Cellular Signal Noise

[Sacha Rosen]

Turn off the God damm phones!

[Steve House]

Originally Posted by Clint Harmon

Anyone have any experience constructing a "shield" for their microphone? When I get some free time I am going to try wrapping the wires in aluminum foil and see if that cuts that signal.

The SMX10 is a consumer mic with an unbalanced cable and unbalanced is more susceptible to electrical interference than are balanced XLR mics. Upgrade to a more professional audio kit.

As for your photographer, tell him that there is no phone call he could possibly take that is more important than your shoot's audio. Tell him to turn it off when you're rolling and don't take no for an answer - that's why they invented voicemail. If he won't comply, confiscate the phone and if he balks at that, fire him.

[Andy Tejral]

Originally Posted by Steve House

The SMX10 is a consumer mic with an unbalanced cable and unbalanced is more susceptible to electrical interference than are balanced XLR mics. Upgrade to a more professional audio kit.

I've got the noise in my house/studio. I think it gets picked up in the actual electronics not the wires. I hear it when my Mackie Big Knob is turned off but the amp is turned on. (and when everything is turned on too.)

Could still be those wires acting as an antenna but they are balanced. I've found Mackie mixers (1402) not particularly good at rejecting RF noise.

[Paul R Johnson]

Plastic cases offer damn all screening, and cell phone interference can get inside kit by so many routes. I can leave me phone sitting on a large theatre style sound mixer, with no problems, but putting it onto the cd player means instant noise. It's also worse when you are further from the nearest cellphone mast, as the phones adjust their output power to suit how far the RF needs to travel. Sometimes it seems clear that the rf gets picked up on the cable screen, and this is it's entry point to sensitive areas of the equipment.

[Chris Swanberg]

I'm with Sacha and Paul. There are innumerable ways that invidious signal can find its way in. I think "quiet on the set" should be finished with "And turn your G*****AM cell phones,blackberries, iPhones and whatever the hell you have on you that receives or sends a signal the hell OFF !!!" I don't mind retakes terribly... but I HATE retakes because of THAT !

[Luke Arndt]

I have run into this problem multiple times. For a while, I suspected the interference was related to cell phones, but it was only recently that I discovered that it was a GSM-only thing, and that CDMA phones seem to not cause any trouble.

Anyway, I have four XLR Audio-Technica lavalier mics. I think the model is AT803B (Audio-Technica | AT803B - Mini Clip-On Mic | AT803 | B&H Photo).

I have them running to a Peavy PV6 (Peavey.com :: PV Series) compact mixer.

From there, I use the 1/4-inch headphone output, and have an XLR cable running as an input to my Sony PD150 camera.

The buzz seems to always be coming from one of the microphones. That is, if I hear the buzz, then I can figure out which mic it's coming from by muting one at a time. So I really don't think the problem is the mixer or the camera.

So basically, I'm looking for any ideas of how to solve this. Obviously, turning all phones off is a sure-fire solution. Short of that, though, I'm just looking for any ideas.

I have heard talk of balanced and unbalanced cables. Honestly, I'm not too familiar with that, but could it be simply a matter of buying different XLR cables to connect the mics to the mixer?

Or would a different kind (or higher-quality) mic be immune to this interference?

I have also seen people mention using ferrite chokers on the XLR cables. I haven't tried this, but I might give it a shot. (Trouble is, I don't have any GSM phones in the house, so it's difficult to test on my own!)

Also, I know that I might be able to avoid the buzz by having all phones placed in an antistatic bag of some sort. (iPhone, the GSM buzz, ferrite shield and static bags - Mac Forums, Stop Cell Phone Speaker Buzz - Home)

So the problem is avoidable by either having people turn their phones off completely, or possibly by placing them in/on a little bag.

What I'm looking for is just some fix on my end, whether it's replacing mics or cables, or modifying mics or cables -- such as wrapping them in foil, lead, or something else. :-)

Like I said, I'm pretty sure all is okay from the mixer on, so it seems like the interference gets picked up on the mic itself or the XLR cable connecting the mic to the mixer.

I'd appreciate any insight! It's not a critical issue, as it's avoidable with asking people to shut their phones off, but it sure would be nice if I wouldn't even have to ask people to do that.

