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Old July 3rd, 2007, 04:12 PM   #1
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Camera noise on wireless mic

Hi all, first post and I'm excited to be here. I just bought a Canon GL2 about a month ago, and we are slowly getting acquainted.

Last night, I was checking out a rental studio which had a wireless lav mic for use. I know it was a Sennheiser, and I can find out the model # if it helps, though I don't remember it now.

While my subject spoke to the camera, I listened with headphones. There was a distinct hum/buzz in the background. His voice came through well, but the buzz was there. We tried detaching the receiver from the shoe, and just sorta letting it hang by its plug, but that didn't really help. What did help a little was taking the camera out of the equation, and plugging the headphones directly into the receiver.

Is that buzz being recorded? It was hard to tell when I listened to it later since again, the camera was on in VCR mode and it hums a bit. I could play it back on a computer and probably answer this myself...

Regardless, this is for a video of interviews which will have a soundtrack the entire time. However, I'd still like to not have that hum/buzz in the background if I can help it. Is the sound coming from the camera? Is the microphone perhaps a cheap model - would a nicer wireless lav work better? Or should I forget wireless and rent a wired boom mike?

FYI, the room was otherwise quiet - we turned off the computer and the building was mostly empty.

Thanks for your thoughts!

Sara
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Old July 3rd, 2007, 06:39 PM   #2
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Welcome, Sara, and congratulations on not recording audio without headphones!

My first guesses would be that either the camera or the wireless was plugged into AC power, or that the levels in either the transmitter or the receiver weren't set correctly. If neither of those applies, I'd be thinking defect.

Yes, I would expect that what you heard in the phones was being recorded.
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Old July 3rd, 2007, 09:27 PM   #3
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Hi David, thanks for this thoughtful response. I took a dv course at Chicago Filmmakers, and the teacher was vigilant about the use of headphones. :)

No, neither element was plugged into AC power, though that's a good watchout. I did adjust the left and right levels on the camera so that the voice wasn't overblown, but I didn't touch the levels on either transmitter or receiver.

I will be in the same situation again in a few weeks. What can I do to set the mic levels correctly? And incidentally, how do the transmitter and receiver each affect the sound differently?

I'm also considering a boom mic, because some of these interviews may have two subjects, not just one, and I'm guessing the wireless lav wouldn't be ideal if only one person could wear it.

If an administrator is reading, should this thread be posted in an audio forum instead of the GL2 forum? I just realized that this is becoming more of an audio question.

Thanks!

Sara
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Old July 3rd, 2007, 11:51 PM   #4
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Sara, good point about this thread probably belonging in the audio forum. Maybe a moderator will move it. Meanwhile ...

It does sound like a boom mic would be a better choice for you. A pair of wired lavs could work nicely too

Regarding the wireless: The transmitter's input level needs to be adjusted for the strength of the signal coming from the mic. Too low and you invite hiss. Too high and you invite distortion due to clipping. The transmitter is not likely the culprit for buzzing though, except maybe if it was supplying phantom power to a mic that didn't want it.

The receiver's output has to be adjusted so as not to overload the input of the GL2. Turning down the GL2's level controls won't correct such a condition. They only control (and the meters only indicate) recording level. Still, though, if the receiver was putting out a mic level signal that was too high, I'd expect clipping but not buzzing. Clipping sounds like scratchiness if you didn't already know that.

However, if the receiver was putting out a line level signal to the cam's mic input (a common error), a buzz could result in addition to severe clipping.
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Old July 4th, 2007, 12:16 AM   #5
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Hi Sara............

Where to start?

At the beginning, I guess!

You didn't elucidate as to where, exactly, the interview was taking place.

Is it possible the mic was so placed/ positioned it was picking up choke hum off a flouro system/ motor noise off an A/C system etc? You wouldn't be at all suprised what a mic will pick up that your ears automatically filter out.

The easiest way to tell if it's the mic picking stuff up not immediately obvious to you, is to disconnect the mic (shorting the I/P to the transmitter is really the way to go but a bit iffy if it's rental gear).

If disconnecting the mic doesn't sort it, turn off the transmitter.

If the reciever is still on and the buzz is still there, turn off the reciever.

