Name this noise...no wireless involved.
 

Hi all.

I was at a hospital in a conference room. Actually a university connected to hospital.

Simple set up. Doing voice overs. A 8 foot xlr wired Rode NT3 cardiod feeding into a Sound Devices 633. I had all cell phones off, even though we were not using any wireless equipment.

So during the record, I was picking up a sound that I can't figure out what it is. Mercifully, it sort of came and went, so it only messed up a few takes which we re-did. I also thought that maybe I was just hearing this through the headphones, but it was in fact recording to media.

I am not sure if this is some sort of periodic signal given off by something somewhere. I also realize that hospitals are not the ideal place to record audio. Especially with wireless.

What has me baffled is that we were not using any wireless, and STILL picking up.....something.

Any thoughts? A short sample is attached.

Jonathan

Re: Name this noise...no wireless involved.
 

There is absolutely no question on the planet that is the classic "ping" sound from a GSM cell phone.
Remember that when you tell people to mute their cell phones, it doesn't prevent them from sending and receiving RF (which is what you are hearing).
They must either put ALL their wireless devices into "Airplane Mode", or power then down completely.

Those strong RF pulses are quite happy to sneak into your SD 633 along with the mic signal. No wireless gear is required.
You can somewhat reduce RF getting into your mic inputs by using Ferrite gadgets around the mic cable.
But you will never completely get rid of such a horrible signal except by elimination of (or distance from) the source.

https://www.freesound.org/people/Rut.../sounds/50699/

Re: Name this noise...no wireless involved.
 

Definitely cell phone(s) hunting for a network signal. Like Richard said, either they have to be in airplane mode or off and I always liked off. I even would shut mine off. Not going to answer or talk or check the interweb while I'm shooting anyway.

Re: Name this noise...no wireless involved.
 

Yup. Cell phone interference.

Interestingly, there are many ways to get RF interference into an audio signal chain. One of my favorites is through a power supply, in particular those wall warts. The cable running between the wall wart and the device it's feeding can often do an acceptable approximation of an antenna. And yes, I've had that happen with a SD MixPre-D. In that particular case, running on batteries eliminated the problem.

Re: Name this noise...no wireless involved.
 

Richard and Don.

Always great to hear from you and thanks for your quick reply.

So a few questions for you:

I've purposely avoided wireless because of the interference issue. I had know idea that this could be a big problem with a wired set up. The audio world just got a little scarier!

I had everyone turn their phones off. (I realize someone may have cheated, but pretty sure all were off.)

Is it possible phones could still still cause any kind of interference even if they are off? May be a dumb ?

The room I was in was small, about 12 x 8 feet. There was only one person speaking their part at a time, thus a single mic. However when this was happening, there were about six of us in that room at once.

Lighting was fluorescent, but turned those off too because they were buzzing. Used incandescent desk lamp.

One guy had the giant IPhone 6 Plus. I am wondering if those give audio a bigger headache.

There is also a well used hallway just outside this room where people are constantly walking by. (Timed my takes between that, what a pain.)

So could someone just walking by with a phone cause this?

Any recommendations for this aside from shut off or at least Airplane mode?

Thanks again fellows.

Jonathan

Re: Name this noise...no wireless involved.
 

Thanks Bruce.

I had the SD 633 running off Sony BL batteries. No AC. The mic is either battery or phantom. There was a battery in the mic, even though I was using phantom on the mixer.
JL

Re: Name this noise...no wireless involved.
 

I GUARANTEE that is the sound from an ACTIVE cell phone. It is possible it might be from the phone of someone walking along the hallway next to the room. Typical office building walls will NOT block the RF signals (or your cell phones wouldn't work!)

A wireless device (remember it doesn't have to be a phone) that is completely powered down, or which is in "airplane mode", will NEVER EVER emit such an interfering signal as you are hearing. You noise came from a nearby, ACTIVE device. It could have been someones tablet or even computer. With 6 people in the room I can almost guarantee that you did NOT get ALL the wireless devices turned off.

Re: Name this noise...no wireless involved.
 

Richard.

You are a genius! Yes, my client who was sitting next to me, and my mixer did in fact have an IPad, which I'm guessing that was on. She also had a bluetooth keyboard attached if that makes a difference.

Can you answer this? How much worse would this situation have been with a wireless mic?

