FAULTY AUDIO CABLE TESTER

[Um...Me123]

Any ideas on what methods to use to check various lengths (max of at least 100') of audio cable for things that may cause line noise or cutting in and out?  I have some ideas but need help.

What specifically causes line noise?

Another question I have that may play a part in this is how do devices detect different things using different voltages...Um how does a panel of buttons work that only have two wires going to it (with diff resistors).  (How does the receiving end differentiate between the voltages??)  Do transistor have a "cutoff point" or something or how does this work?

Thanks for your help.


Edit--- comparators--is that the thing in my last question?  What are they?  Also use for progress bar type thing.