July 27, 2007 4:00 AM PDT

FAQ: The 411 on radio frequency interference

(continued from previous page)

The cell phone interference issue seems to be a bigger problem for people using certain carrier networks. Why?

It's true, customers on AT&T/Cingular, T-Mobile and the old Nextel networks experience this problem more frequently than those on Verizon Wireless and Sprint networks. The reason is that AT&T/Cingular, T-Mobile and Nextel use cell phone technologies that use a radio channel access method known as TDMA (time division multiple access).

Networks for AT&T/Cingular and T-Mobile are built on GSM (Global System for Mobile Communications), while Nextel uses iDEN (Integrated Digital Enhanced Network). These TDMA-based technologies allow several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium or radio frequency channel while using only the part of its bandwidth they require.

Because these networks operate in a "time division" fashion their radio frequency transmitters are turned on and off at fast rates. And this can often be picked up by nearby devices.

"Most people have just learned to deal with it. The alternative is to live without wireless, and who would be willing to give up their cell phone?"
--Craig Mathias, analyst, Farpoint Group

Verizon and Sprint's network use a technology called CDMA (code division multiple access). It does not use TDMA for sharing channels. CDMA transmitters are transmitting signals almost constantly, so they don't cause the interference buzz.

GSM and iDEN are 2G technologies. Will this problem still occur as mobile operators migrate to 3G technology?
It shouldn't be as prevalent. AT&T/Cingular has built its 3G network using WCDMA, which is based on CDMA technology. So new 3G phones on AT&T's network should not have as many interference issues.

Is there anything consumers can do to reduce this problem?
Unfortunately, the answer is no. The best solution is to turn off cell phones when you're near a speaker or some other device that is amplifying the cell phone signal. Or you can try to stay far enough away from speakers and other electrical equipment if they're turned on. For example, if you're on a conference call using a speakerphone don't put your phone on the table next to the speaker.

Hospitals for years have banned cell phones, but can cell phones really interfere with medical equipment?
Hospitals use sensitive equipment such as ventilators and ECG (electrocardiography) monitors for patient care. And just like TVs or speakers, some of this equipment is susceptible to electromagnetic interference.

But some hospitals are starting to lift the ban on cell phones as newer digital cell phone technologies and better shielding on hospital equipment have decreased the potential for interference. Plus the ban is nearly impossible to enforce.

Still, some hospitals have kept the policy in place, mainly to keep noise levels down so that patients aren't disturbed by people gabbing on their cell phones.

Do cell phones interfere with communications on airplanes?
Experts have debated for years whether it is safe to use cell phones on airplanes. Most of the evidence suggesting that it interferes with aviation equipment is anecdotal. But last year, researchers at Carnegie Mellon University concluded that cell phones can disrupt normal operation of key cockpit instruments, especially Global Positioning System receivers, which are increasingly vital for safe landings, according to Bill Strauss, an expert in aircraft electromagnetic compatibility at the Naval Air Warfare Center in Patuxent River, Md., and one of the researchers who conducted the study. Strauss said risks are caused by radio emissions from cellular calls that are higher than previously believed.

Is that the only reason why the FCC and Federal Aviation Administration have banned cell phone use on planes?
There is another reason why the FCC isn't keen on allowing people to chat on cell phones while flying. The problem is that when cell phones are used in flight they are traveling rapidly over hundreds if not thousands of cell phone towers. As the plane flies over these towers, the cell phones inside the plane are connecting and disconnecting from various cell towers much faster than was intended. The rapid signal hand-off from tower to tower of hundreds or thousands of cell phones flying overhead could disrupt service on the ground, affecting millions of cellular customers.

What about other consumer electronic devices such as iPods and laptops? Why do those need to be turned off too?
Actually, the Carnegie Mellon study also found that other electronic devices such as laptops and handheld games can send out potentially harmful signals that interfere with aviation equipment.

The bottom line is that RF interference is a fact of life, says Craig Mathias, a principal analyst for Farpoint Group.

