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Cell phone noise in microphones?
MUFF WIGGLER Forum Index -> Production Techniques  
Author Cell phone noise in microphones?
sduck
So, I play in a symphony orchestra. And most of what we do has some level of miking going on, either for recording or amplification. And it's all high end stuff AFAIK - we win grammy awards for our recordings, so it's got to be pretty high end stuff. Lately we've been getting some flak about our cell phones, and how noise from them is bleeding into the mikes, wiring, etc. Our audio guys say it's happening, some other audio guys say it can't be happening, so there's some debate.

Wonder if anyone here who works with audio is familiar with this and can provide some insight? I'm on the orchestra committee, and we asked for details on this, and this is what our audio guy sent us:

"A little more explanation on the cell phone noise. There is a lot of information available online if any musician would like to research it more.

Phones using GSM standards cause most buzzing. Phones using LTE and CDMA rarely do. GSM networks are less common than they used to be, but are still ubiquitous. (usually Sprint, AT&T and T-mobile)
The interference is caused by the rapid rate of on/off signaling between phone and base station at cellular frequencies. If these “pulsed data bursts” are able to leak into the audio signal path, they are rectified by electronic components within the device, often by the first amplifier they encounter, which introduces false voltages at audio frequencies into the signal and, down the line, into audible interference when the signal is converted into audio by a speaker.
The type of interference can occur if the following things happen together:
1) a pulsing radio transmitter, (cell phone & tower)
2) with relatively strong power,
3) in very close proximity,
4) to a non-linear circuit element. (microphone, guitar amp etc..)

The non-linear circuit element is usually some sort of solid state
device such as a transistor or diode. If the non-linear element is
subjected to a strong pulsing radio signal, it will act as a rectifier
and "detect" the pulsating waveform, i.e., convert the pulsations from
a radio frequency to an audio frequency (if the pulsation rate is in
the pass-band of audio frequencies.)

For example, a hearing aid consists of a microphone, an audio amplifier and a small speaker. If
a strong pulsating radio signal impinges upon the first transistor
amplifier stage, the transistor will be driven into its non-linear
range and detect the pulsations. If the pulsation rate is in the
audio frequency range, the rest of the hearing aid amplifier will
amplify this and deliver it to the speaker, to the great annoyance of
the hearing aid wearer.

Here is an audio example of what the interference sounds like. I’m sure they have all heard this before."



...and yes, that is certainly one of the extraneous sounds we've heard often.
slow_riot
What your audio guy says is absolutely correct. I've never noticed it audibly in my personal studio experiments or on stage (DJs and live synths), except in a Tascam Portastudio that I'm assuming cut all kinds of corners in design.

My own knowledge on the matter would lead me to believe that good design practice can drastically improve the immunity of electronics to RFI. In a mic, I'm guessing the mode of interference is through the long length cable being driven at a high impedance and the high gain amplifier. In that instance both shielding on the cable and a couple of pFs of capacitance to shield at the receiving end should give strong attenuation results.

The level of knowledge given by your sound guy on the matter suggests everything economically possible to attenuate the interference through engineering practice has been done. (rectification of RF signals into the audio band is pretty high level knowledge, it also creates DC bias problems). Maybe like you say, the sheer density of mic-ing necessary backs you into a corner on this. Mobile phones have increased heavily in popularity and complexity, and even when they were first introduced many recording studios banned their use.
rockthomas
Is it audible on the recording or just through the PA? Does it happen only during a performance?
sduck
^this I don't know - I'm not on the engineering side of this.

And thanks slow riot - I also got further confirmation from another trusted engineer, so now it's time to figure out some way of getting 80 people to actually do something about their phones. Hmmmm
slow_riot
sduck wrote:

And thanks slow riot - I also got further confirmation from another trusted engineer, so now it's time to figure out some way of getting 80 people to actually do something about their phones. Hmmmm


80 phones in a heavily mic-ed room is almost certainly going to cause trouble. Lots of phones now have "airplane mode", which I think cuts out most or even all of the interference generating mechanisms.
wsy
Explanation of the original post is absolutely correct.

GSM phone systems work by timeslicing - each phone (even ones not in an active call) have to "give a shout" every N
seconds to tell the cell site they're still alive and operating.