Thanks,

Luke

[Luke Arndt]

Okay, I gotta ask... anybody ever come up with any kind of solution for this?

[Greg Miller]

A chicken wire enclosure is not a true Faraday cage. The mesh needs to be made of a material that is a good conductor, and all the junctions (within each sheet of mesh, and between adjacent sheets) need to be electrically sound (e.g. soldered). Steel-based "chicken wire" would not have good conductivity. The mechanical junctions within each sheet of chicken wire could have a lot of electrical resistance, and if any corrosion is present they might even act as diode junctions, rectifying the cell phone signals. And I very much doubt that the various sheets of mesh, which comprise the different "walls" of the cage, are electrically connected together (e.g. soldered). So the cage you describe is sure to be leaky, with more leakage around its door frame, too.

If you want a valid test, put your cellphone inside a microwave oven (do NOT turn on the oven) and see how well that shielding works. The oven is certainly designed to keep several hundred watts of RF safely inside, so it should completely block any signals from a cell phone.

[Chris Soucy]

Hi, Luke...............

This is one of those "How long is a piece of string" questions.

OK, these devices emit high frequency RF.

Depending on how close they are to a possible receiving device, the power output of the emiitter, the size of the antenna on both emitter and receiver, the efficiency of the demodulation on the receiver and the amount of amplification the resultant demodualated signal is subject to, you will, or will not, have a problem.

Points of entry:

Lav Mics - plastic body = no shielding, unbalanced connection cable - excellent aerial and no rejection from amplification stage.

Pre Amp: Unbalanced input = no rejection of and resultant subsequent amplification of demodulated RF

Pre Amp Destination Device: Further amplification of demodulated RF along with required audio.

Bottom line: Unless everything is shielded (grounded metal case), all connections are balanced (and the further down the amplification chain the receiver is, makes it worse - I'm talking distance away from the actual recording device) you don't have a snowballs chance in the Sahara of keeping this stuff out.

Just a brief explanation of balanced and unbalanced connections and why it matters.

Unalanced: Two wires for a Mono feed, Ground and Live/ Signal.

Any amplifier amplifies the difference between the gound and live wires, ground is always zero so any noise on the live if fair game for amplification.

Balanced: Two wires for a Mono feed, neither of which is Ground, ground is a seperate shield physically surrounding both of the "active" signal wires. Only the difference between the signal on both the "live" wires is amplified.

Any "noise" will be picked up equally by both wires and thus cancelled out by the resultant amplifier.

Result: Well, with electrickery at high RF frequencies, who knows, maybe it'll work, and maybe it won't.

Only takes one dodgy solder connection to undo the entire edifice.


CS

[Richard Crowley]

Mr. Soucy has some excellent suggestions for avoiding and reducing audio-band noise and RF interference (RFI) from distant sources like AM or FM broadcast transmitters, etc. He mentions taking advantage of the common-mode rejection feature of balanced circuits. While very valuable for rejecting audio-range interference (mainly power mains hum, etc.), its effectiveness at radio frequencies is slim to none. Which is why you find people using inductors (commonly seen as cable "lumps" around the wires) to effectively "short out" high radio frequencies. Sometimes you even see a cable looped through the inductor several times for increased attenuation.

But, alas, I have never seen any of these measures have any significant benefit to cell phone interference. The problem is that the receiver of the wireless system just can't handle that kind of (relatively) high-power impulse noise. In fact the kind of signals that cell phones transmit is very similar to what communist countries used to use for "jammers" to prevent their citizens from hearing radio (and TV) signals from free countries.

The ONLY method I have seen that is guaranteed to work is to turn OFF (or "airplane mode") any personal devices that transmit. There are stories about Hollywood film producers who take extreme measures to prevent cell phone interference (RF, acoustic, and attention) on the set. They confiscate the phone of any violator and nail the phone to the nearest fencepost or doorframe.

Of course, it depends on what country you are in, and which telephone service is involved (because they use different frequency bands) and how close those bands are to the frequency used by the wireless mic system(s).

