK
IPC: H04M9/08
Filed: 1967-12-07
Abstract: 1,207,340. Loud-speaking telephones. STANDARD TELEPHONES & CABLES Ltd. 7 Dec., 1967, No. 55741/67. Headings H4K and H4R. In a loud-speaking telephone in which the signal levels in the microphone and loud-speaker channels are compared and the gains of the two channels differentially varied to favour the channel with the greatest signal level, the output of the microphone channel is used to control the gain of the circuit feeding the loud-speaker channel signal to the comparison device to assist the subscriber in an attempt to break into the conversation. As described a loud-speaking telephone set Fig. 1 comprises a microphone 4 and a loud-speaker LS connected by respective amplifier channels 3 and 5, and a hybrid circuit 1 to a telephone line 2. The signal levels existing in the channels are fed via respective switching amplifiers, 9 and 10, to a switching circuit 8 where the amplified signal levels are detected, added in opposite polarity, and the resulting signals used to set a bi-stable trigger ST which differentially controls the gains of the microphone and loud-speaker channels to increase the gain of that channel carrying the greatest signal and reduce the gain of the other channel. To provide a guard against hybrid breakthrough from the microphone channel to the loud-speaker channel operating the switching circuit 8 to switch to receive mode, e.g. when the subscriber wishes to "break in", a winding 15 on the transformer 14 in the hybrid circuit feeds microphone signals, via a detector 12, to reduce the gain of the loud-speaker switching amplifier 10, A.G.C. is provided over the loudspeaker channel by a lamp 25, fed with the amplifier output, which controls the gain of an attenuator circuit 7 in the loud-speaker amplifier input. The switching control signal is taken from the A.G.C. circuit output in order that the switching sensitivity remains substantially constant irrespective of the signal level from the distant talker, see Specification 1,207,339. In order to provide "hysteresis" in the switching the switching circuit 8 controls the gain of the loud-speaker switching amplifier 10 over connection 29, so that when the set switches to the "send" condition the gain of the loud-speaker switching amplifier 10 is reduced so that a larger loud-speaker signal is required to change the switch over, than is required to maintain it once changed. In addition the output from the loud-speaker amplifier is fed, via connection 36, to a detector 11 the output from which controls the gain of the microphone switching amplifier 9 so that a higher microphone signal is required to effect switching to "send" condition when a signal is fed to the loud-speaker than when there is no incoming signal. Fig. 2 shows the microphone switching amplifier gain control arrangement in which the loud-speaker amplifier output is rectified in 51 and controls the base bias of transistor T3 whose collector emitter impedance controls the negative feedback of transistor T2 in the microphone switching amplifier (9 in Fig. 1). The output from the microphone switching amplifier is applied to two rectifying circuits in the switching circuit 8, Fig. 1, the output from one of which 29, is backed off by the output of the other, 33, which is responsive only to the continuous background noise picked up by the microphone so that the continuous background noise will not seize the switch circuit in the absence of speech in the microphone channel.