Computers & Mobile Technology

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On Hackszine, Jason wrtites:

Every GSM cellphone user is familiar with the annoying Bzzzhtzttt noises that tend to emanate from random electronics anywhere you take your device. The iPhone is no exception, but the problem is exacerbated since most people have it sitting on their desk with a speaker close by playing music at reasonable amplification. It sucks.

Mac Life has a solution that may work for you. Just yank the ferrite beads from an old usb cable–they are inside the plastic bulge near one end of most cables–and place them around or taped in-line with your speaker cable. There are a few stories of success with this method, and nobody has mentioned an impact on audio quality, so it’s a cheap fix that’s worth a shot.

iPhone Buzz Kill

28 thoughts on “Killing that GSM buzz

  1. We have $50,000 recording system in courtrooms that are horribly disrupted by these cellphones, and the lawyers, being arrogant %$#$#$s, don’t believe it’s their phone that causes it, so they never turn them off. They’ll even sit their pounding out text messages and swear the phone is off when questioned.

    So, how can I use this technique? We have some 10-20 microphones around the courtroom fed into a mixing device which outputs 4 channels of audio to the recorder, as well there might be some additional mikes feeding directly in to the recorder if there’s an area of the room that needs extra attention.

    Where would I add the ferrite beads, and keep in mind I can’t really mod their sound system and I move around to different courtrooms each day.

  2. Ferrite beads are commonly used on audio signals (line level in, speaker out, and ESPECIALLY Class D outputs) to preven nasty stuff getting in (or out) of devices.

    They are usually quite cheap, much less than the USB cable recommended to be sacrificed. But, sometimes we end up with more USB cables than necessary.

  3. Ferrite Beads are used for blocking high frequency (more than 20 MHz usually) RF emissions. They won’t cut audio hum.

    Ferrite Beads are *NOT* used on audio signals. Ferrite Beads might be used on power supplies for audio amps if they are using a switching power supply, but that would be a very cheap amp.

  4. >Ferrite Beads are used for blocking high frequency (more than 20 MHz usually) RF emissions. They won’t cut audio hum.

    That’s very true. Keep in mind that you’re not actually filtering the audio component of GSM buzz… you’re filtering the RF that is *causing* the audible buzz. What happens is that wires (or circuit boards, if you’re close enough) pick up the RF energy from GSM phones.. this type of phone is called “TDMA” for time-division multiplexing. It’s this time-division business that causes the problems..
    While you can’t hear the radio frequency (RF) coming from the phone directly, the phone is sending out bursts of RF at a rate of 217 Hz, which *is* audible.
    What happens is that the RF energy is being ‘detected’ in op-amps or other active circuits, just like an AM radio, so you *hear* the detected frequency. The best way to eliminate it is to filter the RF, thus the ferrite beads.
    To answer the other question, you’d need to put the ferrites on the inputs and outputs of all cables coming in and out of active equipment, especially runs that go near the pesky lawyers. Try to place the ferrites as close to the active equipment as possible (this includes phantom mics, so those would require one at each end of the cable)

    ..Also, RF can jump across cables, so watch how you route them.. don’t route an unfiltered cable section near a filtered one!

    Hope that helps!

    Cable ferrites on Digikey:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=3408554;keywords=ferrite

    EDN article on eliminating buzz in circuits (for circuit designers):
    http://www.edn.com/contents/images/498768.pdf

  5. Most electronic devices must comply with part 15 of FCC rules – you probably see the reqired compliance notification on lots of products:

    “This device complies with part 15 of the FCC rules. Operation is subject to the following two condidtions: (1) This device may not cause harmful interference, and (2) this device must accept any interference recieved, including interference that may cause undesired operation.”

    I consider the GSM buzz to be interference.

    So how do the cellphones/manufacturers get away with it? Are cellular phones not subject to these rules, or is the FCC slacking off, and not enforcing the rules?

  6. Dude these $5 “products” are actually antistatic bags that come with all computer products. Computer stores pay people to haul these away. They actually do help though, but so does foil!

  7. I did view the video of speaker buzz on the stopthebuzz website. That is the exact noise I hear. There’s also no wires going to that speaker execpt the power cable, so even if I put ferrite beads on every audio cable in the room, would that really help? The mixer and the recorder also both have many unused inputs since they can handle more mikes than we have.