"Most people have just learned to deal with it," he said. "The alternative is to live without wireless, and who would be willing to give up their cell phone?"

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28 comments

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It's not a problem for all carriers
Well, "carriers" mis-characterizes the issue, its the network technology not the carrier. Having a CDMA phone since before verizon was verizon, I had never experienced this. I have upgraded to phones with increased capabilities through the years and no "buzz" from ANY phone. Then, I was issued a Blackberry from work that had Cingular service. Different freqs, different coding scheme, etc etc. Now I have the problem, computer speakers, car stereo, everything.

Want to get rid of it? Get a CDMA carrier.
Posted by GOV_Stooge (2 comments )
Reply Link Flag
Does appear to be GSM phones only
I'm a great fan of GSM phones, but I also have noticed that they seem to be the only ones that give this sort of interference, although it varies from phone to phone. I constantly have a CDMA LG VX8300 next to my monitor at work, and don't have any interference, not even when it rings.
Posted by exjoburger (13 comments )
Link Flag
100% Correct!!!
I have also had Verizon Wireless Service which uses CDMA Technology. I've had it for about 10 years and NEVER had that problem, that's why i prefer CDMA over any other Technology, because that buzz is really annoying, prior to verizon i use to have AT&T and then NEXTEL and had that problem with both carriers. i think is something with GSM and EDGE Technology.
Posted by conazo (2 comments )
Link Flag
This is correct, but here's why:
All cell phone systems need a way to allow multiple users on the same tower. GSM uses TDMA (Time Division Multiple Access), so each user transmits a short burst of information at a different time, so they don't interfere with one another. A GSM phone sends one packet (burst) of data every 4.615ms, or about 217 per second. Your speakers are being interfered with at 217Hz, causing the buzzing sound.

This does not occur with CDMA (Code Division Multiple Access). In CDMA, each user gets a different code applied to his signal, so the base station can separate the users apart that way. This allows all users to transmit at the same time, so the radio isn't switching on and off at 217Hz (or any other rate in the audio spectrum).

I used to have Cingular, and I amazed everybody when I would pick up my phone to answer the call before it started ringing :-). Can't do that anymore with Verizon, but I'm glad the interference is gone.
Posted by silenthorn (5 comments )
Link Flag
I have Verizon CDMA and still have the interference problem. There is a partial solution though. Put your phone in airplane mode. You won't be able to get phone calls but 4g still works.
Posted by patentcolo1 (2 comments )
Link Flag
Amiga 1200 frozen with GSM interference
I remember having Motorola 68020 CPU Amiga 1200 and when
cell phone rung, it got frozen. Freezing (e.g. BSOD) wasn't even
known on Amiga unless you get somehow infected by one of
rare viruses.

Of course Amiga 1200 was a computer from 1993. The
electronics engineer forgot to tell ONLY the electronic devices
having special workarounds in GSM age isn't effected from cell
phones. They are designed in a way not to fail or get real actual
effect from that huge interference.

