Here are some basics to keep in mind:
1. Unbalanced lines such as RCA home entertainment cables and home owner 
style coaxial cables have one conductor. The return path is the shield which 
is supposed to be at ground potential. Ground being the common in the 
system, but ground having a huge amount of background noise from natural and 
man made sources. If the desired signal level is fairly high then modest 
background noise is not a problem. The desired signal over rides the ambient 
noise level. In cases where the ground is carrying a lot of noise or the 
there is a ground differential between the equipment in use, then you have 
noise injected into your system. The more gain you add to pick up the 
desired signal the more noise you bring into your system. This is one of 
several reasons why ignition noise and "10-4 good buddy" gets into audio 
systems. Ferrite beads and by pass capacitors can filter some of this out.

2. Balanced lines are a little different in that the you have two conductors 
independent of ground so picking up low level ground noise is reduced. The 
balanced line concept cancels out a lot of low level ambient noise. If the 
pair is twisted then even more low level noise is cancelled out. If you add 
shielding then even more noise is cancelled out. If you have a big check 
book and want double shielded cable then you can rest assured you have done 
your best. One point: Read the next paragraph and keep in mind having spent 
a ton of money on cable will not eliminate bad engineering practices on 
either end.

3. Recording and broadcast studios have used balanced line for generations. 
Between equipment audio sources are usually handled as balanced line with 
XLR type connectors with the signal levels of about .5 to 1 volt. This means 
the signal feeding the line is usually are a fairly high level so that the 
receiving equipment does not have to use a lot of gain to pick up the 
signal. By keeping the feed signal high and the receiving end gain low you 
minimize noise pick up. In these types of installations ambient and ground 
noise is very high so you need to beat the nose by using good engineering. 
Going one more level: High quality microphones use balanced lines. To keep 
the signal relatively high on very long audio lines the microphones have 
built in mini-amplifiers to feed a higher signal down the line to the mixer 
or audio board. The mixer or audio board introduces a small power source on 
the signal line called phantom power. The phantom power is what powers the 
audio amp in the microphone.

4. It is far better to feed a relatively high level low noise signal down a 
line to be processed by another piece of equipment with a relatively low 
gain setting. You are beating the ambient noise on the line by keeping the 
desired signal level well above the ambient noise. Having stated that, 
shoving an extremely high level signal down a line will cause interference 
to adjacent circuits and make life real bad. Balanced lines will not 
compensate for bad karma. Keep the line signals in the .5 volt range and you 
will not have a lot of bleed through.

5. In commercial communications systems and broadcast studios just about 
everything is shielded and balanced line inputs are used to prevent 
interference. In home entertainment systems cheap is what sells.

6. One closing point: Home PC's can have their own degrading issues which 
can reflect into other systems. Keep this in mind:. Unless you have a well 
shielded PC and monitor then keep it away from critical low level signal 
systems.

Jim


_____________________________________________________________________--
Yes, I think I understand a tiny bit about this.
But can you tell me about common mode verses differential.
as referenced to ground.

It seems to me AC can leak both ways into a system then through some
mysterious force infiltrate a system in a differential form
causing a dc or (low freq ac) drift.

It seems to me at Seismic frequencies you never operate
a proper balanced transmission line system because the wavelength is too 
long.
You need like 1/2 wavelength or multiple thereof.

Twisted lines only insure that common mode signals
are distributed evenly on both differential wires.
It does not stop common mode or even attenuate them.
You must rely upon the CMMR ratio to attack the
unwanted common mode stuff.

Use twisted pair along with 100% shielding
in my opinion is the only way.

This is all laymen talk since I'm not an engineer.


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