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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?"
Just to add to that, there are quite a few people that can hear the extremely high-pitched whine of CRT's scanning - we can tell if a TV is on in a room without looking with it on mute. The channel changing sound is much lower pitch so almost anyone can hear that, not to mention the static discharges that can occur. It's no surprise to us that electronics make noise, in fact sometimes it can be downright painful. I used to have this 32" inch TV that would whine to the point of pain until it warmed up 5-10 minutes later. Of course as I cover my ears everybody else doesn't notice a thing. Good ears can be a curse :(
"I don't know that atheists should be considered citizens, nor should they be considered patriots." George HW Bush
Presumably you're refering to this event at the Homebrew Computer Club back in 1975... (snipped from this article)
The Altair may have been frustrating, but it drove the nerds to experiment, finding real uses for the useless box, turning it from a curiosity to a computer.
Lee Felsenstein
Steve Dumpier set up an Altair, ehm laboriously keyed a program into it. Somebody knocked a plug out of the wall and he had to do that all over again but nobody knew what this was about. After all, was it just going to sit and flash its lights? No.
Roger Melen
You put a little eh transistor radio next to the Altair and he would by manipulating the length of loops in the sofware - could play tunes.
Lee Felsenstein
The radio began playing 'Fool on the Hill'....Da da da, da da da....and the tinny little tunes that you could tell were coming from the noise that the computer was generated being picked up by the radio. Everybody rose and applauded. I proposed that he receive the stripped Philips Screw Award for finding a use for something previously thought useless. But I think everybody was too busy applauding to even hear me.
Roger Melen
It was a very exciting thing, it was probably the first thing the Altair actually did.
On a related note, my old BBC micro used to pick up interference on it's internal speaker, which could actually be used for some basic debugging. You could tell if it had crashed, or whether it was still running round a particularly heavy maths loop, etc...
Properly designed hardware should not do this.
A few weeks ago, I thought my Soyo Dragon motherboard had gone flaky because I was getting massive fs corruption when copying between drives. I panicked and went out and bought a new motherboard without having done any research. I told the guy I wanted to replace a Soyo Dragon and had 5 IDE devices, including 3 7200 RPM drives. The moron gave me an MSI KT3 Ultra2. That is not a replacement for a Dragon. The onboard sound doesn't even have digital audio outputs. I was using the Dragon's SPDIF to connect to my speakers. It sounds very nice.
I tried it out anyway. One thing I noticed right away was that I could hear noise whenever I selected text or moved a window. I took it back (for other reasons as well) and got my money back. The fs corruption was caused by the power supply unable to put out the power so I got a new one.
Right now I'm using the Dragon's analog out and there's no noise at all at any normal volume. If I turn everything up to maximum, I can barely hear something above the fan noise, but if I play something at that level, my ears would hurt.
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=