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This question is for all those home theater and computer specialists out there. How does EMI from power cables affect analog signals in audio cables and digital signals in cables like Ethernet/HDMI/etc? The inductive electromagnetic energy from the power cables will interfere with the signals being carried in the analog and digital signals in other cables correct? I know audio signals can suffer from a hum in such a situation, but haven't been able to find information about how digital signals are affected. What exactly happens regarding constructive/destructive interference in both analog and digital mediums, how does it affect the signal being carried, and what consequences does this pose to technicians?

riverofwind
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  • May be of interest: https://electronics.stackexchange.com/q/145989/2028, https://electronics.stackexchange.com/q/290310/2028 – JYelton Nov 02 '23 at 23:59
  • Note that power cables (I assume you're talking about household AC power, 115 VAC) carry both the outgoing (hot) and return current on 2 wires in close proximity to each other. Hence the magnetic fields from each conductor cancel each other pretty close to 100%. – SteveSh Nov 03 '23 at 00:11
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    Digital signals are essentially immune to signal degradation due to EMI; to oversimplify things a bit, the signal either is completely recovered and works perfectly, or it doesn't work at all--there's not much of an intermediate region of degraded signal quality like you can get with analog signals. This is one of the major reasons digital (audio/video/everything) took over so completely. – Hearth Nov 03 '23 at 03:51

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To be short, audio is analogue and the whole signal waveform is the useful data, any external disturbance that adds up will become part of the signal and it can't be removed any more.

Digital signals are also analogue waveforms on the wire and may be affected by external disturbance, but the difference is that the waveform is interpreted to be e.g. high enough to be considered as digital 1 and low enough to be considered as digital 0, with some hysteresis and margin so that small enough amount of noise does not matter much.

Also like balanced audio, high speed digital communications is mostly differential, usually transmitted over twisted pairs. So the signals are relatively immune to disturbance that affects both wires, like how ground loop hum is less of a problem with balanced than single ended audio.

The impedances of digital signals are also much lower, around 100 ohms, so while the signal voltages are low, any disturbance will cause less noise in a 100 ohm impedance than in a 20 kohm audio signal input impedance.

Some digital buses are not even DC coupled. Ethernet is a transformer isolated interface that can have more than 1000V voltage difference between device innards and cable. USB3 high speed lines are capacitively coupled so again DC gets filtered out. So only high speed signals pass the transfer medium, and problems like mains hum has little effect.

Any disturbance happening on the wire will just slightly adjust the detection between logic thresholds and that may be seen as jitter and data transmission error.

Justme
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  • When you say "that small enough amount of noise does not matter much", and also regarding the jitter/transmission error, does that mean the digital signal might suffer at longer lengths? Will shorter lengths be unaffected? – riverofwind Nov 03 '23 at 19:29
  • @riverofwind every signal suffers at all lengths due to losses in transmission lines, digital signals start to have bit errors when length and disturbance affects the signal enough to be received incorrectly. – Justme Nov 03 '23 at 19:47
  • Excellent thx that's the explanation I was looking for! – riverofwind Nov 03 '23 at 22:12
  • One more question I thought of - does the transition from complete digital signal integrity to experiencing bit errors happen very rapidly over the length of the cable once you start to experience bit errors? Or does the digital signal degrade gradually as the length increases once you reach that point? – riverofwind Nov 04 '23 at 22:53
  • @riverofwind Digital signal is correctly received until it is at the threshold of being received correctly or incorrectly. Depending on what the signal is, it may tolerate no errors, some errors, or a lot of errors, until there is just too much errors. Think of an audio CD; it can't be read pefectly without errors even when brand new and it can still withstand a few scratches and play without errors until it is too degraded and fails to play. It has built-in error correction so some amount of errors can be corrected. – Justme Nov 04 '23 at 23:03
  • So what precautions does a technician need to take when organizing cables? Is it safe to bundle digital signal cables alongside power cables without concern at shorter lengths? Are analog signal cables the only ones for which it would be prudent to avoid placing them parallel to power cables? – riverofwind Nov 05 '23 at 19:38