TELEPHONE CONFERENCE AMPLIFIER

摘要

AN A telephone conference amplifier consists of a plurality of identical channels, one per subscriber participating in a conference, connected to a common point. Each channel includes a transformer having a primary winding and two secondary windings. The primary winding is connected to the corresponding subscriber's line to send and receive speech signals to and from that subscriber. The first secondary winding is connected to an active rectifier and smoothing circuit which produces when the subscriber is talking an envelope signal corresponding to the speech signal being received from the subscriber. This envelope signal is added to the speech signal to produce a unidirectional signal which is applied to the common point via an ideal diode circuit. Signals on the common point are picked off by a second ideal diode in each channel, and only if the signals have not originated from that channel are they transmitted to the subscriber via the transformer. The signals picked off from the common point which have not originated in the particular channel are also used to establish a threshold level for the active rectifier so as to reduce the level of the envelope signal and hence suppress incidental sounds from the subscriber while another subscriber is talking. A transistor is used in the threshold establishing circuit and the resultant temperature dependence of the threshold level is compensated by a temperature dependent resistor. Each channel also includes an automatic level control circuit which compensates for the wide range of signal levels incoming from subscriber's lines.