Echo cancellation for speaker phone

[efendi]

Referring to previous thread https://www.edaboard.com/threads/313327/
Hello
I want to design speakerphone but I get echo and howling is this circuit useful and the are many schematic any one is the final and that operate correctly.
thanks

 

[FvM]

Re: Echo canceller Schematic

I want to design speakerphone but I get echo and howling is this circuit useful and the are many schematic any one is the final and that operate correctly.

I fear you misunderstood the meaning of the thread title. It's about hybrid circuits used with telephone lines and partly about microphone squelch.

Feedback cancellation for "speaker phones" is a different topic. The best solution is careful setting of the microphone gain, using a good directional microphone and in some cases an equalizer.

Some people tried to apply a frequency shift with a single side band modulator circuit to suppress feedback, but sound is rather ugly.

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I finally noticed that I misunderstand your question. The original thread is discussing in part circuits that can be used for speaker phones.

 

[Audioguru]

The howling is not caused by echoes. Instead the howling is caused by the speaker sound being heard by the microphone then is amplified. Since the telephone line has only two wires with transmission plus reception on it then the telephone circuit must use a hybrid circuit to cancel the feedback. Most cheap speakerphones use "voice switching" that mutes the microphone when receiving distant telephone sounds and mutes the speaker when the local person is talking.

You can have a speakerphone without switching but its hybrid circuit cancels its mic to speaker coupling, talking to another speakerphone the same and have long distance feedback where a sound is transmitted by the local telephone and played on the distant speaker then the distant microphone hears it and sends the same sound back to the local speaker that is heard by the local microphone and is sent back to the distant speaker that is .....