Mjordan2nd

My friend's cell phone, a Motorola V235, makes her monitor appear to degauss and give off a sequence of beeps when it is near her monitor and she gets an incoming call or text message. The thing is, I don't think the monitor acually degausses, but the effect is similar. It makes her television do something similar, with the sequence of beeps sounding exactly like the ones that came out of her monitor. I've been thinking about it for a while, but can't quite understand why it would do this. My guess would be some sort of EM radiation, but it's honestly a stab in the dark? Anyone know how to explain this, or experienced anything like this before?

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grumpy

It's electromagnetic interference.

A mobile phone communicates with the network via electromagnetic waves.

Electrical devices rely on currents travelling through circuits, and currents generate electrical and magnetic fields. Monitors are no different: there needs to be some movement of electrical charge (i.e. current) through a medium. Electromagnetic waves interfere with those fields, resulting in modification of those fields. Changing those fields therefore affects how the current is transmitted. If there is a change in the current supplied to a device, the device behaves differently. With monitors, in practice, it happens to be a sequence of sounds but can also be a disturbance of the generated display.

One reason that airlines often insist on mobile phones being turned off in flight is that the electromagnetic waves from the phone (eg as it tries to find a network) can interfere with cockpit displays, flight sensors, and flight instruments. Such disturbances can affect pilot situation awareness.

This is a very over-simplistic description: the mathematical and physical description is not that difficult, but tends to make laymen's eyes roll back.

DaWei

Exactly. Here's some additional information, essentially worthless. It's precisely the same phenomenon as degaussing. A CRT has an electron gun that shoots a stream of electrons at the center of the tube. This gun is surrounded by plates that generate electromagnetic fields that displace the stream. These plates are driven with sawtooth signals. The horizontal rate is high, the vertical rate is relatively low (in traditional American TV, about 15,750 Hz and 60 Hz). The result is that the beam scans the face of the tube from upper-left to lower right. The frame rate is 30 Hz (again, American). The 60 Hz means that you get two fields (scans) per frame. They're interlaced in the best monitors. The strength of the beam, thus its resulting intensity, is modulated by the content signal in sync with the scan. This reproduces the picture signal.

In a color device there are RGB pixels. Hitting these tiny targets precisely is aided by a physical mask that prevents the beam from striking in improper places.

In the course of operation, these magnetic fields tend to magnetize permeable materials present in the device. These permanent fields displace the calculated trajectory of the beam, an effect that accumulates over time. This results in deconvergance of the beam, and is why some devices have convergance controls.

Degaussing applies another rapidly varying electromagnetic field (60 Hz, "wall frequency", American) to the device. This tends to reorient the magnetic domains randomly, thus restoring performance to its unperturbed, original state.

Your phone does precisely the same thing. It's the low-frequency information modulated onto the carrier that causes the most visible effect. Anything that produces a relatively strong field, whether it's electromagnetic or just magnetic, will garfle your picture. Try a hair dryer or electric mixer or even a vacuum cleaner.

The beeps have nothing to do with your monitor. It's just the same effect applied to the circuitry carrying the audio signals.

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Mjordan2nd

I see. Thanks for the responses.

I would be interested in learning the mathematical and physical reasons behind this phenomenon. I knew that all electrical devices give off EM waves, but that's as far as my knowledge on the subject goes. My Physics book does not seem to describe EMI at all, nor does my introductory engineering book. The links I found on Google seem to offer a description of the phenomenon and the hazards associated with it but not the reasons behind it, and wikipedia offers only a very simplistic explanation to the cause of the phenomenon. Do you know anywhere I could read more about this?

Once again, thanks for explaining this to me.

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"Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower

DaWei

Simplistically: electrons have a charge, and this charge produces an electric field. When you move the charge (through a conductor), you get an electromagnetic field. Conversely, when an electromagnetic field passes through a conductor, it induces a movement of electrons.

An AC current is moving a varying number of electrons, with respect to time. This means that the electromagnetic field is varying with respect to time. If there is a closed-circuit conductor in this field, the moving field will introduce a time-variance in the current that is present in that circuit. This variation sums with the intended variation, strengthing or weaking it according to its own variation. If this were an audio signal, say, then the waveshape of the audio would vary with the interference, as well as with its intended variation. This is distortion (noise).

This is a traditional or 'conventional' view, without regard to quantum-mechanical views.

I don't personally have any reference material or links to offer, or currently in my possession (they crumbled to dust about the time we landed the ark). I don't recall what your major is, but if it's EE, or physics is in your path, you're gonna see it before those Aggies turn you loose .

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Booooze

Good Question and answers, overall good post. Thanks for the explanation guys. I get the same thing when my cell phone is around my speakers. Never really thought much of it though. Interesting to see how it all works.

kruptof

lol..i have the same issue with the speakers and the phone but when i put the two next to each other i can hear what seems like people talking but really fast if you listen for some time you can properly make out one or two words.

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