Why Does a Screen Re-Draw Make Noises?

grungy asks: "On several computers I have owned, I have noticed an audible noise related to large screen re-draws. A hardware guy once hypothesized that the large memory-move operation was creating electronic 'noise' which was then picked up and audibly amplified by my speaker. I unwired my speaker, removed it from the machine and put it in a different room, and the phenomenon still occurred. At this point I assumed it was something going on/emanating from the monitor itself. Now I have a TiBook laptop with an LCD panel. At quiet moments I can still hear it when I drag windows around. I have tried doing big memcpy's & the like, I don't get the same noise. I've been wondering about this for years. Anybody know what gives?"

Comments

Electromagnetism

[Mad Quacker]

Whenever you have a flow of current, you will have a magnetic field generated, and that field reacts with the environment to cause motion. Even though computers are 'digital' doesn't mean that some how they are immune from the all the laws of electromagnetism we use to design analog devices like speakers - it's all the same thing. If nothing else, there is always the earth's magnetosphere to react with like a speaker's coil to it's magnet.

It's the same reason electrical transformers hum, and fluorescent lights buzz.

Re:Electromagnetism

[Smidge204]

I wasn't aware that humans could 'hear' electromagnetic waves. :)

The reason transformers hum is because their cores are vibrating in response to the magnetic fields they're subjected to. Same with flourecent lights (which have transformers in them).

But yeah, just about every AC power appliance gives off a "Hum" of electromagnetic waves, and digital devices, with their constant pulsing, do it as well. And preventing the two from interacting is big business.

Some good examples from personal experience:

Trying to record some audio clips, but when I play them back, half of them have a STRONG buzz in the background. So loud you can barely hear the recording. Turns out my mom turned on her ceramic kiln in the basement (which sucks a lot of juice), creating strong interference. Sure enough, when the kiln turned off, the problem went away.

If I have the volume up, not only can I "hear" the screen redraws, but the mouse move, my keyboard pulse, and my network card go to work. If I have the headphones on and the volume all the way up, I can hear the hard drive working, too. (Interestingly, I'm reminded of this one government "safe room" that was specifically sheilded to stop these pulses, since it would be possible to catch and decode them to figure out what the input devices are doing. eg: passwords and other text)

=Smidge=

Noise....

[keoghp]

They are all wrong...
It's a little guy in the computer with chalk, drawing the pictures on the screen. Sometimes when the screen "freezes" - it's him taking a break.

After a long spell at the coomputer you can usually hear him gasping for breath.

When he has run out of colours and he only has blue left - that when you get the BSOD.