Michele,
You're turning me back into a geek. :-( However I always say the final
test is by ear :-)
I run a pair of 150 metre cables with MKH416's or MKH 816's and can't
hear any degradation in level or noise. There is a BBC Engineering
story about a radio OB engineer using a mile long barbed wire fence as
a balanced line and getting transmission quality.
a) how do you decide if a cable is "good enough" for this application,
based on technical specifications?
Any audio cables with a proper specification are about the same in
practice. I use cheaper "fixed installation" foil screened cable which
is less flexible and needs more care while laying. Helical laid
screens can theoretically be sensitive to RF interference but I only
ever observed this once while near the powerful Daventry longwave
transmitter. Don't pay big money for fancy screening. I've used
conductive rubber screens with no problem.
The practical cable parameter differences are robustness, flexibility
and ease of wrapping - like cold weather butyl fleximic, sleeve
colours for line identification, and cable microphony if it is going
to be trodden on. I use heat shrink bands on the XLR's to identify L &
R.
You are using a balanced line with two push-pull signal lines and a
screen. I converted two cheap ECM mics to balanced line by using a
balanced transformers box and even they work fine at a distance.
> What sort of numbers should I be looking at in terms of resistance
> and impedence? Or is that not an issue at all?
Theoretically the "characteritic impedance" of the cable should match
the mic output impedance and the mixer input impedance but in
practice, many mics output at 50ohms (yours are 30-40) and inputs are
2K or even more (the Fostex FR2-LE is 6K but that is no problem) and
they all work fine with 100 or 150 ohm cables.
Now for some technical stuff. If you try to send a mid or high
impedance signal down a low impedance cable you will lose HF quickly.
This s because the cable looks like a capacitance at high impedances
and higher frequencies.
Now for the good news, if just one end of a low impedance cable is
matched or nearly matched, that is good enough. What you are looking
at is a long "resonant pipe" which will "reflect" at an impedance
mismatch. You won't hear any frequency losses until the cable length
gets towards a quarter wavelength at the speed of light. At 10KHZ,
this translates to 7.5 kilometers, so several hundred metres is
negligible.
The full cable spec will give an attenuation of all frequencies in dB
per 100 metres, but this is negligible with my 150 metre cables.
b) does is it make sense to have an xlr to xlr connection at the
middle, or would that compromise sound quality?
That way I could have 2 x 50m cables instead of 1 x 100m, which would
be easier to handle and would allow for 2 recording options: stereo @
50m, or mono at 100m
No problem but don't connect the XLR shells to the screen as this can
form a "ground loop". I found volts between my electricity ground and
the real earth ground. For the same reason don't let the mic bodies
touch on a stereo pair. Only ground anything once.
If outdoors, you have to guard against moisture getting into the
XLR's, hence my long cables. I keep them on garden hose drums -
cheaper and lighter than metal mic drums. You don't need to unroll the
whole length if you can get at the inner plugs. Wrapping 50+ metre
cables by hand can finish up with a mighty tangle.
The first things I used to teach sound trainees was how to tear gaffer
tape cleanly and how to wrap cables.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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