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In article <1992Apr13.170559.8310@athena.cs.uga.edu>, mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:
|> What was the impedance at the end of the cable?  That is, what were you
|> using for a load?
|> 
|> The circuit looks like this:
|> 
|>              R1
|>        -----//////--------+
|> Signal                    / R3
|> source                    /
|>        -----//////--------+
|>              R2
|> 
|> Here R1+R2 is the resistance of the cable, and R3 is the resistance of the
|> load (the thing you're measuring the voltage at).
|> 
|> Let V represent the voltage at the source.  Then the voltage across R3
|> will be   V * R3 / (R1 + R2 + R3).
|> 
|> If R1+R2 = 5 ohms and R3 = 10 million ohms (the normal input impedance of
|> a digital multimeter) then R1+R2 will be negligible.

I think the poster may have wanted the AC factors to figure into the
frequency response considerations. At audio frequencies, the cable
length versus frequency response is governed more by the
impedancse of the source and load, along with the capacitance of
the cable. As noted above, with a high impedance *AC* load and
a relatively low source impedance (figure 1K ohm for most consumer
outputs), there will be little change in signal amplitude noted
over 5 meters of cable. With a low-impedance load there will be
even less *frequency dependent* loss in the audio range.

To approximate the loss for short cables (less than 100 feet or so),
take the above diagram, but make R1 the source impedance, R3 the
cable impedance (round-trip, DC), and R2 the load impedance. Now,
parallel the total cable capacitance across R2 and measure across R2
with a scope or similar AC device (note that most DVMs
have decreasing AC response above 1 KHz).

The real term of the capacitive reactance is given by:

	Z = 1/(2* pi * frequency * C)

Use this value as "resistance" in the DC Ohm's Law equations. Note
that although the capacitance is distributed along the length of
the cable, we're treating it as if it were all lumped in the
worst possible location. The results will show that for low impedance
sources, a reasonable cable length is no problem from the signal
conduction standpoint. It may pick up more interference or noise the
longer it is, though...

Inductance is not much of a factor at audio frequencies, using
regular cable.

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