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Old March 5th, 2010, 01:43 PM   #16
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Originally Posted by Salvis Are View Post
How long cable do you use for shotgun mics, guys???
Does the lenght of the cable influence the quality, too...?
6 meter long cable is not too handy for camera attached mic, do you know what I mean!!! ;)
Hi Salvis.

Just reinforcing some of the comments here:

Keep in mind that issues of loss and mismatch affecting signals in audio bandwidths are not the same as those experienced in the video bandwidth.

Speaking purely in signal fundamentals (ignoring sidebands) we are looking at audio as 20hz to 20khz, and video as DC to 6mhz. Therefore there are critical factors for one and not the other:

That is, resulting losses in cable length due to capacitive and inductive reactance which are the accumulative results of the losses incurred due to the length of the cable. Simply - more losses the higher the frequency and longer the length.

1. Apply the thought to impedence, source generator into input, particularly to source impedence of the mic into the load of the input. Rule of thumb, the lower the source into a higher load - should be okay.

2. Balanced/Unbalanced. This is the more crictical element for longer cable runs. Again, simply stated, operating in a balanced mode increases the amount of rejection of spurious "noise" elements to your input (in addition to the common mode rejection ability of the input circuitry). The main noise element in a sound rig is often hum (and it's harmonic).

Maintaining balanced mode inputs you can operate to greater cable runs - 100s of metres is not uncommon (given a good cable specification of low capcitive reactance). Twisted pairs assist in the rejection.

So in your situation with a shotgun of probably 200 ohms source (and much less) into even a standard XLR balanced input (with standard cable) of 1K ohm (typical) and either balanced transformer input (or what's called a differential input pair which is an electronic balanced input) you would have no problem running 100 metres.

And to answer your question about affecting the quality, capacitive reactance (Xc) is inversely proportional to (f) frequency - the higher f results in lower Xc (bad) - lower Xc means a virual short-circuit to your higher frequencies - a bandpass filter. At 20Khz tops, you would need some pretty naff cable to noticeably impact upon your audio - unlike video!

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Last edited by Claire Buckley; March 5th, 2010 at 01:51 PM. Reason: more typo!
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Old March 14th, 2010, 03:07 AM   #17
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many appreciations :)
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Old March 20th, 2010, 01:33 PM   #18
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Hi Salvis,

Aside from impedence mismatches and unbalanced cable limitations, XLR cable lengths isn't a real concern with a high-end frequency of only 20,000 hertz. Heck, high-end manufacturers even sell great cables in lengths of over 500 feet:

Canare | L-4E6S Star-Quad Microphone Bulk Cable | L-4E6S-656

Warm Regards, Michael
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