Abstract: A control unit for controlling operation of a plurality of audio teleconference terminals comprises a plurality of inputs which are connected each to a particular terminal. A speech detector detects from which terminals speech signals are received and actuates switching means to connect those terminals in open loop teleconference.

Abstract: A control unit for establishing a four-wire connection between two terminals via a pair of telephone lines comprises switching means which are operable to provide a connection to a pair of telephone lines, signalling means connectible to said lines via said switching means to enable another control unit to be signalled via a line, and line send and receive circuitry connected to the switching means. The line send and receive circuitry has an input for connection to the send channel of a terminal and an output for connection to the receive channel of the terminal. A central control controls operation of the switching means and line send and receive circuitry to automatically establish a four-wire connection and adjust parameters of the line send and receive circuitry to compensate for loss on the telephone line.

Abstract: An electro-acoustic terminal unit for use in an audio teleconferencing system comprises a loudspeaker and one or more microphones each having a sensitivity which is directionally dependent and exhibits at least one null or substantially null position. The loudspeaker and microphones are rigidly mounted on a boom and the microphones are so located and orientated relative to the loudspeaker that the null position is directed towards the loudspeaker.