Wireless Cell Phones and Wireless Microphones?

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I had something interesting happen tonight, but first I'll set up the scenario-

I am a high school student lighting operator working tonight in our auditorium for a show put on as a fundraiser for one of our clubs, but I am also familiar with our sound setup. Our wireless microphone receivers (8 SLX4 and 8 ULXS4) are on a shelf above the lighting console, so it usually falls onto my lap to look into signal problems.

Just before the show began, I looked up to check that we were receiving signal from all of our mics (2 SLX1, and 3 SLX2). In the 30 seconds between when I checked the receivers and when the National Anthem soloist walked on stage with an SLX2, the channel assignment of the mic had changed to one being used by one of the other SLX2s, leaving us with two mics on one receiver. I attempted to contact a backstage crew member to have them give the soloist the last SLX2 that still worked, but they had abandoned their headset and gave the soloist the second SLX2 on that frequency. She finished and did great, but all we could do was sit back and watch. When I reached a crew member, they brought back to me the problem microphone and that is when I discovered the problem. I switched the channel back and all was good for the rest of the night.

After the show, I had found out that the microphone, just before it was given to the soloist, was being held in the same hand as a cell phone. This then reminded me of a time when I was acting and the channel had changed on my SLX1. We thought nothing of it and moved on. But as I recalled that happening, I realized I had my cell phone in my pocket.

My question is- Would it be appropriate to make the assumption that the presence of the cell phone caused our frequency changes?
 
The short answer? Probably not.

Edit: It's more likely that the transmitter (read: microphone) wasn't locked out and was inadvertently re-programmed. However, if you read Page 8 of the user manual, it does warn that cell phones *can* cause interference; regardless, everyone should be focused enough on the task/event on hand to not be glued to their cell phones.
 
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Yes, it's possible that RF induced into the mic from the cell phone caused the channel to change. Cell phones don't transmit with much power, but in very close proximity the signal density is quite strong. Holding both in the same hand is certainly close proximity.
 
Yes and no. Did the transmissions of the cell phone cause a frequency change, no. The amount of power so specific that it is able to rewrite a logic chip is is not from your consumer grade cell phone. Most likely, the hard edges of the phone hit the buttons in ways flesh couldn't and changed the frequency. Make sure to freq lock and power lock your transmitters to avoid things like this from happening.
 
Yes and no. Did the transmissions of the cell phone cause a frequency change, no. The amount of power so specific that it is able to rewrite a logic chip is is not from your consumer grade cell phone. Most likely, the hard edges of the phone hit the buttons in ways flesh couldn't and changed the frequency. Make sure to freq lock and power lock your transmitters to avoid things like this from happening.

I'll grant you that an accidental push of buttons could explain it, but I don't know how you can be so certain that RF didn't cause the change. It doesn't have to affect the memory of a chip directly. I'll bet the control portion of the wireless mic circuitry is unshielded. It isn't much of a stretch for strong RF to induce a signal into a circuit board trace that looks to the microcontroller like a button push or a series of button pushes telling it to change the channel. It's especially easy to have happen in battery powered stuff, because there is so little voltage difference between a logic low and high, and the current level are tiny. I've seen RF fields cause all sorts of odd behavior in equipment. No manufacturer is going to spend the money on components or use the physical space required to keep RF from every circuit trace in their product, unless normal use is expected to subject it to high RF fields.

GSM phones are much more likely to cause problems with nearby electronics than CDMA phones. Verizon and Sprint use CDMA. ATT and T-Mobile use GSM. If you don't believe me, try placing one of each next to a corded condenser mic, receive a phone call, and take a listen to the mic.
 
GSM phones are much more likely to cause problems with nearby electronics than CDMA phones. Verizon and Sprint use CDMA. ATT and T-Mobile use GSM. If you don't believe me, try placing one of each next to a corded condenser mic, receive a phone call, and take a listen to the mic.

Huh, really? Can you explain why the frequency division GSM uses is more likely to cause interference than CDMA's digitizing?
 
Huh, really? Can you explain why the frequency division GSM uses is more likely to cause interference than CDMA's digitizing?

GSM is a time-division multiple access technology. On a given GSM frequency (of which there are many), eight users transmit in a round-robin fashion. 1 goes, then 2, then 3, and so forth. Each transmits for exactly 576.9 µs, and is then off for 7*576.9 µs (4.038 ms) before transmitting again. If you look at the envelope of the signal (basically, when the RF is on v. off), it ends up making a 216 Hz sound wave with lots of buzzing (harmonic content), primarily because of the 1/8 duty square wave. CDMA, on the other hand, is constantly transmitting--hence, it does not cause this issue.

BTW, UMTS/HSPA ("3G" GSM) are CDMA-based. So is EvDO/CDMA2000. LTE is OFDM-based, and like CDMA it is constantly transmitting. Both CDMA and ODFM signals are "whitened" before transmission, btw, to make the signals look like noise.
 
I guess I'll clarify, the transmitter was locked. I've learned that people like to play with the mute setting then wonder why it doesn't work.
 
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