Topic: Cell phone interference: how can I shield against it / what's wrong with my rig?

[whatboutbob]

Ok folks, this is going to be a bit of a novel, because there's a bit of detail to go into.

Short story is: I'm getting a lot of cell phone interference on about half of my recordings and I'd like to stop it, cos its ruining otherwise perfectly good recordings and driving me crazy.  Any help would be appreciated.

I've read all the threads on this subject on this board, and almost revived an old thread but decided to create this one because the old thread hasn't seen a post for 6 months.

I suspect this is more of an issue for me than most of you lot because most of the cell phones in Australia are on a GSM network...I believe that's not the case in the US (pls correct me if I'm wrong).

First point to make is that the interference is not caused by my phone, which is off at all times during recording.  The interference is from those lovely people around me.  I've tested at home and am picking up cell phone interference from up to 10 meters away (I haven't tested any further than that).

I've run hundreds of tests using the following equipment in different combinations:

Mics: at943 bodies with at853 caps (mini xlr ends)...I believe the cable is standard (it has 'audio technica' written on it), but they're only 4 feet long.
Power: homemade 3 wire battery box (as per Grame Cogger's schematics) and Church Audio 3 wire bb / preamp
ADC / pre: AD20
Recorder: iriver H120 * 3 (yeah, I've tested on all three of 'em)

The tests werent terribly scientific, but I tried to be as consistant as possible...I basically recorded silence with different combos, placed a phone nearby, made lots of calls, and noted the levels in Asobe Audition before and during the resultant interference.

I won't post all the data, unless someone wants it, but what I've found most enlightening from these results is that a particular mic into a particular channel sees more interference than when the mics are switched around.

So for example, with this config:

Mic 1 (R), Mic 2 (L) > Church audio bb/pre > H120 (with 20dB gain both channels): Base Levels: -57dB (L), -60dB (R).  Levels w/ interference: -36dB, -58dB

...whereas if I switch the mics around:

Mic 2 (R), Mic 1 (L): Base Levels: -58dB (L), -60dB (R). Levels w/ interference: -48dB, -40dB

The same bears out with all other combinations - ie if I apply the 20dB gain using the Church audio Pre, or replace the pre with my homemade bb, or add the AD20 after either of the bb's.

I can't figure out why this would be the case, so would appreciate any advice.

One other thing to throw into the mix, which i'm not sure is pertinent...one of the lead Rockbox developers (LinusN) has mentioned that some iriver units are not grounded correctly, though he was talking with regards to  tiking introduced to recording by plugging in the remote: "...the problem is that the analog and digital ground are connected on the ticking units...most likely a mistake, since there is still a resistance.".

If you've gotten this far, thanks for reading.

[rokpunk]

turn your phone off.

[it-goes-to-eleven]

I always get nervous when the person next to me starts taking pics with their phone or, worse, calls someone so they can hear the show.. So far I have not noticed anything that I would attribute to phones.  But interference from a phone 30 feet away.. Wow!

First question...  What gear are you using that is not shielded?  The plastic cases on battery boxes and small preamps may be an indication that their is no shielding. There might be a shield under the plastic but probably not.  You might try repeating your test while running the mics into a shielded device like a ua5, v3, whatever.

At this point I'd try and isolate it to your mics, your bbox, your cables, or the iriver. Maybe repeat the test with no mics and just the iriver+patch cord.

Have you verified that all of your cables have shields and that the shields are well grounded?

[guitard]

turn your phone off.

the interference is not caused by my phone, which is off at all times during recording.

[nic]

only thing I can think that will keep your gear interference free is to build something like a faraday cage and keep your gear in it during a show.

[whatboutbob]

Thanks for your advice thus far guys.  I just broke out a few other knick-nacks to test with.  Something I've just found that is particularly galling, is that my Giant Squid Audio Omni mics > matching battery box > iriver gets no interference. At first I thought it may be because the bb is in a metal box, so I tried leaving the top of it off, but still no interference.  Now, onto Freelunch's questions:

I always get nervous when the person next to me starts taking pics with their phone or, worse, calls someone so they can hear the show.. So far I have not noticed anything that I would attribute to phones.  But interference from a phone 30 feet away.. Wow!

First question...  What gear are you using that is not shielded?  The plastic cases on battery boxes and small preamps may be an indication that their is no shielding. There might be a shield under the plastic but probably not.  You might try repeating your test while running the mics into a shielded device like a ua5, v3, whatever.

At this point I'd try and isolate it to your mics, your bbox, your cables, or the iriver. Maybe repeat the test with no mics and just the iriver+patch cord.

Have you verified that all of your cables have shields and that the shields are well grounded?

The AT853 mic cables have shields. I did the soldering on the mini-xlr ends myself, and it looks ok...but...there's every chance I could've screwed something up.  In my favour though is that I've got another pair of older AT853's that I haven't taken a soldering iron anywhere near, and I still get interference running them > AT8505s > PS2 > AD20 (also w/o) > iriver.  The interference is definitely louder with the mics plugged in tho.

Homemade 3 wire bb: Definitely not shielded. I built it in a plastic box cos I couldn't find a metal box to fit it in.  Because of the interference I thought I had screwed up the grounding somehow and I don't know enough about this stuff to troubleshoot very well, so after ripping things to piece time and again I got the church audio bbox/pre.