  8. >Most electronic devices must comply with part 15 of FCC rules – you probably see the reqired compliance notification on lots of products

    Cell phones are not Part B devices.. they are intentional transmitters. Different section.

    >There’s also no wires going to that speaker execpt the power cable, so even if I put ferrite beads on every audio cable in the room, would that really help? The mixer and the recorder also both have many unused inputs since they can handle more mikes than we have.

    In that video he is *very* close to the devices picking up the buzz. In that case the circuit board can directly pick up the radio energy. That just requires a better-designed product! (to be fair, most people haven’t had to consider digital cell phones when designing their cheap electronics for the past 50 years)

    If you have access to the system during off-hours, try isolating the audio gear that is picking up the noise… get somebody with a GSM phone browsing the internet wandering around until you hear the problem, then start unplugging cables, mics from cables, etc. until you find the source… you might get lucky and narrow it down to just the mics, just the mixer, some other random piece of equipment… you get the idea. Once you find it, get some ferrites and start experimenting.

    Something else worth mentioning is that different GSM carriers use different frequencies in an area. It would be a good idea to determine if you’re having problems with one and not the other. In my experience, the 830MHz band is *much* worse than the 1800 MHz band, but YMMV.

  9. > I consider the GSM buzz to be interference.
    >
    > So how do the cellphones/manufacturers get away with it? Are cellular phones not subject to these rules, or is the FCC slacking off, and not enforcing the rules?

    Cell phones are “intentional radiators”, so they automatically fall in the Class B device. The warning that you see on Class B devices is as follows:

    “NOTE: This equipment has been tested and found to comply with the limits for a
    Class B digital device, pursuant to Part 15 of the FCC Rules. These limits are
    designed to provide reasonable protection against harmful interference in a
    residential installation. This equipment generates, uses and can radiate radio
    frequency energy and, if not installed and used in accordance with the
    instructions, may cause harmful interference to radio communications. However,
    there is no guarantee that interference will not occur in a particular installation.”

    Nothing about “harmful interference” in there. In fact, it says that there is no guarantee that interference will not occur. But, on another matter, there is a difference between “harmful interference” and interference. “Harmful interference” is defined as follows in 15.1.m:

    “Harmful interference. Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radiocommunications service operating in accordance with this Chapter.”

    Unfortunately, speaker buzz does not qualify as “harmful interference.”

  10. Intentional radiators are subpart C, not subpart B, and in any event a cell phone is definitely neither a Class B Digital Device nor a “cordless telephone system”. Applicable bits of Part 15 can be found at http://www.fcc.gov/oet/info/rules/part15/PART15_07-10-08.pdf

    That said, in the case of the all-too-familiar GSM interference the defect lies entirely within the sound system, and is no fault of the GSM phone. There’s a long history of everything from police radios to CBs to AM broadcast stations getting into public address systems, stereos, and the like.

    The solution is adding L (inductance) in series with, C (capacitance) in parallel with (or both), the point at which the RF is getting into the system. This can be either the input side (the mikes or the CD player or whatever), the output side (the speakers), or both. Additionally, the RF shielding afforded by the enclosure may be inadequate.

    Good further reading for those who are interested in getting rid of RFI is here:
    http://www.arrl.org/fcc/tvibook.html

  11. dont forget you need the correct bead for the correct freq. in the us thats 850 and 1900 but its the 850 band that is the culprit. nextell and att uses 850 att uses 850 and 1900 but mostly 850 tmobile uses 1900 and very little 850. this is why nextel and att phones are the ones mainly complained about. as far as the reason is occurs is because of product design. stop buying cheap imports or modify the cheap imports to block the buzz. beads can help but shielding on the amp section of the speakers is the key. if you also have your phone connected via use ensure the usb cable has the bead on it. do not hack the bead off.

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Gareth Branwyn is a freelance writer and the former Editorial Director of Maker Media. He is the author or editor of over a dozen books on technology, DIY, and geek culture. He is currently a contributor to Boing Boing, Wink Books, and Wink Fun. And he has a new best-of writing collection and “lazy man’s memoir,” called Borg Like Me.

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