Posted by Matt (148.197.176.139) on January 16, 2004 at 06:59:23:
In Reply to: XLR Wiring Help PLease ASAP posted by George McNelia on November 01, 2002 at 09:06:24:
Hi,
Peter's comments seem the most useful here. There are 5 conductors in the cable (four coloured and sleeved ones plus the braid/shield)
The shield is the first layer of protection for your audio signal as it goes along the cable. There's lots of electromagnetic radiation that naturally occurs in the environment. This can cause hisses and hums in your mic signal. The shield picks up some of this and dumps it to ground.
A shield is highly effective for small distances (up to about 10M) in most situations. But sometimes you'll find yourself using a mic around other electrical equipment. So, it's time to up the anti in how you protect the signal in the cable.
Here comes the science…
I know the cable described in this question as "star quad", denoting it's four signal conductors. There's a simpler version of this cable with two conductors and a shield, lets think about this one first. In this simpler “balanced” cable, the shield does the same job, It’s the negative or ground side of the circuit by draining away interference. There’s also a live or positive conductor (pin 2) but pin 3 carries the same audio signal, but inverted.
An inverted signal in this case means that when there is say, 0.5 of a volt on pin 2, you wil find there is –0.5 of a volt on pin 3. If the voltage on 2 dips to –0.2v you will find 0.2v on 3. These signals pass along the cable and when they get to the pre-amplifier in the equipment you’re using, the equipment makes the inverted signal normal and combines it to the positive signal.
Why?
Well, when the signal is passing along the cable it can get boosted by other interfering signals. Imagine on our positive cable there’s 0.3 of a volt. Interference cuts in and adds 0.2 of a volt. The interference will add the voltage to both the +ve and –ve signals. This means the +ve side jumps from 0.3 to 0.5 the –ve side will jump from – 0.3 to –0.1. Then the pre-amp gets hold of the signal and makes it into a 0.1 and a 0.5 the when it combines these, it gets an average that’s much closer to the 0.3 of a volt that came out of the mic in the first place.
Of course in real life, an audio signal never stays still long enough for you to get a voltmeter on it. An audio signal voltage goes up and down, that’s why speaker cones go in and out.
So, that’s balanced line explained. “star quad” is a development of this. In the quad cable, conductors are paired together. (pair up the diagonally opposed conductors, same colours paired at each end) this is an additional way of averaging out the interference .
I hope that’s helped people understand what you’re playing with, it was something that confused me for years… I normaly explain this with a pen and paper in hand so I can draw pictures, it’s harder explaining this in just written words. E-mail me for any clarification that might help you.
Happy recording and soldering!