Why do phones mess up monitors

Raphael

I've always wondered this, why is it that when you leave a mobile phone next to a CRT monitor, the monitor goes demented when you receive a message or a call. By going dememnted I mean the lines that go across the screen. It does it with speakers as well.

Anyone know why this is?

Karoma

Electromagnetic interference.
In CRT monitors, it's due to the large coils for focussing the electron beam & the tuner which catch the signals.

whiskeyman

Electromagnetic interference.

bah - bet me to it... at least I'll show you where ^^^ got his info
http://www.deccanherald.com/deccanherald/apr42005/102924200543.asp

Sparky

funny enough it doesnt happen to my tv when its through sky

Civilian_Target

First -> http://www.boards.ie/vbulletin/forumdisplay.php?f=391

The lines you see are caused by electromagnetic interference from your phone. The monitor uses a set of electromagnets to focus a beam of electrons that hit a flourescent coating, effectively drawing horizontal lines on your screen. An average 17" monitor draws about 70,000 lines a second. When your phone rings, the electromagenet in the aerial interferes randomly with this beam, as it passes near the phone, causing some of the lines to be drawn more towards the phone more than others, hence the interference you see.

The reason you're less likely to notice this on a TV is because a TV is a much more high-powered, less sensitive device. A typical TV draws only 29,000 lines per second, so a more powerful beam must be used to keep the line visible for longer. So more powerful magnets are used, and these are less susceptible to interference from your phone. And the interference itself is much less visible because TV singals are interlaced (only alternate lines are read from the signal on each pass) and co-axial cable is generally better shielded than the D-sub cables used in monitors.

Red Alert

Microphones are also very sensitive to it (not the cable mind, if they're the of the balanced type as most pro ones are). Try it in the pub, church or supermarket.

whiskeyman

Red Alert wrote:

Microphones are also very sensitive to it (not the cable mind, if they're the of the balanced type as most pro ones are). Try it in the pub, church or supermarket.

For the love of God, dont try it if you go to a gig in a small intimate venue...
So many people think it's fine if they just put their phones on vibrate.... turn 'em off!!
Nothing worse than hearing it on the speakers as a slow acoustic set comes on

Binomate

If you put speakers or magnets (I think) near your television screen it will either turn purple or green. I had speakers on top of the telly for about a month. I was nearly loosing hair because I couldn't understand why the picture had all of a sudden been tinted greed at the top.

billy the squid

It has something to do with interfierence coming from the GSM encoder. if you hold your phone to a speaker when you get a call it can drive the speaker equally demented.

If you put speakers or magnets (I think) near your television screen it will either turn purple or green. I had speakers on top of the telly for about a month. I was nearly loosing hair because I couldn't understand why the picture had all of a sudden been tinted greed at the top.

there are magnets in speakers.

Binomate

billy the squid wrote:

there are magnets in speakers.

Yes, I know.

Sparky

used to be able to tell when a call was coming in by the biping on the telly before it was a full on bipppppppp.

I hope you get what i said there^^^^^^^

jimmycrackcorm

On a side note I'm curious why hospitals have this thing about mobiles interfering with sensitive equipment whwn I'd imagine that a PC is more complex and doesn't get affected by mobiles around.

Red Alert

it's because medical signals like from your heart or a scalp EMG sensor are really tiny (in the microvolt range). they need huge amplifiers to magnify this big enough to do any sensible processing on it. the amplifiers are rather dumb and don't know a heart signal from the pipping of your mobile so the analyser doesn't know the difference either. even the 50Hz hum from the mains can be a problem with these devices and much of their electronics is actually there to remove this.

Stephen

Some hospitals have relaxed their no-mobiles rules. I guess modern equipment can tell the difference between interference from a phone and the real signal from your heart/lungs/whatever.

Mr. CooL ICE

jimmycrackcorm wrote:

On a side note I'm curious why hospitals have this thing about mobiles interfering with sensitive equipment whwn I'd imagine that a PC is more complex and doesn't get affected by mobiles around.

Reason: Digital! The majority of PC electronics 1s and 0s, which are roughly equal to 2 different signals: 5V and 0V. The circuits are designed to disregard any signals within 2 thresholds. I cant remember the exact figures, but the thresholds are something along the lines of 0V-0.25V and 4.75V-5V, so anything below 0V, between 0.25V and 4.75V, and above 5V is disregarded. Read Red Alert's post for the analogue version of things.

Red Alert

so when you've got

(Analogue Signal) -> (Analogue Processing) -> (A-D Conversion) -> (Digital Processing) -> (Digital Display/Recording/Printing)

there's loads of places for a mobile to interfere. you wouldn't let a broadcasting studio into an intensive care room now would you?

incisor71

Sparky_S wrote:

used to be able to tell when a call was coming in by the biping on the telly before it was a full on bipppppppp.

I noticed (as a GSM mobile owner since 1996) that older TV tuners were much more susceptible to the spread-spectrum interference of a call signal - one cheapo TV I owned from the mid-80s would blare out the loud "motorboating" B-note sound, as some of the call signal fell within the frequency range of the tuner. Filter technology has come a looooooong way since then!

Civilian_Target: should that 29,000 lines per second figure that you quoted not be 15,625, for PAL (15,734.26 for NTSC)? 625 lines per frame x 25 frames per second = 15,625Hz. (I used to be able to hear the high-frequency flyback whistle when a monitor was turned on, but not any more, now that I'm over 30.) Interlacing just has the effect of scanning 312.5 lines in one field, and the remainder on the other.

Binomate: televisions are sensitive even to the Earth's magnetic field, not to mention a big speaker magnet on top, so the picture tube is surrounded by a de-gaussing (de-magnetising) magnet which activates when you switch on the TV. That's one of the reasons why you hear the tonal "thump", and momentary dimming of lights, when you start up your TV or computer monitor for the first time.

ok, ok, I'm nitpicking, and straying off-topic, but I did a Masters degree on the stuff, and can't help it!

incisor71

Regarding the rippling of the monitor on-screen image when a call comes through:-

I believe this is partly a consequence of the critical convergence requirements for a monitor, namely, the three CRT colour guns must be brought to convergence at a single pixel, for every visible pixel on the monitor.

Each pixel has a much smaller area than its TV counterpart (and you sit much closer to a monitor than you do to a TV), so the effect of electromagnetic disturbance is magnified. The interference could also affect the behaviour of the convergence circuitry itself, not just the signal being fed to the CRT.

LCD monitors don't have a need for convergence circuitry - is it true that they do not suffer from this rippling effect? I haven't tried it myself but I imagine they wouldn't.