View Full Version : LCD screen getting interference from modem?

30-12-2005, 02:33 AM
Hey guys,

Some friends of mine have an LCD screen (15" HP) and whenever they connect to the internet on it, (dialup WINternal modem), it seems to start intefering with the monitor (damn I'm good at speeling ;)).

The interference is like thick lines going up the screen, almost like the picture is about to "roll" like you get on tvs.

I thought this to be very strange as I didn't think lcds were even susceptible to inteference, and that there is no way a modem could cause inteference like that. Connecting to the internet wouldnt even pull that much more power would it?

When the screen was upstairs, it didnt do it, however now it is downstairs which leads me to believe it could be wiring in the wall? But surely wiring cannot cause that much interference either....

I don't know of any PABX's, transformers, or anything nearby that could cause it either.

There is a laudry next door, could the interfrernce (goddamit!) be multiplied by the huge metal drum in the washing machine...

very strange....

30-12-2005, 09:16 AM
Probably interference caused by the washing machine. Does the signal cable of the lcd pass near any power cables? I doubt the modem would be causing it. It could be that the power cables in the downstairs area are not shielded properly.

30-12-2005, 09:30 AM
What you're describing sounds like the interference you typically get from an electric motor in the vicinity, or some other source of magnetic interference.

I guess in theory an internal modem that's not properly shielded could do it.

When you say the screen is upstairs, do you mean on a different PC? Have you tried moving the PC and monitor together to see if the interference goes away?

A washing machine sounds like a very good candidate for EM interference.

30-12-2005, 10:29 AM
An LCD display (the screen itself) is not really susceptible to any EM interference, but its possible that the A-D circuitry in the monitor electronics (Analogue to Digital) could be.

Or in severe cases it could be induced into the monitor cable, if unshielded, but it's not something I have ever seen.

30-12-2005, 11:27 AM
yeah, thats the stuff I was thinking... the washing machine is in the next room on the other wall I think... and turned off...

But the weirdest thing is it only does it badly when you connect to the net. Otherwise its just the odd noticabl flicker...

30-12-2005, 11:35 AM
It not the washing machine, if its turned off.

What happens when you shift the position of the LCD as far in any extreme as the cords will allow?

Graham L
30-12-2005, 02:28 PM
Do you get sparks when you plug the telephone line into the modem?

That sounds like big bad 50 Hz hum bars. Something's not properly earthed. Or you've got something like an extension cord with a neutral to earth swap (or phase to earth :( ).