LTE and CDMA do this too, but the CDMA and LTE use a long duration, low power signal (i.e. the signal ramps up in power, stays
for a while, then ramps down). The signal is barely above background noise; spread-spectrum techniques are needed to pull the
slowly-moving signal out of the background noise. If you could hear the digital CDMA signal, reading the numbers from one to five, it would be like this:

oooooooooonnnnnnnnnnnneeeeeeeeeeettttttttttwwwwwwwwwwwwwooooooooootttt ttttttthhhhhhhhhrrrrrrrreeeeeeeeeeeeefffffffffooooooooouuuuuuuuuurrrrr rrrrrrffffffffffiiiiiiiiiiiivvvvvvvvvvvveeeeeeeee


but GSM does it by putting a very hign amount of energy into a very short burst, which is actually more power efficient but tends to
interfere with other sensitive electronics nearby. It would sound like someone shouting in your ear:

12345!!!!


It's the envelope of the short burst of powerful transmission that your microphone circuits are picking up.

As to fixing it - tell them to switch their phones into "airplane" mode. Or get a 4G phone and make sure you have good 4G
service in the studio.

Shielding is difficult due to the very short wavelength of GSM and the relatively high instantanous power
(usually several *watts* - see the charts down page on:

http://www.radio-electronics.com/info/cellulartelecomms/gsm_technical/ power-control-classes-amplifier.php)

So yeah - airplane mode when recording a "take" may be the best you can hope for. Also if you can _play back_ a
particularly bad instance of "GSM Ticks" recorded in that particular venue to your orchestra, that might help
compliance considerably.

Sorry, no easy answers....

- Bill
felixer
there is one way to get rid of that noise: switch 'm off, get a big metal box and put all the cellphones inside. then close the box hihi if you want to be sure nobody is listening in, remove the sim-cards in the phones. this methode is used by the military ... they use an empty ammunition box ...

anyway, cellphones are a pest. why would anybody have one of those while recording or even rehearsing? switch 'm off at the very least ...

in schools the cure is often a bucket of water. whenever a cellphone goes of, it ends in the bucket Mr. Green that is a rather final way of dealing with the problem, but it is a problem and sometimes you have to be ruthless cool
Hainbach
sduck wrote:
^this I don't know - I'm not on the engineering side of this.

And thanks slow riot - I also got further confirmation from another trusted engineer, so now it's time to figure out some way of getting 80 people to actually do something about their phones. Hmmmm


I wish you strong nerves with that. Used to be that everyone had to switch of their phones, that has gotten wiped by modern life vey quickly.
wsy
Hainbach wrote:
sduck wrote:
^this I don't know - I'm not on the engineering side of this.

And thanks slow riot - I also got further confirmation from another trusted engineer, so now it's time to figure out some way of getting 80 people to actually do something about their phones. Hmmmm


I wish you strong nerves with that. Used to be that everyone had to switch of their phones, that has gotten wiped by modern life vey quickly.


Yeah, people have become normalized to "instant contact anytime" as well as "terrorists are under EVERY ROCK!!! OMG!!!"

Silent / vibrate mode is perhaps acceptable because it doesn't actively disturb others, but that won't help your recordings - it's not
the audio of ringing, but rather the RF as the cellphone radio turns on it's transmitter to sync with the cell tower.

Sorry that I don't have a better answer....

- Bill
felixer
seriously: is anybody going to answer an incoming call in the middle of a recording or rehearsal? so there is your argument.
it's just silly. you get yer thee/coffee-break regularly so that is the time to do those things if you have to ... and i think quite a few people will secretly love you for taking a stance agains cellphones: finally they are 'off the hook'. and they can blame someone else for it hihi
sduck
felixer wrote:
seriously: is anybody going to answer an incoming call in the middle of a recording or rehearsal?


It's happened. Pissed me off - it was my stand partner. And a sub. I would have thought they wouldn't be asked back, but it was ignored.

The biggest part of the problem are the various parents in the orchestra, who mostly put their phones into silent mode but keep them on to receive texts, so they can instantly respond if their kids have some kind of issue. They're "supposed" to have such contacts notify the personnel manager, but that rarely gets set up.

Anyway, this is getting off topic - my technical questions have been answered, and the sociological part is a whole different kettle of fish. So thanks!
felixer
sduck wrote:
felixer wrote:
seriously: is anybody going to answer an incoming call in the middle of a recording or rehearsal?


It's happened. Pissed me off - it was my stand partner. And a sub. I would have thought they wouldn't be asked back, but it was ignored.

well, there you have it! you are bringing this upon yourselves meh fight back!
ignatius
every studio i ever worked in the engineer/producer would make everyone turn off their cell phones before recording started. it ALWAYS would cause noise in the speakers/mics. crackling, noise etc.

edit: oh shit i sucked in by a thread bump from a spam bot. sweet. gonna turn off my brain now.
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