If, with everything but the camera off, the buzz is still there, it's the camera.

It goes without saying that whichever the culprit is it will become pretty obvious if you follow this routine.

And as David said - if it's in the cans, it's on the tape.

Hope this helps.

CS
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Old July 4th, 2007, 08:21 AM   #6
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Chris makes a good point that I forgot, about noises filtered out by our perception in a setting, but which stand out in an audio signal. Just last week I was shooting an outdoor performance of Pirates of Penzance. During the quieter moments I noticed a buzz. I took of my phones and it took a moment to pick it up and identify it as a gently buzzing walkway lamp about ten feet behind me. And this was being picked up by the rear lobe in the polar pattern of a shotgun mic pointed the other way!
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Old July 5th, 2007, 08:22 AM   #7
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Using headphone also makes any noise tend to stand out more so than in a typical listening environments thanks to the tight coupling to the ear and reduced ambient sounds reaching the ears. lTake a close listen to the sound coming off the tape to judge how bad it really is.

Also, it maybe that routing of wires from the wireless to the camcorder could be adding some noise due to pickup of stray electrical fields around the camcorder; e.g., head switching noise or vertical frequency noises. I've encountered this with some camcorders where in battery current actually coupled to the audio with some batteries, but at a rather low level.
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Old July 5th, 2007, 10:50 AM   #8
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Hi guys...

Thanks to all of you for these helpful responses.

David, how would I know if the transmitter was supplying power to a mic that didn't want it? Does this happen sometimes? I'm not sure if I totally understand this yet, but I'm wondering how I'd know if it was happening, and more importantly, how to stop it.

You said a common error is for the receiver to be putting out a "line level signal." What does that mean? There was no clipping after I turned the camera's inputs down, certainly not severe, so I'm guessing that wasn't the problem. But regardless, I'd like to understand this a little better.

The whole sound-perceived versus sound-picked up by the mic is really interesting. Chris, thanks for 'starting at the beginning.' I was in a small artist's studio on the third floor of a large building which (to my ears) was silent after we turned off the desktop computer that was on in the back. However, there were two hot lights in softboxes, a larger and a smaller one, shining right on the subject. Do lights send out signals that might cause a hum or buzz? I don't recall if there was A/C on, but I'll check that next time too.

Your test is a perfect diagnostic tool. I'll go through these steps when we set up next time, regardless of which mic we end up using. If it's the camera, then I suppose I'll simply have to work around it.

David, based on your experience with the walkway lamp, it sounds like lights could indeed be the cause of a buzz. Hmm, I'm not sure how to correct that since we need lights on our subjects. Assuming the lights are the source of the offending noise, are there 'quieter' lights? Or mics less sensitive to ambient noise/more sensitive to the nearby voice? Is the lav mike any more or less sensitive than a shotgun or boom, or is it just the quality of the product that matters?

Finally, can anyone with a GL2 confirm or deny that the camera may itself be the source of the buzz?

Thanks so much for your help.

Sara
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Old July 5th, 2007, 12:17 PM   #9
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Discharge lamps, such as fluorescent, low pressure sodium, and mercury vapor can put out RF noise, which if strong enough can 'leak' into a wireless system, or conventional radios for that matter. Same for electronic lamp dimmers on conventional incandescent lamps. It is situational, and depends in part of the specifics of the setup you have, including potentially the specific equipment you are using (due to manufacturing variances).

(Kind of like cell phones causing noise in conventional telephone receivers if near by as they handshake with the local cell tower.)

As to the transmitter providing phantom power to the mic - will depend on the system used - you would have to look at the manual. However, most moderate cost wireless systems do not provide phantom power.

From what you say, "line level output" does not sound like an issue. Wireless mic receivers for video typically have two outputs. One for head phones, and one for the camcorder. The camcorder output is often MIC level, meaning an output voltage on the order of 0.01 volts for normal voice levels, that plugs in to the mic jack. But some may have a "line level" output which is more like 0.3 volts or more. And on some this may be selectable with a switch, from a menu, or a small volume control. Again, this should be described in the receiver manual.

