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I'm wiring up a new radio and intercom system on my plane. The installation manual suggests:

5.10.3 Audio wiring

For any wires carrying an audio signal, a screened twisted pair should be used to avoid any unwanted signals interfering with the radio installation. Unscreened wires can act as small antennas and transmissions from other avionics on the aircraft could be picked up by the radio and heard as interference in the connected headsets. For example, Mode S transponder transmissions can be audible as clicking sounds in the headset and cause an annoying distraction to the pilot.

Correctly terminated screened wiring will limit any unwanted transmissions being picked up by the radio and should be used on the MIC and PHONE lines.

All makes sense, however:

Is it sufficient to combine the headphone and microphone signals into a single screened cable, or should I route two separate cables?

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Microphone signals are tiny, so small amounts of noise can be big relative to the actual sounds signal you want. Microphone wires should be shielded, with nothing else inside the shield.

Signals going to a headphone are much lower impedance and much higher voltage. Noise that totally overwhelms the audio signal coming out of a microphone might not even be audible on top of a headphone signal.

If in doubt, run separate shielded cables for the microphone and the headphone signals. You definitely don't want them together inside the same shield. The strong headphone signals can then cause feedback into the sensitive microphone lines. This can lead to whistles and other bad effects.

You may be able to get away with twisted pair for the headphone signals instead of a shielded cable, depending on how strong the RF is and how the headphone amplifier reacts to RF being fed back into its output.

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