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Why do you need to remove your cell from your pocket while mic'ed with wireless pack?

Discussion in 'Microphones' started by ShrinkToobe, Feb 18, 2014.

  1. ShrinkToobe

    ShrinkToobe Active Member

    Currently on a corporate show. I ask my clients to take their cells out of their pockets and one guy says "You know, cell phones operate in a higher band than these microphones currently do. It might just be safe to leave them in your pocket with these on". I disagreed with him and still insisted he take it out or shut it off and he said he trusted me. Then I thought, he's right?! Why the hell DO we need to take that precaution then? Any thoughts?
     
  2. paulears

    paulears Well-Known Member

    Because they're nasty interference producing devices that rarely achieve the technical specification the initial ones that were produced did. Everyone who uses sound equipment regularly has heard the nasty digital rubbish that leaks out of the damn things - especially in fringe areas where cell size is greater and the phones ramp up their output power when they constantly poll the bass stations. In the UK, we lost our historic allocation in TV channel 69, and are now in channel 38, but license free users have a few channels in 70. We now have cell phones in 68 and 69 and they are very wide and lacking any proper filtering, so close in, they radiate significant power in a radio mic band. The license free users have no protection whatsoever, and as 4G rolls out, it will get worse.

    Cell phone RF doesn't really cause the pocket transmitters any problem, but in the pocket, with the typical unbalanced lav mic the RF can get into the pre-amps and make nasty noises. You hear it in the theatre, on broadcast TV and on stage - where it gets into guitar pickups. Cell phones are nasty interference causing devices - but handy things. We always used to try to get everyone to turn off phones in the wings in theatre, but everyone uses their phones for so much now, it's a lost cause - and interference is a direct result.
     
  3. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    The RFI may be getting in through the mic or the wiring to the transmitter. That has nothing to do with the phone and wireless mic using the same band.
     
  4. Boswell

    Boswell Moderator Distinguished Member

    Boulder is right - transmitter frequency has little to do with it when you are pumping radio waves into the immediate area, even at the relatively low power of a normal mobile phone. The RF output saturates the inputs of almost any electronic gear in close range, and non-linear effects cause rectification and low-frequency blocking. A microphone and pre-amp can make a good crystal set when saturated with RF waves. You were right to insist on the phones being switched off.
     
  5. dvdhawk

    dvdhawk Well-Known Member

    Ask him, "Why are we discouraged from using personal electronic devices on airplanes especially during take-off and landing?" Same basic reason. What the FCC would call "Spurious Emissions". Unintended harmonics, or other signals radiated outside the designated carrier frequency - which can interfere with other delicate electronics regardless of specific band or frequency. Take off and landing are times when there is little room for error, so let's make sure all the pilot's instruments are working properly - and turn those PEDs off. Simply silencing the ringer is not the same thing. I can also tell you that a wireless mic has little to no internal shielding compared to the airplane cockpit and its avionics, yet interference is still a concern. And now let's consider the proximity of these "nasty interference causing devices" (as paul called them) to the wireless transmitter/receiver….. If it was just a simple frequency issue, we'd only have to worry about it leaking into the receiver - truth is, as the other guys have said, it's as likely garbage [EMI & RFI] getting into the transmitter.
     
  6. paulears

    paulears Well-Known Member

    I always think about how many times we hear the very easy to spot mobile phone interference signature, and see people almost shrug and say to themselves, damn phones. I was in my studio this afternoon with a client, working on some of his music, and halfway though recording a small overdub, came the chirp chirp noise - yep, his phone in his pocket, getting in through his guitar.

    At a session I went to last year, the guys from Sennheiser were chatting about radio mics in the break, and were telling about one of their ex-colleagues who had left them too work at another manufacturer, and his job was to cut production costs, by swapping components for cheaper ones, or sometimes removing completely certain items - and apparently filtering was an easy target. You can remove filtering with no audible effect - but of course, when interference is present, there's nothing to filter it out. It saves pennies, but on big production runs, pennies mount up. Digital systems with their hard edged waveforms produce quite a bit of harmonic radiation way outside their operating frequency, and as I mentioned - here in the UK, we're going to have fun (not) as band occupancy increases as 4G is rolled out. Worse still, in our case, the problem isn't the base stations, which could be a way away, but it's the phones themselves, because it's their TX frequency that is only just outside one of our bands. In the US, it sounds like you have more distance, but for how long?
     
  7. dvdhawk

    dvdhawk Well-Known Member

    Several years ago, the US made the push toward digital TV broadcast in an effort to free up the 700MHz band for another wave of potential wave of wireless communications. Anyone with a wireless mic in the 700MHz band was essentially told, "tough-luck, dispose of your now illegal transmitters and receivers responsibly". Now just a couple years later, I've seen a trade magazine article indicating that Sennheiser has filed a complaint against the FCC for preparing to grab up the 600MHz band as well - in the very near future. Sennheiser citing the undo burden put on the wireless mic consumers, who due to their (the FCC's) lack off foresight and planning, would be forced to buy new units again. Surely finding out big corporations would pay obscene amounts of money at auction for a slice of the 700-band wouldn't have them eyeing the 600-band so soon ??

    Shure (and probably some others) already manufacture a few wireless systems up in the unregulated 900MHz X-band overlapping into cell-phone frequencies, and they make digital 2.4GHz systems directly competing with a wide assortment of short-range gadgets. Over-crowded broadcast spectrum is only going to get worse from a technical standpoint - before you even factor in the harmonics.

    And from a cultural standpoint, I'm afraid many people these days are so self-involved they can't imagine how the world would get by if they couldn't be reached for an hour or two. I've seen people light up a cigarette standing by a gas (petrol) pump, so I doubt these people could be convinced that talking on their cell-phone at the pump has any risk of causing an explosion.
     