However, be sure you are not connecting to a headphone jack on the wireless system. The output from the headphone jack can be lower quality and may be problematic. And if your system is line level output, someone may have adjusted the transmitter input level to something other than optimum for use with the GL2, which could result in unwanted noise creeping in.

A normally functioning GL2 should not be inserting objectionable noise into the audio.
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Old July 5th, 2007, 02:00 PM   #10
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Quote:
Originally Posted by Sara Cantor View Post
Sara,
1. ... how would I know if the transmitter was supplying power to a mic that didn't want it? ...
2. ... You said a common error is for the receiver to be putting out a "line level signal." What does that mean? ...
3. ... Assuming the lights are the source of the offending noise, are there 'quieter' lights?...
4. ... can anyone with a GL2 confirm or deny that the camera may itself be the source of the buzz? ...Sara
1. 1/8" mic input minijacks on many devices are designed to automatically supply 2-3 volts to connected mics. It normally can't be turned off. Most mics that don't need the power aren't bothered by it, but it's a possibility. More serious is "phantom power." Many professional mics require phantom power, which is 12-48 volts supplied by the mic jack (normally an XLR type jack) and carried by the same wires that carry the mic signal. Many pro devices are capable of supplying that power. It's usually switchable. Some self-powered mics can be bothered or even ruined by phantom power. We'd have to know what Senn wireless system and mic you were using to comment further.
2. Microphones put out a very small voltage on the average. It's typically on the order of 0.003 to 0.01 volts. This is called a mic level signal. The circuitry connected to a mic jack, of course, expects a mic level signal. But other pieces of audio equipment share audio signals at a higher voltage, called line level. Line level is way to "hot" for mic inputs. There are several standard line levels, but they're all in the neighborhood of 1 volt. The levels are usually expressed in decibels (dB) rather than volts. Mics are typically in the -40 to -55 dB range, and line levels range from -10 to +1.8 dB
3. Buzzing isn't normal for most lights I've been around (but I'm not a big lighting guy). To me it indicates some kind of defect. The ballasts that are used in fluorescent lights often buzz, though. At any rate, if you deliberately listen for them, you can hear any noises in the room that a mic will pick up.
4. Never say never, but it's unlikely that the GL2 is the problem.
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Old July 7th, 2007, 02:29 AM   #11
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Hi again...........

Sara,

What I left out of my first post, but which should really be the first thing to do with the diagnostics, is, of course.......

...................short circuit the mic acoustically,

- by which I mean, leave it connected electrically, but stuff it into something that shields it from all, or as much as possible all, acoustic vibration. This will tell you whether you have an acoustic or electrical problem.

What that "thing" is to stuff it (so to speak) depends on what is to hand - with a lav, your hand is a good start - just grab a handfull of t - shirt, wrap it round the lav and close your fist over it, job done. If the interference stops - it's acoustic. If not, it's probably electro - magnetic.

Another thing to note is that a bad electrical connection anywhere in the chain can wreak havoc. It's worth tearing the entire chain apart and checking for frayed/ loose cables, dirty contacts, bent pins, missing insulation OR a severely, permanently bent cable - this may indicate you've lost your earth/ shield and that the hum/ buzz is induced mains hum - from just about any mains cableing within 100 yards. Also check that every single cable exit/ entry point has adequate cable restraints that work - if the cable is loose at any of these points, one or other of the conductors may have been stressed into parting company with it's proper connection point.

Also bear in mind that rented gear takes a REAL hammering, and even with the best will in the world, the rental companies aren't mind readers when it comes to finding/ fixing faults with stuff that has been er, "over extended' shall we say.

With wireless gear it IS possible for that dinky little aerial to get fatigued and fail where it enters the unit - bloody hard to spot but can give rise to some interesting sleuthing (and even more interesting symptoms).

Going back to the percieved by ear/ picked up by mic thing for a moment - it's my experience that setting up a mic just about anywhere, through a system of any sort, and listening to it on cans will, in 99.9% of situations, pick up a totally different "picture" to the one your unaided ears do. Take the cans off and you wonder just where all that sound is coming from.

Whatever, get back to us with your final vedict, if you ever have one, on where, exactly, the culprit was.

CS
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