PURPOSE: To effectively prevent howling for a telephone set with speaker and a sufficiently assure the quality of communication signals, by estimating detour characteristics with an echo canceller for the cancellation of the detour signals.
CONSTITUTION: At the initial operation of an echo canceller circuit 5, an audio switching circuit 4 interrupts an input signal from a microphone 1 over a prescribed time and gives a training signal from a signal generator 9 to the circuit 5 via a level adjuster 6 in place of the input signal. Thus, the circuit 5 detects the magnitude of change in amount of detoured signals at a hybrid circuit 3 due to the change in line conditions including the initial line connection. A level discriminator 8 detects a signal level control value from the magnitude in this change, drives level adjusters 6 and 7 selectively and controls the level of transmission or reception signals.
KAWASAKI TADAMICHI
MINAMI SHIGENOBU
TOKYO SHIBAURA ELECTRIC CO