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?"

Re:Static electricity?

[Mad Quacker]

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 :(

Yep, me too. Bad RF shielding.

[arekusu]

I've heard this on every computer I've owned, going back 14 years to my Apple IIgs. Any CPU operation (tight loops reading memory especially) could be heard as pitched tones on the audio out, especially noticable when listening with headphones. On the IIgs, I had a stereo "Audio Animator" card and always figured it was crappy RF shielding. You could hear a scale, tones changing as you moused over each item in a menu.

But it happens on my other computers too, to larger or smaller degrees. On my TiBook, it's pretty noticable when the fan/drive are spun down. I program in Cocoa/GL and you can hear the tone change just by creating an NSTimer with different frequencies, and using it to do graphical operations. Most of the time, this results in a low 60Hz "hum."

I think it's due to RF interference between the audio portion of the board and whatever else is nearby. It seems more prevalent on laptops where everything is packed closely together but it's not limited to laptops, or LCDs.

Somebody should write an app that plays one-channel melodies with the RF noise... ;)

Oh man, me too

[eamonman]

Before, with CRTs and cheap LCDs, anytime that I moved a big window (typically bright, say all white), and wiggled it around the desktop, I heard something similar to a, "wheeeee, weeeeee" kind of sound (the 'wheee' matching the window movement). Of course, this was a very very high pitch sound; a quieter and higher pitched version of the whine that TVs and crappy old CRTs make. Now I have a new and really nice LCD... I can't really hear anything, but then again, maybe I'm just getting old.

Isn't it great to know you have good hearing though?

This is a little OT but... back when I was a kid, I think I had even better hearing... I used to stay at my grandparents' house, and I could sense people walking down the hall to my room, no matter how quiet they were. The floor didn't squeak, and my grandmother used to walk around softly. But I could tell when she was coming. Basically, I would hear what seemed to me a lack of noise approaching; there was a lot of ambient noise from the living room (the windows were open which means lots of trees, birds, wind, etc. to hear), so someone walking down the hallway towards my room from the living room seemingly blocked some of the sound. It was very slight, but it was enough so I would usually be looking up at my grandma when she turned the corner to my room. I've had other experiences, like hearing if someone was sticking their hand in front of my face when I was blindfolded (it had to be in a fairly quiet room however).

Sigh, I miss having my good hearing. 25 years and lots of concerts, New years festivals and 4th of Julys will do that. It would be so helpful now to have that hearing, especially when my boss walks to my cubicle ;)

Re:Yep, me too. Bad RF shielding.

[Scorchio]

Presumably you're refering to this event at the Homebrew Computer Club back in 1975... (snipped from this article [pbs.org])

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...

Re:Static electricity?

[teamonkey]

I can hear that too. I can tell if a TV's on nearby - in another room or even downstairs. Always have been able to, although it was much more noticable when I was younger. High frequency devices are less noticable though. What about the details on screen? I find that a white screen makes a higher-pitched noise than a blank one. On another note, my old monitor used to make high-pitched noises every so often which could be solved by giving it a sharp whack.

Could be the power supply

[Gordonjcp]

When you do stuff, the CPU draws more current. This could affect all the chokes and stuff in the power supply. Small chopper PSUs such as those used to regulate down the 5v/3.3v rail to the 1.5~odd volts that modern CPUs need have several inductors, which could vibrate and make noises. Especially when attached to a big wobbly fibreglass sounding board.

Crappy hardware

[jfunk]

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.

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=

iMac too!

[gabe]

I have the same problem with my LCD iMac. It's not attributable to simple CPU usage, but rather the interface. When I drag windows, move scroll bars, access menus, etc., I hear a slight grinding sound (sort of like a hard drive but considerably quieter).

Then again, when I've got iTunes blaring it doesn't really matter.

RF noise

[halfelf]

My vote is for the RF noise being picked up by the unshielded soundcard. Just think --- the companies making the cards are constantly trying to find ways to make them cheaper. I mean, just how far can they go adding new features before the average person can't tell the difference anymore. The only thing left at that point is to find ways to make your product more cheaply than any of your competitors, and one way to do that is to not bother with the engineering involved in making an RF shielded card... "Hate my people? I love my people! PULL!"

Re:Static electricity?

[Anonymous Coward]

i saw this website once where a guy was using his screen refresh rate to generate AM signals and transmit music to a radio nearby.

Re:Electromagnetism

[david duncan scott]

Looks like milspec gets tougher than screen, judging by this, [lindgrenrf.com] which describes a honey-comb waveguide approach. (That company also makes welded steel rooms, with no fasteners used in the seams -- just solid steel everywhere.)

Re:Just wait a while...

[JabberWokky]

Not necessarily. My grandfather, father and I can all hear it - we can also hear the so called "silent alarms" used in some banks and a few dog whistles (presumably those with lower frequencies that were out of the manufacturer's hearing but not ours). We all have 100% Irish ancestry, to give a genetic background.

--
Evan

Re:Just wait a while...

[JabberWokky]

We're Presbyterian, and while neither my father nor I are abstainers, I started drinking well past my 21st birthday and I've only gotten drunk on less than a handful of occasions. I have a Guiness maybe once a month, and only really drink at New Years and X-Day.

My hearing isn't all that great - I think I've suffered damage from listening to music too loud in headphones and later playing in a few bands (the amps went up to 11!). It manifests itself as muddy hearing of words, though - I tend to snap to alertness at a small noise.

As a music lover, it's depressing. As someone who worked with the blind (DOS users might remember txt2b and txt2b2, text to grade 2 braille translators I wrote), I'd rather lose my sight than my hearing.

--
Evan

Re: Double Wow!!!! !!!!

[nito]

I'm amazed at the number of answers from the "Slashdot experts" and yet I don't see the correct one yet.

Yet you also fail to give a correct answer too.

Even though some claim they (dogs probably can) hear their video card HSync signal, most of the time what you hear when the speakers are off is static electricity discharges like when you degauss your monitor.

On the other hand the sounds the original poster refers to are most probably caused by the induced voltages in the speakers from **changes** in the nearby electromagnetic fields emanating from the front and back of monitors (especially the cheap ones with crappy farady cages), as the screen content and colors change. Conditions that maximize this would be high contrast patterns, like alternating bands of bright and dark, since those cause more change in the electromagnetic field, which maximizes inductions in nearby conductors, like the coils fo your speakers.

Mousing noise?

[BillX]

On several computers I have noticed sounds (it's a clicking or thumping sound) that occur when the mouse is moved, proportional to how fast it is moving. Does the sound only occur when moving a large window, or does mousing while the window (or similar screen content, e.g. white background etc.) is visible?