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Old January 29th, 2014, 09:41 AM   #1
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3,5mm cables and interference

I need to shoot an interview. Two persons will be standing or sitting - so I dont need an expensive wireless system. I will buy two AUDIO-TECHNICA ATR3350 lavs and I am wondering, is it ok if the 3,5mm cable will be longer than 3 meters? I heard that with 3,5mm cables longer than 3 meters could cause some problems with interferences. My camera Canon HV20 has 3,5mm input too.
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Old January 29th, 2014, 09:46 AM   #2
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Re: 3,5mm cables and interference

I typically use a 20' (6-meter) "headphone extension cord" to extend the lavalier to the camcorder and have never encountered any issue.

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Old January 29th, 2014, 09:58 AM   #3
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Re: 3,5mm cables and interference

Note that the ATR3350 comes WITH a 6 m (20') cable. Do you really need longer than that?
The ATR3350 is unbalanced. And THAT is the factor that makes it more vulnerable to interference. It is practically impossible to predict whether you will experience interference, as much depends on the local conditions where you are recording. Best to try everything in the location before shooting.

Remember that you will need a SPECIAL kind of Y-cable to connect two micorphones to the stereo mic jack on you camcorder. Something like this....

Hosatech Stereo Breakout, 3.5 mm TRS to Dual 3.5 mm TSF - 3.5 mm TRS to Dual 3.5 mm TSF
or
Beachtek MCC-2
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Old January 29th, 2014, 10:24 AM   #4
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Re: 3,5mm cables and interference

Quote:
Originally Posted by Peter Berger View Post
I heard that with 3,5mm cables longer than 3 meters could cause some problems with interferences.
There's no get-out-of-jail-free length. I've had a 0.1m unbalanced cable pick up RF interference from a lamp ballast and feed that into a camera. Sounded like a 60Hz hum, but it wasn't an analog sine wave, it was a series of spikes that occurred at around 1/60 of a second intervals. I traced this it an HMI lamp ballast.This little exercise taught me the value of a fully balanced audio chain.

What I'm saying is, regardless of length, an unbalanced cable is much more susceptible to RFI noise pick up than a balanced (XLR) cable. Which is why professional audio rigs use XLR cables.

Can you get away with a long run of unbalanced cable? Maybe. No guarantees. Try it, listen to your capture with headphones, and see. And... good luck with it.
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Old January 29th, 2014, 11:31 AM   #5
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Re: 3,5mm cables and interference

Always monitor all audio with headphones. Well, any audio that matters. Any noise *must* be caught in the shoot.

If you do hear interference or noise, try moving the mic cables away from power cables. The worst is when mic & power cables are close and parallel. If cables must cross, have them cross in one place, and cross at a 90-deg angle.

Nearby power cables aren't the only potential source of noise on unbalanced audio cables, but they are the most common. The more equipment you can run on batteries, the better, especially camera.
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Old January 29th, 2014, 12:30 PM   #6
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Re: 3,5mm cables and interference

If possible, test the extension cable beforehand. Connect the mics, then move the extension cable around in proximity to power wiring, lamps, etc., and listen for hum and buzz.

Local stores (Rat Shack, Worst Buy, etc.) often sell "headphone extension cables." I have found that some of these actually are shielded, and work OK as mic extensions. But some are not shielded (shielding is not necessary for headphones) and those do not work as mic extensions. You don't want to get to the shoot, only to ascertain that your particular extension is unshielded!
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Old February 2nd, 2014, 07:17 AM   #7
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Re: 3,5mm cables and interference

So I shot the interview and the Audio Technica mics are suprisingly clean. And you were right, the power cord of my camera caused noise, so I had to throw it away and shoot with batteries.
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Old February 2nd, 2014, 12:32 PM   #8
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Re: 3,5mm cables and interference

Those switch-mode power supply "line lumps", like the ones that come with laptop computers, etc. are NOTORIOUSLY noisy (electronic noise). It is not at all surprising that you had to run on battery.

However, balanced microphone connections (with XLR connectors, etc.) can often handle even that kind of gross electro-magnetic interference. So that is one of the tradeoffs we get when we use low-end equipment with unbalanced mic-level connections. But you have proved that you can still get decent quality if you live within the limitations.
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