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
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    \$\begingroup\$ Look at a 1Wire protocol. \$\endgroup\$ – MadHatter Nov 3 '15 at 18:13
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    \$\begingroup\$ I'm voting to close this question as off-topic because the asker abandoned it four years ago without ever stating the necessary details of their need. It is unclear how the term "audio" must apply to the signalling or if the signalling must avoid distorting ongoing audio usage. \$\endgroup\$ – Chris Stratton Aug 12 '19 at 1:59

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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[low speed digital communication over an audio line]


[very basic and low speed]

I'll bite at assuming you want to add digital data to the existing audio without "modifying" the audio. This is not going to be "basic."

My thoughts about this are to put an active low-pass filter after the audio source, to limit the audio frequencies to <20kHz. Then have a pair of 40kHz oscillators "inject" bursts across the audio link, which are picked up by the other device. Another 20kHz lowpass removes these pulses from the audio before exiting. If microcontrollers are used to time the bursts and interpret them, along with a bi-directional protocol of your design, then this just may work. The capacitance of typical audio cable will likely prohibit long runs however.

The same could be done with sub-audible frequencies and high-pass filters, such as 10Hz, but then the bit-rate must be very low. However long runs would not be a problem.

This is a far easier approach than professionally-encoding data over audio, which likely uses a DSP.

Another method could be to inject a +/-DC offset representing a zero or one and filter it out afterwards, but this seems technically more difficult to filter and prone to errors depending on audio content.

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