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I'd like to transmit digital data together with an audio signal along an instrument cable.

In more detail, I'd like to send MIDI data and the audio signal of a guitar on a single cable. The rationale is that audio cable is ubiquitous and saves me the trouble to find the specific kind I need whenever it breaks.

The original idea I had was to simply send the MIDI data raw along the cable, but its low bitrate (32 kbauds) makes it dangerously close to the audible range, and in my experience even though the data in and of itself is not audible, it produces whines whenever evenly spaced packets are sent (as in CC messages).

I then thought of producing a 1Mhz carrier that would be modulated by the MIDI signal, that would then be decoded at the receiving end: this solution is supposed to solve the bleed into the audio signal.

I now find two main issues in my idea: first and foremost, I have no idea whether the audio cable has enough bandwidth to let a 1Mhz signal through. I found this spec sheet that mentions some parameters, and according to this answer the resulting impedance is around 400Ω, which is a standard impedance in the audio industry, IIRC. I don't know how this affects the ability to transmit RF signals, though. Also the capacitance is specified for 10KHz, not 1MHz.

The second doubt I have is that the audio signal is low-voltage, high-impedance: could this be a problem when decoding downstream? Will a simple passive lowpass filter be able to eliminate the data carrier leaving the audio (relatively) intact?

Final note: I have zero experience in RF circuits. I have no idea if I would have to do stuff like terminating the connection or things like that, I am not really sure how important it is at 1MHz frequency.

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  • \$\begingroup\$ I think 400 Ohms is miscalculated. The datasheet says ~40 pF/ft (0.04 nF/ft) and 61 nH/ft, which makes it as ~40 Ohm of characteristic impedance. Which is much more likely. \$\endgroup\$ – Ale..chenski Sep 4 '18 at 22:34
  • \$\begingroup\$ Why not just try it and see if it works. \$\endgroup\$ – Chu Sep 4 '18 at 23:07
  • \$\begingroup\$ @Chu I like doing an assessment to see if something is feasible rather than try doing something that's going to be impossible :) \$\endgroup\$ – Francesco Bertolaccini Sep 5 '18 at 6:17
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Rather than messing around with RF modulators and demodulators, I would recommend simply encoding the MIDI data differently, so that its bandwidth does not impinge on the audible range of the analog signal.

For example, you could Manchester encode the data at 2× the MIDI baud rate — i.e., sending each async bit twice. This would give you a signal that's centered at 62.5 kHz, with most of the information in a band that's within ±15.625 kHz of that.

On the transmit side, send the signal through a 45 kHz - 80 kHz bandpass filter before mixing it with the audio; this will eliminate "splatter" into the audio passband.

On the receive side, use the same filter to block the audio before sending the signal to a threshold circuit ("data slicer") and then to your Manchester decoder. The output of the decoder will then feed an ordinary MIDI input with no additional work. A simple low-pass filter @ 25 kHz or so will eliminate the MIDI signal from the audio path.

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  • \$\begingroup\$ This seems like a nice solution. I am not familiar with the techniques mentioned, I will look them up and try this approach! \$\endgroup\$ – Francesco Bertolaccini Sep 5 '18 at 6:20
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It may be possible with a controlled impedance shielded twisted pair (120 Ohm STP) with balun using FM with a wide deviation ratio to improve SNR by the ratio of deviation to bandwidth above the threshold.

I have used a single coax to mix full-duplex baseband audio, 150kbps biphase on 10MHz FM and 1Mbps bi-phase. But the audio source was poor quality from a throat mic, so not HiFi.

Wireless senders seem to be more popular solutions. So without RF experience, you ought to shop for this. Or figure how to interface CAT5 cable with the twisted pair for audio and twisted pair for MIDI.

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  • \$\begingroup\$ Cat5 was indeed going to be my second choice, since they are still quite flexible and cheap, but have the severe disadvantage that they are not in the "standard toolbox" of most musicians, so if something happens to the cable and I need to replace it, I'm done unless I have backup. As for the twisted pair, did you actually try the audio coax cable first and then switch to the STP, or did you just figure it out by intuition that it wouldn't work? I'd like to understand the reasoning, thanks \$\endgroup\$ – Francesco Bertolaccini Sep 4 '18 at 21:00
  • \$\begingroup\$ Common mode noise depends on inteference like triacs , motors and such so rejection of CM requires good balance (Balun transformer), line filter style and shield or coax cable with 50 or 75 Ohms. \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '18 at 21:03
  • \$\begingroup\$ So you're saying that it's not actually the cable in and of itself that's the issue, it's the background noise that would be picked up, right? In that case, would I have any better chance by using a stereo cable and differencial signaling? \$\endgroup\$ – Francesco Bertolaccini Sep 4 '18 at 21:06
  • \$\begingroup\$ Yes of course , "balanced differential' is better but often still not enuf. That's why Ethernet and Telephones have Balun transformers. \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '18 at 21:18
  • \$\begingroup\$ I assume that by "audio cable" you mean XLR microphone cable. Audio signals on such cables are normally balanced, using pins 2 and 3 for signal, and pin 1 for ground - there are no spare wires for your MIDI signal. \$\endgroup\$ – Peter Bennett Sep 4 '18 at 22:30

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