[Andrew Smith]

Was shooting a conference recently and we had low levels of a radio station signal coming through the PA system. Given that I was the cameraman, I automatically became the AV expert. Tried everything at the PA cabinet.

It was only later in the day that we found a handheld wireless mic in the hall and switched it off. Problem suddenly solved. Yea, a quality conferencing venue it was. :-) (The staff there were excellent, I must add.)

Thank goodness I had my own clean feed off a wireless lapel mic on the talent. I'd hate to be faced with removing a radio station programme from my recorded audio.

Andrew

[Steve Game]

Originally Posted by Richard Crowley

Of course, it depends on what country you are in, and which telephone service is involved (because they use different frequency bands) and how close those bands are to the frequency used by the wireless mic system(s).

Its only whether the local mobile phone system is digital or analogue that makes much difference. Analogue systems don't seem to have the same effect other than raising the overall noise level in the band. Digital systems (GSM,PCN) do however have a major impact and it is not necessarily relevant whether the band is near that of the microphone system RF frequency (or a harmonic if it).
The problem is that high levels of RF produced by handsets, (up to 2W) pushed through indifferent antenna systems produce spurious couplings into any local conductors, including headsets, mains wiring and speaker cables. So there can be unexpected levels of RF injected into sensitive equipment anywhere in an area where there are people with phones in their pockets. When GSM type systems transmit, they modulate the RF with digitised audio resulting in RF pulses as well as streams of control pulses. This saves battery power and allows multiple handsets to multiplex into each RF channel.
So every piece of sensitive equipment must be designed to prevent these bursts of RF appearing at semiconductor devices whether their bandwidth extends into the RF spectrum or not. Even audio and power supply devices can act as detector diodes, injecting baseband pulses throughout the system. The the basis of good EMC design is to create immunity from all frequencies that aren't wanted, not just catering for the wanted ones. So a VHF microphone receiver must be protected from all RF signals except those in the specific VHF band it is designed for. Furthermore, that immunity should be high enough to prevent breakthrough from all likely sources in the area. So if mobile phones are to be assumed, the receiver's EMC qualification test should cover such radiation and conduction levels.
One final point, You can buy the kit with the best EMC performance available, but if it is integrated with inadequate cables and ancillaries, e.g. power supplies and audio devices, the system EMC performance will be compromised. System EMC performance is the responsibility of the on-site engineers.

[Cole McDonald]

I upgraded my audio capture kit to help solve these problems... the issue is that the cable acts like an RF antenna, the longer your cable run, the more it'll pick out of the air and put in your soundscape.

Make the move to balanced XLR. You'll be very happy. The XLR uses 3 cables, one for round and two for signal. It inverts the signal on one lead, then inverts it back at the other end. When the two leads are merged back together, any inverse waves cancel each other out... specifically, anything that has entered the cable through induction.

Cells turned off on set, absolutely, but helping the problem with upgrading your audio setup is a good thing... I specifically upgraded due to switching to Fluorescent lights (CFLs) that threw a ton of RF around my set.

[Luke Arndt]

Cole,

My Audio Technica mics (AT803B, http://www.bhphotovideo.com/c/product/68272-REG/Audio_Technica_AT803_AT803B_Mini_Clip_On.html) connect via XLR to the mixer, then the mixer via XLR to the camera.

I'm pretty sure that the noise is entering on the mic "head" itself, and not the power pack or the XLR cable. Meaning if I set the actual microphone on top of an offending cell phone, then I'm going to get a LOT of noise. However, if It set the power pack on top of the phone or the XLR cable on top of the phone, the noise will be significantly less or gone... leading me to believe that the point of entry is the head itself.

If this is true, then would balanced XLR cables still help? I guess I'm wondering if a balanced XLR cable (or anything else) can take the cell phone noise out of the mic/cable once it has already entered the stream, so to speak.

Thanks for replying!

Luke

[Nate Haustein]

The other day, I was shooting with a shotgun mic on a pole connected via XLR. I was also running about 25' of HDMI to an external monitor for to the director. Everything was working fine all morning, but all of a sudden, we started to hear talk radio in the headphones. Turns out, the power supply to the Marshall 7" monitor was causing the issue - switching back to battery power solved the problem. Maddening.