Echo cancellation circuit

An echo cancellation circuit in a full duplex two-way communication system in which a first communication device and a second communication device make two-way communication with each other through a same signal cable, the echo cancellation circuit comprising:

• an input/output terminal for transmitting a first transmission signal from the first communication device to the second communication device or receiving a second transmission signal transmitted from the second communication device;
• a subtractor having a positive input terminal and a negative input terminal, in which the first transmission signal is inputted to the negative input terminal as a pseudo echo signal, the first transmission signal is inputted through an output buffer to the positive input terminal as an echo signal, the pseudo echo signal inputted to the negative input terminal is subtracted from the echo signal inputted to the positive input terminal;
• and a result of the subtraction is outputted to the first communication device as an echo cancellation output;
• a detection circuit unit for performing any one of 0 value detection, non-zero value detection, and 0 value non-detection for the first transmission signal, and for performing either one of the 0 value detection and the non-zero value detection as well as amplitude detection for the echo cancellation output;
• and
• a control circuit for controlling at least one of echo signal and pseudo echo signal that are inputted to the subtractor or the echo cancellation output outputted from the subtractor, based on a detection result of the detection circuit unit, so that the echo cancellation output amplitude or a value regulating the output amplitude is adjusted to be optimized.