Loudspeaker telephone system

Abstract

A loudspeaker telephone system is comprised of a voice switching circuit, hybrid, and power supply circuit. The voice switching circuit includes comparators for comparing speech through the main paths with a side tone through auxiliary paths, time constant circuits for controlling exponentially a pair of variable gain amplifiers for making or breaking the sending and receiving signals, and band pass filters arranged in each input of the comparators for rejecting indoor noise as well as the line noise coming into the loudspeaker telephone system through the lines. Accordingly, the howling effect between microphone and loudspeaker is negligible; transfer from a sending mode to a receiving mode is easily made while maintaining naturalness of the voice. By inclusion of impedance matching elements in a conventional hybrid, the hybrid can be used in common either in the present loudspeaker telephone system or in a conventional telephone system. In the loudspeaker telephone system, line current is used as a bias voltage by the power supply circuit; there is no need for providing an additional power supply source.