Is this all info you can give?
Klaus
I worked with Philips, Stentofon and TOA large high quality intercom systems. They were all half-duplex that used sound level for communication direction and none of them used AGC.
Years ago, Motorola introduced two intercom ICs that were also half duplex but they averaged then cancelled continuous background noise so that it did not affect communications direction.
Later I worked with high quality boardroom telephone-conference systems. They began to use an echo-cancelling IC for full duplex and sometimes it got confused and made funny noises.
Today, Polycom intercoms use an echo-cancelling IC for full duplex but they are VERY expensive.
The circuit I posted is old so it uses simple transistors instead of opamps. The trimpots R7 are adjusted for perfect cancellation so there is no feedback howling or squealing.usually a local opamp is used to subtract the local microphone signal from the line signal leaving only the remote signal into the local speaker - of course if there is too much audio connection from the local speaker to the mic at each end you will form a loop which can squeal if the total loop gain is too high ...
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