Full duplex telephone set using echo cancellation and side tone management for high gain stability

In a full duplex telephone comprising a portion of a closed loop in which an analog signal transmitted by the telephone is fed back onto an analog signal received by the telephone as a line echo and side tone signal, and in which the received signal is fed back onto the transmitted signal as an acoustic echo signal, a method for increasing the amount of gain that can be applied to the received signal by at least 50 dB without exceeding a total closed loop gain of unity, and without sacrificing the full duplex capability of the telephone, the method comprising:

• canceling the line echo and side tone signal from the received signal to leave a remaining received signal;
• canceling the acoustic echo signal from the transmitted signal to leave a remaining transmitted signal;
• conditioning the level of the remaining transmitted signal to match the level of the remaining received signal;
• combining the remaining received and conditioned remaining transmitted signals to form a modified received signal,
• applying at least one of amplification, compression, and soft limiting the modified received signal to form an enhanced received signal;
• detecting first periods during which the remaining transmitted signal is zero and the remaining received signal is non-zero;
• detecting second periods during which the remaining received signal is zero and the remaining transmitted signal is non-zero;
• measuring the level of the enhanced received signal during the first periods and storing the levels measured;
• and,
• increasing or decreasing the level of the remaining received signal during the second periods such that the level of the enhanced received signal matches that of the enhanced received signal during the first periods.