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I'm trying to transmit data(bytes) from a speaker (out) to microphone (in) via an FM modulated link. Now I have the problem that both will be used simultaneously (full duplex) and next to each other (so I have one transceiver) so the microphone (in) will catch sound generated by the speaker. Now I want to cancel out the received audio so that it is not transmitted back to the sender.

For clarification: The data modulation will be FM (Frequency modulation) and I'm a total noob in this case. The 'Data' will be sent in Bytes not voice recording. I use audio waves because those travel underwater way better than radio waves.

I hope I described my problem enough else here is another clarification:

A module is made up by one speaker and one microphone. Two or more of such modules will communicate with each other via audio waves (underwater). Problem: The microphone will catch audio waves that are sent by the speaker of the same module. I need to cancel out those.

Bad Shematic:

schematic

simulate this circuit – Schematic created using CircuitLab

As I tried to explain Mic1 will record SPKR1 I don't want this and need to filter out what SPKR1 sent.

How can I do this ?

PS: sorry for my bad English.

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  • \$\begingroup\$ Why not just turn off the microphone while it is transmitting? Are you concerned about echo's? \$\endgroup\$ – Ron Beyer Jul 23 at 15:37
  • \$\begingroup\$ If you have a schematic of your circuit then you should add it in. There is a CircuitLab button on the editor toolbar and you don't need a CircuitLab account. We can then copy your schematic into our answers and edit it. You will have problems with more than two units, I think. \$\endgroup\$ – Transistor Jul 23 at 15:49
  • \$\begingroup\$ Can you use different carrier frequencies (i.e., can you have an 'A' station and a 'B' station), or must all your stations be identical? \$\endgroup\$ – TimWescott Jul 23 at 16:15
  • \$\begingroup\$ @TimWescott basically what I want to do is to replace a cable (one receiving channel and one sending channel) by something wireless and working underwater - regardless what exactly is send \$\endgroup\$ – Lucas Jul 23 at 16:19
  • \$\begingroup\$ Basically what I tried was forcing full Duplex into half Duplex ... @RonBeyer yes I will need to shutdown the reciever while transmitting .... Thanks all for the help and the ideas ! \$\endgroup\$ – Lucas Jul 23 at 16:50
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Choose two different carrier frequencies, one for the right-left link, and a different one for the left-right link. So the left-side box transmits on frequency 1 and receives on frequency 2, while the right-side box transmits on frequency 2 and receives on frequency 1. Choose them so that they're reasonably far apart (think several times the bandwidth of the FM signal), and not harmonically related (i.e., don't make frequency 2 equal to some integer times frequency 1).

If you're lucky, you'll be able to filter out the signal coming from your microphone electronically. If you're not, then your local speaker will swamp out your local microphone, and you'll need to acoustically isolate the speaker and microphone, with careful case design and with acoustic filters before the microphones -- and you'll need two filter designs, one for each frequency.

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  • \$\begingroup\$ I already had the idea of using Fourier transform .... but that would prevent the 'plug-and-play' design. Basically I will transmit IP/TCP like data. So making it sound extra nice making an underwater wifi with audio waves ... so to speak (thats not entirly right but it describes it somehow) \$\endgroup\$ – Lucas Jul 23 at 16:34
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You may have some success using something based on an old "telephone hybrid" circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

The idea being that you adjust R5 such that the signal received by the Mic is cancelled out.
I"m not sure how well this would work at higher frequencies though due to the delay introduced by the separation of the Mic and Speaker

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  • \$\begingroup\$ Thanks a lot i will this \$\endgroup\$ – Lucas Jul 25 at 13:30

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