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I was watching a short tutorial video about balanced audio, and the guy claims for unbalanced transmission even there is shield in coax cable "hum and noise still gets true". But in wiki it says: "Coaxial cable also provides protection of the signal from external electromagnetic interference."

So I was wondering does a coax cable is worthless to shield electromagnetic interference? If so what is the benefit of using it rather than a zip cord.

Basically what I'm wondering is imagine a single ended(non-differential) signalling. What kind of noise coax shield would reject better comparing to a zip cord? Or should we say coax mitigates the noise but still passes some? I observe myself 50Hz noise when I use coax cables. So why are they used instead of zip cords besides the reason impedance matching in RF?

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    \$\begingroup\$ I don't know what a zip cord is, but if the coax shield is properly connected to ground, it should do a very good job of keeping out radiated noise. It will not be perfect, however, so some noise will still manage to get in, just far less. Additionally, I don't think you ever need to match the cable impedance in an audio system. The frequencies are just too low. (Matching for optimal power point is a different matter tho) \$\endgroup\$ – Joren Vaes Sep 20 '17 at 18:26
  • \$\begingroup\$ @JorenVaes matching is for to make the induced noise on lines to equal I guess. \$\endgroup\$ – Genzo Sep 20 '17 at 18:28
  • \$\begingroup\$ That still does not require the cable characteristic impedance to be matched. \$\endgroup\$ – Joren Vaes Sep 20 '17 at 18:29
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    \$\begingroup\$ en.wikipedia.org/wiki/Zip-cord \$\endgroup\$ – Genzo Sep 20 '17 at 19:17
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    \$\begingroup\$ The hum may be more likely due to ground loops and other systemic problems, The co-ax is sure to behave better than a twin-flex (zip-cord) for low level unbalanced signals that you can ground appropriately. \$\endgroup\$ – KalleMP Oct 2 '17 at 13:29
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The coax shield protects against incoming electromagnetic fields.

However, if a current flows in the shield, for example due to the parasitic capacitances of power supply transformers in equipment on both ends, this current creates a voltage by Ohm's law due to the shield not having zero impedance.

Since the shield is also used as a voltage reference for the signal, this noise voltage is added to the signal.

The shield protects against electromagnetic pickup, but not against ground loops.

Balanced (differential) transmission uses two wires. The idea is to transmit both the signal and its voltage reference, via two paths of equal impedance (this is why it's called balanced). These paths are not connected to ground at the receiving end. Instead the receiver performs a substraction, and any noise picked up by both conductors will appear equal on both and thus be nulled.

A shield is usually added also, it both protects the conductors from picking up EMI, and provides a way to connect the grounds on both sides.

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At low frequencies (too low for EM wave behaviour to be significant) there are basically two ways for noise to couple to a cable. Capacitive coupling and magnetic coupling.

I will assume that by zipcord you mean a flat twin type cable.

First lets consider capacitive coupling.

The "ground" conductor is generally a relatively low impedance node, it may be tied hard to ground. Even if it isn't it will likely be connected to a relatively big bunch of circuitry that has relatively strong capacitive coupling to ground. The signal conductor in contrast likely has a relatively high impedance.

With the coax capacitive coupling will couple to the ground conductor where it will have a relatively small effect. With flat twin cable it may couple to the signal conductor where it can have a larger effect.

Now lets consider magnetic coupling.

With a flat twin cable magnetic fields can couple strongly to the cable because the two conductors form a flat loop. With coax you actually get two loops, one on each side of the coax which in a uniform magnetic field cancel each other out. In a non-uniform field of course this cancellation is imperfect but it's still generally better than a flat twin cable.

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