Automatic impedance appts. for telephone network - momentarily samples impedance magnitude and stores that data to provide stable information to adaptation system

摘要

The telephone has a microphone (6) coupled through an amplifier (7) to a telephone line (11). The amplifier(7) also feeds a network (8), connected to the telephone line, which subtracts the signals received from the amplifier and the line and applies the result through an amplifier (9) to the earpiece. The network ensures no microphone signals reach the earpiece. To adapt the unit to the telephone line impedance and ensure efficient operation, a circuit (1) measures the line resistance and records its digital conversion in a memory (3), via a digital processor (15). The processor controls the network (8), and the gains of the two amplifiers in dependence on the line impedance. The control is effected sufficiently rapidly to have no affect on user operation.