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I am trying to figure out a way to achieve low speed digital communication over an audio line. I have two devices that are connected with a two wire audio cable. Audio and Ground. From my research, I can't find any current solution to achieve bidirectional digital communication over a single audio signal.

Does anyone know of any IC's or designs that can accomplish this task? I only need something very basic and low speed.

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  • \$\begingroup\$ Morse coding, that is. Oh, well, the modem/fax communication is working this way. Upd: Or you mean modulating a signal in addition to the audio? Than DSL is doing this. It's called FDM (frequency division multiplexing) \$\endgroup\$ – Eugene Sh. Nov 3 '15 at 17:01
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    \$\begingroup\$ Does this actually have to be audio? If it's just a 2 wire cable, you could use any number of digital encodings - the fact that you're using a cable normally used for audio is irrelevant. \$\endgroup\$ – Nick Johnson Nov 3 '15 at 17:06
  • \$\begingroup\$ Bi-directional could be a real problem due to contention over who is driving the line. I think you want some kind of simple modulation technique. There is simple on-off keying, frequency shift keying (FSK), and all kinds of other stuff. What are you doing, anyway? Do you have control of the design of both devices? \$\endgroup\$ – mkeith Nov 3 '15 at 18:01
  • \$\begingroup\$ Look at a 1Wire protocol. \$\endgroup\$ – MadHatter Nov 3 '15 at 18:13
  • \$\begingroup\$ do you want to record it, so must fit within audio channel? With bidirectional, could suggest two different frequency ranges, one for out, one for return, seperated by filtering. \$\endgroup\$ – Neil_UK Nov 3 '15 at 18:48
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It is not clear from your question whether the audio cable is to support an audio signal simultaneously. Assuming it does not, and you already mentioned the data rate is low, you can adapt the hybrid (in telephone terminology) concept.

Check out this link for example: ESP AN-010

I have copied figure 2 here: enter image description here
I assume the figure is copyrighted by ESP.

You can just feed a low speed (low enough that it can be handled by opamps) signal to "In A" and it should come out at "Out B". Correspondingly, "In B" to "Out A". $$$$

Alternatively, if you have microcontrollers at both ends, it is not that much work to use the built-in UARTs, put in RS485 transceivers and come up with some packetizing and handshaking scheme that works well at low data rate.

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