Church Audio 9100 bb/pre: Again, plastic...I haven't opened it up but I doubt it is shielded.

PS2: The interference running everything through this isn't as bad, but it is still there.

Would love to run into a fully-shielded device, but damned if I know where I could get one.

So...where does that get me?  Because the GSA mics are clean, does that make it safe to rule out the iriver as the problem?  I also tested with a patch cable running out of the iriver...clean again.

If so, that leaves us with the mics and/or power.

I guess I could build a Y cable (or whatever its called) to run from my mic mini-xlrs to a 1/8 jack so that I could run them directly into my iriver to isolate them?  If that gets a clean bill of health then its definitely the power supplies...right?  Can y'all think of any other way of going at it?

I've got some copper shielding tape lying about. Could I potentially use that to shield the power supplies?  If so, how would I ground it?  If its not grounded it would act like an antenna, wouldn't it?

Thanks again.

[it-goes-to-eleven]

Cool - I think that mostly rules out the iriver for now.

At this point.. I'd try and put everything except the at853's in a big metal box that is gounded to a water pipe or electrical ground (careful).  Maybe an old tool box, or PC case. Maybe do it two ways - once with the electronics in the box and again with the mics and cables in the box.

There could still be something more funky going on with grounds in the cable, etc, but hopefully this will narrow it down further.  I suppose it is possible that the bbox/pre is sensitive and maybe the mic cables are carrying the signal into it.. If so, putting it into the box may not help.

[whatboutbob]

Ok, a cooking pot served the purpose.

Whole kit in the pot: no interference

Mics/cables out of the pot, iriver/church audio b/pre in: interference (though this *could* be attributed to an incomplete seal)

I suppose I should do it the other way around (mics in the pot) just to be sure. I'm off to bed though.  Just woke my gf by pottering around in the bathroom.  Does it say something about me that she walked into the bathroom @ 2am to see me putting my mics in a giant pot in the sink and she didn't say a word...just turned around and went back to bed...

[Brian Skalinder]

Just woke my gf by pottering around in the bathroom.  Does it say something about me that she walked into the bathroom @ 2am to see me putting my mics in a giant pot in the sink and she didn't say a word...just turned around and went back to bed...

   She's a keeper! 

[Humbug]

Brilliant, just briliiant +t for the girl

[whatboutbob]

Ah.. A cooking pot.  Of course!

That's good progress.  I think a reverse test would be the next step to make sure the seal wasn't to blame - mics in and boxes out.

Also, a test where the mics and bbox/p are in the pot but the mic cables are out and exposed might help. Another test would be poking just the mics out with the cables/bbox/pre in.  Might tell you whether it is the cables or the mics.

Just tried mics in, the rest out. Interference resulted.

Could I wrap stuff in aluminium foil for a better seal?

Will try again with the other tests tomorrow night...I think I'm pushing my luck with my better half. ;-)

Thanks again for all your help.

[SparkE!]

Your problem could be due to grounding of the shield to signal ground at both ends of the cable, thereby causing a ground loop.  You could try lifting the shield ground at the mic end of the cable.  If that's not possible, try lifting the shield from signal ground at the other end of the mic cable.  If that reduces the interference, then you know you've got a ground loop issue and we can continue troubleshooting from there.

[Digital Quality]

First thing I would try is taking mic cables completely out of the mix. Hook the mics directly into the pre and give it a shot.
Next put the cables back and hold the phone close to the mics or maybe borrow different mics from a friend.
Same thing for the pre.

Seems like you should be able to move your phone up and down the line while someone is ringing you and the interference will get stronger as you get closer to the leak.

[whatboutbob]

Your problem could be due to grounding of the shield to signal ground at both ends of the cable, thereby causing a ground loop.  You could try lifting the shield ground at the mic end of the cable.  If that's not possible, try lifting the shield from signal ground at the other end of the mic cable.  If that reduces the interference, then you know you've got a ground loop issue and we can continue troubleshooting from there.

I'd rather not do this at the mic end...I think I'd break something.

To 'lift the shield' at the mini-xlr end, do I just snip the shield in the mic cable at the mini-xlr and tape it up?

First thing I would try is taking mic cables completely out of the mix. Hook the mics directly into the pre and give it a shot.
Next put the cables back and hold the phone close to the mics or maybe borrow different mics from a friend.
Same thing for the pre.

The mics aren't detachable. They've got 4 ft of cable attached to 'em.  Don't know any other tapers in this country.

Seems like you should be able to move your phone up and down the line while someone is ringing you and the interference will get stronger as you get closer to the leak.

Moving the phone was my first thought...the problem is the interference isn't a consistant strength/length.  I can leave everything exactly as is, call 3 times, and get three different levels of intereference.  Maybe if the mic cables were longer than 4 ft...

[Digital Quality]

Can you feed it a line in (no mic or cable) whilst Sheila calls you? Turn it off and back on may be another good way to get a signal but I think incomming call would still be the best. The phone sends stronger signals depending on distance to the tower and other traffic.

I do know some aussie sim racers but no tapers.

It's most likely in the cable because it's also the biggest antenna.