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From: myers@hpfcla.fc.hp.com (Bob Myers)
Newsgroups: rec.audio.high-end
Subject: Re: Cable questions- wire wonderings
Date: 4 Apr 1996 00:28:02 GMT
Organization: Hewlett-Packard Fort Collins Site
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Mr One Eye (mroneeye@aol.com) wrote:
> Bob Myer wrote:

> > Ideally, at least to me, the cable should be a means for getting a
> > signal from HERE to THERE with no change whatsoever - a perfectly
> > transparent transmission path, in short.

> [ quoted text deleted -- jwd ]

> My question is this: what numerical values (or range of values) should
> the series inductance and shunt capacitance be???

As low as possible in keeping with the restriction that the cable be
practical. Unfortunately, there is a tradeoff to be made here - cable
constructions which minimize capacitance can increase inductance, and
vice-versa. For example, coaxial cable represents a very
low-inductance geometry, but has relatively high capacitance. That's
why coax has a much lower characteristic impedance than twinlead. (I
know I shouldn't be bringing characteristic Z into this, 'cause
someone will now start to worry about it...)

> Also, what are the wavelengths delat with in audio cables, such that
> they either are or aren't transmission lines???

The wavelength of a signal in any medium is found by dividing the
velocity of propagation through that medium by the frequency.
(Meters/second divided by cycles/second gives meters/cycle.) Most
electrical cabling has a velocity factor of 0.6-0.8, meaning that
signals travel this fraction of the speed of light within them. So,
if you think audio includes something like 100 kHz, you'd be worrying
about wavelengths of AT LEAST....

(0.7 x 300,000,000 m/s)/(100,000 c/s) = 2100 meters/cycle.

We usually don't worry about transmission line effects until we're at
least at the 0.1 wavelength point - so unless your cables are 210
meters long, AND you're worried about the effects at 100 kHz, I
wouldn't spend too much time thinking about characteristic impedance,
reflections, and so forth.


Bob Myers KC0EW Hewlett-Packard Co. |Opinions expressed here are not
Workstations Systems Div.|those of my employer or any other
myers@fc.hp.com Fort Collins, Colorado |sentient life-form on this planet.