> The total impedance of a cable is dependent on the length, so as the
> capacitance accumulates with longer cable length high frequencies
> will be rolled off. Accumulated resistance is generally minimal but
> will cause slight loss of signal level, & accumulated inductance
> will smear transient response. So there are limits before cable
> length will cause unacceptable degradation
This is going to be technical :-( Technophobes - don't read it. :-)
This is a misconception which is often repeated. A cable is a
transmission line whose characteristic impedance is set by its
distributed inductance and its distributed capacitance. Its
distributed resistance causes signal loss with distance.
Ideally a cable, whether video or audio, should be terminated at each
end by a matching source impedance and a matching load impedance. This
is critical with video and digital lines. Fortunately with audio,
impedance matching in not critical within a factor of about 10 until
you get lengths which resonate near an audio frequency. For instance,
a 3km line will resonate at 100 Khz or 50 KHz (re: speed of light)
depending on how it is mismatched. At a fraction of that frequency you
will get a fraction of a mismatch.
If a cable is terminated too high, at lower than resonant frequencies
it will act as a capacitance load. Thus if you feed a 50,000 ohm
microphone into a 100 ohm cable you will lose high frequencies.
However, if you terminate a long 100 ohm cable with, say, 10 ohms, it
will act as an inductance and lose lower frequencies. With a
goldilocks matching impedanace, you will maintain a flat frequency
response. That's why long distance land lines used to work.
Practically, to get a full line resonance you need a mismatch at both
ends. With a low impedance mic feeding into an input such as 1000 ohms
at the other end, you may start to hear the mismatch at about 3
kilometers, (but don't quote me) However if you terminate the
receiving end with the correct value of resistor, you will get a flat
response, however long the cable. I have never come across a situation
where this was required, but that's the theory.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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