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The technical term that you're looking for is acoustic echo cancellation[1]. It's a fairly common problem in signal processing, and comes up in "simple" device

The technical term that you're looking for is acoustic echo cancellation[1]. It's a fairly common problem in signal processing, and comes up in "simple" devices like telephones too. [1] https://www.mathworks.com/help/audio/ug/acoustic-echo-cancellation-aec.html# https://www.mathworks.com/help/audio/ug/acoustic-echo-cancel...

mananaysiempre 2y ago
I seem to remember analog telephone lines used a very simple but magic-looking transformer-based circuit of some sort for this purpose. Presumably that worked because they didn’t need to worry about a processing delay?
- ajb 2y ago
  That's slightly different. analog phone lines suffer from 'line echo', which is an echo generated by the phone line itself - because the same pair of wires is used for signals traveling in both directions. If you're thinking of the 'hybrid' , which connects the phone line to the speaker and mic, that matches the impedence of each of the three pairs of wire (to the mic, speaker, and phone line) to avoid generating echo in the first place. But they weren't perfect (and echo could be generated at other points in the line) so later, digital electronics was used to do first "echo suppression" (just zero any signal below certain magnitude) and then echo cancellation, which is very similar to acoustic echo cancellation (an AEC can probably do anything an LEC can do, but the LEC is less capable). I'm not aware of anyone doing echo cancellation using an analog circuit, but that doesn't mean no-one did. I guess it's theoretically possible but I don't see how the adaption could work.
- chadcmulligan 2y ago
  There used to be a transformer in phones for side tone, a small amount of what you say is piped back into the earpiece, they did this because they found people would shout if they couldn't here their own voice. I've often wished mobiles would do this.

mananaysiempre 2y ago

I seem to remember analog telephone lines used a very simple but magic-looking transformer-based circuit of some sort for this purpose. Presumably that worked because they didn’t need to worry about a processing delay?

ajb 2y ago
That's slightly different. analog phone lines suffer from 'line echo', which is an echo generated by the phone line itself - because the same pair of wires is used for signals traveling in both directions. If you're thinking of the 'hybrid' , which connects the phone line to the speaker and mic, that matches the impedence of each of the three pairs of wire (to the mic, speaker, and phone line) to avoid generating echo in the first place. But they weren't perfect (and echo could be generated at other points in the line) so later, digital electronics was used to do first "echo suppression" (just zero any signal below certain magnitude) and then echo cancellation, which is very similar to acoustic echo cancellation (an AEC can probably do anything an LEC can do, but the LEC is less capable). I'm not aware of anyone doing echo cancellation using an analog circuit, but that doesn't mean no-one did. I guess it's theoretically possible but I don't see how the adaption could work.
chadcmulligan 2y ago
There used to be a transformer in phones for side tone, a small amount of what you say is piped back into the earpiece, they did this because they found people would shout if they couldn't here their own voice. I've often wished mobiles would do this.