For example, if you got a car with ABS made before cell
phones, e.g. a very good quality Mercedes you can't give up
from 1991, you better ask your car manufacturer whether to
use ordinary cell phone while driving or buy a car kit (real one
with ex. antenna).
Posted by Ilgaz (573 comments )
Reply Link Flag
Not a problem for all phones
I had my Nokia E61 sitting on a conference table next to my colleague's new iPhone. The Polycom on the table (which seem to be even more susceptible to this interference than many other devices) kept buzzing periodically as both our devices checked email. When the buzzing got louder, I moved my phone away... but it didn't stop until my colleague moved his iPhone. I then did an experiment and made a call with my phone and there was only slight interference. I guess the "i" in iPhone stands for "interference".
Posted by SnidleyWhiplash (61 comments )
Reply Link Flag
It's about the noise...
At the end of the day, it doesn't matter to as an Producer and audio engineer whether a particular phone "works" with something or not... All phones are banned in the studio, period. I'm surprised anyone cares if the iPhone produces interference or not. People need to learn to turn their cell phones off when appropriate anyway.
Posted by mattumanu (599 comments )
Reply Link Flag
FCC seldom acts on this type of RFI
The FCC considers that it doesn't need to regulate manufactured electronics, and allows sloppy designs to be imported to the US. It could act, but wants to "deregulate" wherever possible. Imagine what would happen if FDA did that (oops, it does!).
Posted by tpobrienjr (15 comments )
Reply Link Flag
Finally, someone explains this!
This is extremely irritating and it must be stopped from becoming more prevalent! I have been increasingly hearing the "dit-dit-dit-dah-dit-dit-dit-dah", through my desktop PC speakers, even when the PC is turned off, on my car radio on an AM station, and in a customer's conference room on their speaker phone. So - the culprit is GSM cell phones, which fairly recently came onto the U.S. scene? So, CDMA is better after all! We are in San Diego, so I was thinking it was military radio interference... It is very irritating, especially when you are recording any audio into your PC for your MP3 player...
Posted by MTiddens (2 comments )
Reply Link Flag
Sounds like Blackberry's & I-Phones have the problem!
Based on the comments so far and me also having a Blackberry with Cingular service, it sounds like the Blackberry's and I-Phones have this problem. They should have not passed FCC testing! The FCC must step on these companies (the designers) before "smart phones" proliferate further! Can't blame this one on "China"...
Posted by MTiddens (2 comments )
Reply Link Flag
why?
I just happen to own a set of cheap made in China speakers.
They start to 'oscillate' a few seconds before the Motorola
actually rings. So I can pick up the phone before it rings---
freaks out people around me. ESP.

On the other hand, I can't believe they devoted an article to RF
interference. There will always be cross-talk. I would have
thought readers who frequent CNET have at least a very basic
scientific or technical background and understand exactly what
the sticker from the FCC, which adorns all of our electronics,
actually means.
Posted by Rick Cavaretti (216 comments )
Reply Link Flag
Missing the point...
Lots of folks seem to be missing the point - cell phones and
other similar devices (two way radios) are designed to emit RF;
that's how they work. And since we are all looking for better
performance out of these devices, the manufacturers will
continue to ramp up their output power and cause even more
interference. The real fault here is crappy shielding and
grounding schemes that are so prevalent in 'consumer' & 'pro-
sumer' level audio devices. The RFI is only heard if the
interference has a path into the amplification circuit - block the
path and the interference disappears.

There are audio mixing consoles out there that a five watt radio
can be right beside without a problem, and other (less well
designed) ones that the radio can't be within ten feet without
causing RFI problems.

Not the radios fault -- its the audio manufacturer's.
Posted by SonicL (3 comments )
Reply Link Flag
its not just gsm
its cdma as well, and any rf device
Posted by zrogerz69 (20 comments )
Reply Link Flag
just kidding
guess i'm an idiot haha
Posted by zrogerz69 (20 comments )
Link Flag
it's common for phones
My phone, a nokia 6101, makes these weird radio frequency through my speakers whenever someone calls or texts me. Most phones should do that because of a signal that is going through your speakers into your phone. It should be anything to be worried about. At least I know when about to get a call or text now!
Posted by snypervictim (3 comments )
Reply Link Flag
it's always been here but no one made an issue out of it
The issue has always been here but no one has really made an issue out of it. The problem is the comsumer electronic devices that are cheap and not properly shielded. The cheaper they get the more interference they get. Also, in general, the better the phone gets a singal the more likely it will cause such interference.
The problem should have been more obvious years ago with the first generation GSM phones should it had been GSM's fault because back then there were not strict regulations with regard to the RF emission of those phones, and I believe they did emit much more RF than modern models do. I suppose back then we had more quality comsumer products. The cheaper the computer parts were getting over the years the more I was noticing this interference problem. In fact, the first time I personally noticed this issue was when I changed an old but quality Sony monitor for a newer but cheaper Samsung model.
I believe we'll see more and more of this issue in the years to come because the world only is going to see more and more of RF emiting devices, such as phones.
Posted by skareta (13 comments )
Reply Link Flag
happens on my desktop speakers i pick up emergency band
every once in a while my local fire dept down the street or the police parking depot behind my house, will cause my speakers on my desktop computer (altec ) speakers to pick up their band and i would clearly hear the communication from a car or truck.

