Two-way loudspeaker telephone circuit - has two way amplification control with speech current in one direction affecting coupling in other direction

摘要

The two-way loudspeaker telephone enables working in both transmitting and receiving directions and there is an amplification control device which operates in both directions of transmission. In each transmission path for the microphone in the transmitter and the loudspeaker in the receiver mode of working there is at least one optical coupler (OK1, OK2) which consists of a light emitting diode and a phototransistor. Each of these is controlled in dependence upon the speech current flowing through the other. The base electrodes of the phototransistors may have applied to them a positive voltage dependent upon the speech current.