Seth Bloombaum |
December 17th, 2007 11:18 AM |
Quote:
Originally Posted by Peter Moretti
(Post 793965)
Is there a certain length that should be avoided because its too long a distance for the signal to cleanly travel, or is this really a moot point (practically speaking) with XLR cables?
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Well, not quite moot for many of us who are doing work in larger rooms. I recently had to go through the math for a 420' run of line level...
There is a formula:
-3 dB Frequency = 1 / (cable length x capacitance per unit length x output Z x 2 x Pi)
for determining the high-frequency cutoff point of a particular cable. Longer cables tend to affect the signal as a low-pass filter (cut off high freqs.) Output Z refers to the output impedance of the source. This effect is known as "line loading", and is all about the total capacitance of the cable. The manufacturer supplies a spec, for example "60pf/foot", pf referring to picofarads of capacitance.
Suffice to say that (using pro low-impedance devices) any pro cable ought to get you at least 100' at mic level and 150' at line level, and usually more.
Radio frequency interference (RFI) is quite another matter, and may occur with any cable length. Here the remedies are: monitor always, balanced cables and circuits, test the cables, shorter cables, longer cables, different cables, move the cables, don't run audio cables parallel to power cables or other metal, etc. which sounds intimidating but in practice only occasionally occurs with pro equipment.
I'd certainly agree with those who've posted above that 25' is a very practical length for audio-for-video in field work.
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