  8. paulears

    paulears Well-Known Member

    Did you in the US get compensation when the last changes was made? We did. I got rid of all my ch 69 kit and got very reasonable compensation to buy the new kit.
     
  9. dvdhawk

    dvdhawk Well-Known Member

    Not a dime Paul.

    Last time, the manufacturers all seemed to offer various rebates to offset a portion of the cost to people buying new systems, but the balance was still out-of-pocket for the consumers. The people who profited from clearing the frequencies didn't offer any compensation, or buy-back program to the consumers. The compensation issue/strategy is part of the petition Sennheiser filed (supported by Shure, Audio-Technica, Lectrosonics, and CP Communications) - or if you prefer the legal jargon.
     
  10. paulears

    paulears Well-Known Member

    Wow! Here, HUGE numbers of users did not bother to license their equipment, so 'officially' the bands were very unoccupied, so the people who had been paying licence fees were offered compensation - there was a big equipment list with values. You decided if you wanted to keep it, and only use channel 70 (our 'free' channels), or to surrender it and buy new kit on channel 38. I kept a few odds and ends and gave the rest to the Government's agent. I got a few thousand pounds - and some of my kit was 8 years old, and really owed me nothing! Then, there was a delay - they didn't need the channel 69 as urgently, as the phone systems weren't sorted, contract wise, so we were offered the chance to buy back some kit for bargain basement prices. I bought some IEM systems, and use them in ch 70.

    The big hire companies sold back all their well used hire kit, and we were pretty pleased with how it was done. The clear out made money for the manufacturers as we all re-equipped, and the only losers were the people who didn't have licenses and were ignorant. Many of us are now waiting for the complaints when somebody turns on a 4G cell in their area, and all the 4G equipped phones burst into life. A bit evil, I guess, but when you DO pay a license, it's annoying to see so many people who don't, even when they don't know!
     
  11. dvdhawk

    dvdhawk Well-Known Member

    Very interesting, that is another big difference here. Due to the (relatively) limited range of wireless microphones, the FCC doesn't require a license of any kind to use transmitters under 50mW. So as long as you don't have a powerful UHF television transmitter in the area, and the church across the street isn't on the same frequency everything is usually good. If you were a touring artist, or rental outfit that travelled outside the region, you just needed systems that had a wide enough range you could find a clean frequency.

    And to tie it all back to the original topic, As someone who installs and services church sound systems, I still hear tales of churches (one just this week in fact) who are just a few feet from heavily traveled roads and have issues with truck drivers' CB radios (typically 500mW around 27MHz) randomly cutting into their sermon. One even had problems with the nearby McDonald's drive-thru. Again, less of a tuning / frequency issue than a sheer overload of transmitted energy. That's got to be annoying.

    … still mildly amusing to consider the possibilities…. "and the serpent said to the Eve… would you like an apple pie with your combo?"
     
  12. rmburrow

    rmburrow Active Member

    Dvdhawk: The typical CB radio is 5 watts (typically 4 watts TPO), not 500 mW. Some of the CB antennas (Trucker twins, etc.) have gain and directionality, which increases the effective radiated power somewhat. Some truckers run illegal CB power amplifiers, which makes the RF problem worse. Unshielded speaker wires on stereo systems, PA amplifiers, etc. on the order of 1/4 wavelength (or a multiple) at 27 MHz (approx. 9 ft) can act like an antenna at CB frequencies. Terminate one side of this "antenna" to the output of a amplifier (especially solid state) and audio rectification may occur (through the P-N junction of transistors). The reactance of the speaker coil, etc. at 27 MHz is high, so the antenna essentially is a doublet.

    The term "audio rectification" has been around for years....regardless of whether the source is broadcast, CB, cellular, etc. The front end of a receiver can be overloaded by a nearby transmitter, regardless of frequency. It's the properties of the receiver front end that makes the unit vulnerable to nearby transmitters. If the offending transmitter is sufficiently close to the affected receiver, the induced signal may be so strong that the rectified signal changes the operating point of the receiver front end. Alternatively, speaker leads from a amplifier operated near a broadcast station antenna can couple RF directly into the amplifier output. Sometimes interference comes through the AC line, and one of those plug in filters is needed in that case.

    Most of the time, a couple of RF chokes and/or some 0.01 uF (200 volt) ceramic or mylar capacitors should keep "Break 19, how does it look over your shoulder" out of the minister's sermon...Electronic organs are also vulnerable.... One word of warning...manufacturer warranties. Contact the equipment manufacturer before modifying any equipment or bypassing/RF choking the output. Most equipment sold these days has a clause voiding the warranty if the equipment is modified or the speaker load is "nonstandard"... Unfortunately, a lot of commercial equipment is not "hardened" for RF interference, so one has to be careful selecting equipment for use in a RF environment.

    In the USA, the location of broadcast antenna sites is public record. Use Street Atlas or some other software to get the geographic coordinates of your location, and run those against FCC records for AM, FM, and TV for starters. If your location falls within a km or so of a transmitter, extra precautions may be required. (shielding of speaker leads, bypass capacitors on speakers and/or amplifier outputs, RF choking, AC power filters, clearing "ground loops", etc.) The local cable carrier must satisfy interference problems traceable to their equipment...and you shouldn't be charged for that, also.

    Just my $0.02 worth...
     
  13. dvdhawk

    dvdhawk Well-Known Member

    Thanks for the correction and clarification rm. Of course, you are absolutely right. I found the 500mW number in a rush on Google/Wikipedia, trusted my search query would turn up the right answer, and clearly didn't read the reference page well enough. Worse yet, now that you've got me actually thinking… I should have known better. I used to have cheap walkie talkies that put out 500mw. Again, thanks for catching the glaring mistake.


    Excellent advice on counter-measures too.
     

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