It tends to be really loud and it actually cranks up the volume on its own. Happened on a few night where I would get freaked out when all of a sudden I would hear this screaming sound right next to my head almost. Learned my lesson to turn the speaker off at night...then it won't pick up anything.
Posted by dondarko (261 comments )
Reply Link Flag
Shield the plane's equipment !
I can't believe my safety online is put at risk of someone tripping their WIFI, bluetooth, or cells on (even while bouncing in shaking luggage at the worst possible moment when flying through air pockets); so either a) it is not really an issue and so the spurious warnings need to stop, or b) it is and issue and sensitive instruments should be shielded rather than have their continued operation be dependent on luck.
Posted by Rants&Raves (199 comments )
Reply Link Flag
Happend all the time
My friends and I used to have SPRINT/NEXTEL phones and that interference happened all the time when we were watching tv, listening to music at home or in a car, pretty much whenever we were near any device that had an audio output.
Posted by plogue87 (14 comments )
Reply Link Flag
Now you know why airlines are worried
Just imagine the number of phones and mfgrs types that could be onboard your next flight. Then imagine what kind of interference would be created and the multiple harmonics that would be generated when half of them are in operation. I would be almost impossible to sheild any equipment from all that RFI generated by those onboard devices. I think the airplane is the one place these devices should be forbidden from operation.
Posted by mjd420nova (91 comments )
Reply Link Flag
Killeen Texas Computer Nerds - Copper Foil
A very inexpensive way to eliminate this problem is to use copper foil and create an RFI shield around the emitting components. This can be done without causing thermal damage to surrounding components. The Government uses similar principals in RFI shielding of Mil. Spec. equipment.

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Posted by thetopnerd (17 comments )
Reply Link Flag
OMG My subwoofer...
I think my home theater subwoofer has been making random noises because of this! Now that I think about it, it started about the time I got the iPhone, and I have been keeping it in my pocket when we watch movies...
Posted by count0--2008 (38 comments )
Reply Link Flag
Thanks soo much for the informative story!
Thanks soo much for the informative story!
Posted by shmody (71 comments )
Reply Link Flag
This is news?
GSM Blackberries have always had this problem, many quite badly. Most BB users know instinctively to keep it away from speakers to avoid the annoying tehcno if EMI. Plus the running jokes about not stowing it in your front pants pocket if you want to retain the ability to reproduce.

Glad to see that industry appears to be moving towards eliminating this annoying issue.
Posted by madmatt (6 comments )
Reply Link Flag
No, its not just GSM....
Ive been a Nextel subscriber for almost 10 years now, and been through about a dozen phones. Throughout that time, ive always had to deal with this kind of thing. In case you dont know, Nextel is using a technology developed by Motorola in the early 90s called iDEN, which runs at a fairly low frequency as far as cell phones go. The kind of RF interference experienced by many cell phone owners is a disruption in the RF band, and it doesn't only happen with frequencies that are close together either, it can happen all over the RF band. If a device isn't properly shielded, it can disrupt all sorts of devices, but speakers are especially susceptible because of the exposed magnet and their overall design.

Just wanted to dispell anyone believing it was ONLY GSM phones. iDEN is older than any current US technology by the major carriers ( It just took awhile for a carrier in the states to pick up teh technology, ie Nextel.
Posted by usb123 (1 comment )
Reply Link Flag
haha
One time in church, I was giving a talk and i got a very large
amount of interference as a text message was being delivered. I
had to step away three feet before it would stop, lol.
Posted by mineralwalker (5 comments )
Reply Link Flag
There is a partial solution. Put your phone in airplane mode. You won't be able to receive calls but emails will still come in since 4g still works.
Posted by patentcolo1 (2 comments )
Reply Link Flag
 

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