And in case anyone new is reading this: ALWAYS monitor your audio!!!!!! Not doing this is like taking pictures or shooting video without looking through the viewfinder!

Would you happen to know at what distance someone has to be with a cellphone for this to NOT occur? For example, someone walking by about 10 feet away with a cell device, can you count on that interfering?

And lastly, what is actually picking up the interference? The mic, mixer, cable, all three?

Very interesting thread.

Jonathan

Re: Name this noise...no wireless involved.
 

The only other thing I can think of off hand is to avoid being too neat. I have a tendency to want to leave cables coiled up as much as possible, to make things look neat. Unfortunately, coils can make antennas. Best practice seems to be to fully uncoil cables, not allow power and signal cables to run next to each other (instead cross them at right angles). You've probably done all that.

Even when you've done all the practical things you can, RFI can still enter your system. Perfection isn't possible. Which is why we monitor! At least then you know when it happens so you can either stop and fix it, or stop and run another take. Both are much better than trying to fix junk like this in post, which is often impossible.

Re: Name this noise...no wireless involved.
 

Wireless microphones are often resilient to this kind of noise, because as an RF device, it's already designed to reject out of band interference. Cell phones use brute force to interfere with sensitive electronics - close in they simply saturate the preamps and electronics, and you hear the noise. Wireless mics suffer from occasional interference can annoy you with drop outs and other nuisance noises. Apart from both using RF, they're very different beasts!

Your best protection is to do what you did - ask people to turn them off, and many will, but plenty won't. Hospitals used to ban radio devices because they said they interfered with machines like ECGs/EEGs and imaging equipment, but this was debunked when somebody pointed out that many hospitals run quite high power cellphones from towers on the roof, plus other radio systems, and banning mW equipment when hundreds of Watts are coming from just above the ward didn't make sense.

Sound people call a halt to interviews all the time because of noises - odd coughs, chair scrapes, passing ambulances and planes - a bit of interference from somebody passing has to be another stop. Just how it is?

My worst issue was an induction loop in a theatre - leaked into all sorts of stuff - plastic box on a mic phantom adaptor, electric guitar pickups, and a boundary mic - all discovered to be very susceptible to these strong fields.

Re: Name this noise...no wireless involved.
 

It is possible you might have a damaged cable (shield XLR pin-1 broken).
But even with a good cable the mic input to the mixer/recorder is a primary port of entry for RF interference.

There are ferrite "beads" (rather large "beads") that filter out most of the RF noise. You can get "split" versions that can be snapped around your mic cable. But if you have control of the situation eliminating the source is always much better than trying to filter it out.

Hollywood movie sets traditionally have a very strict Zero Tolerance Policy for cell phones. There are legends of famous-name directors who carry a hammer and nails. If anyone is caught with an interfering device (whether audibly ringing, or someone talking, or RF interference), the offending gadget is confiscated and literally nailed to the nearest door frame or fence post.

I have seen this kind of RFI from > 100m / 300 ft away. But it is much more likely within 20m / 60 ft

My signature line over on another forum is...
"Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder."

Re: Name this noise...no wireless involved.
 

Hi Paul. Thanks for that info.

Richard, I knew I couldn't have been that creative to come up with the photo and video analogy myself!

Those are some eye-brow raising stats you present there. Interesting that the interference like the inputs on the mixer. My 8 foot xlr cable is just about new. I never coil anything and avoid even getting near standard AC electric corded sources.

I also try to use just the length of xlr cable I need, even though, as I learned from Richard that you can run a hell of a long xlr cable run before problems.

Audio people are the reason psychologists are still in business I'm convinced...

JL

Re: Name this noise...no wireless involved.
 

Just curious....what venue were you at when this happened? Some in Chicagoland are better than others.
For example, I always hated the Inter-Continental on Michigan Ave for a number of reasons not the least being I seemed to get creamed with interference more there than many other places...The Marriott on Michigan was fine. Like I say, just curious.

Re: Name this noise...no wireless involved.
 

Hey Don.

I was at Rush Medical Center. I've received so much good info here that at least I will know to take a few extra measures to ensure a proper audio session...until my next gotcha. I got lucky this time.

Re: Name this noise...no wireless involved.
 

I never shot there but I have shot in other hospitals. Some of them were turrrrible with all kinds of interference no matter what you used or what you did. I'm talking about run N gun. Stuff in conference rooms were generally OK if you pulled a feed off the PA system but even then you had to be careful.