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?"

Static electricity?

[EvilMal]

Is it associated with a large change in brightness? Like in drawing a white box on top of a black area?

It could be static electricity, as it is suddenly going from one number of electrons to a very different number of electrons hitting the screen.

Gee, it's great to have an electrical engineer as a dad...

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.

Possibly the power supply

[toybuilder]

The hissy-screechy-screech-screech that you're hearing might also be coming from the power supply. To the extent that it would carry into your audio circuit, electrical noise would easily translated to acoustic noise through your speaker/headset.

However, it's also possible that you have a marginal power supply that operates at switching frequencies that approaches human-audible frequencies; or the actual current draw changes from high-speed memory transfers within the graphic sections (board) has a human-audible frequency component to it that actually emanates from (say) the torroids in the supply.

People with very sensitive high-frequency hearing can sometimes tell the brightness of a television screen just by listening...

This could also happen from other activity -- I once had a 386 PC which, when running DOS, would emanate the tell-tale sound when it was waiting for keyboard input. It was kinda neat, actually -- I could go read other things while waiting for a program to finish its calculation -- and I didn't have to keep looking up at the screen...

Re:Static electricity?

[Goldsmith]

Not quite true.

Remember that light itself is electromagnetic radiation. The change in screen color (as suggested by one comment) might be enough to make some small amount of noise.

More likely, it's an induced response from the screen material to the changing charges across the screen. An LCD works by putting an electric field across a liquid crystal to allow/block light. As you redraw, those electric fields are changing. Those fields might have a small effect on actual material the LCD is made of. Those materials might have a small sensitivity to electric fields, and there is almost certainly some small charge even from dust on the screen.

So... after all that, it's my guess that the clicking noise (I've heard it too, you're not insane) is portions of the plastic on/in the screen either expanding out or contracting in with the field, essentially "popping" in an out.

Another thing it might be is electrically charged dust (dust does not have to be neutral) moving around on the screen. Try dusting your screen and seeing if that does anything.

Re:Static electricity?

[You're All Wrong]

LCD actually change physical state, i.e. there is a mechanical change. Enough of those at the same time, and there will be some sound. A little like piezo-electric shriekers, but much smaller, and with much smaller movement, and only a single pulse of movement rather than repeated oscillation.

My Psion 5 used to sing a merry song to me all the time as things changed on screen. Almost everything LCD makes these noises in some quantity. I do have very sensitive ears though, so perhaps not everyone hears them.

YAW.

Singing Capacitors

[Anonymous Coward]

When capacitors are charged and drained quickly, they can emit sound. As the charge changes, the two plates that hold the charge will try and move closer or further apart, in a similar way to a speaker. Unlike SRAM, DRAM is actually made from banks of tiny capacitors. In older machines, you could often hear the memory singing while the bootup memory check was in progress.
While you can't hear a bit changing here and there, when changing large amounts of memory very quickly, such as changing/redrawing a screen, the sound soon mounts up, and you can hear it.

--[me]

Re:Crappy hardware

[shepd]

You got an MSI for a reason.

From my experience, here's what I've found as to why to buy from different motherboard manufacturers:

Soyo: A value board. Lots of stuff crammed on for a good price, or hardly anything on it for a bargain basement price. Not a bad board, but not top notch.

MSI: The die-hard motherboard. Might not be a cadillac, might even have some annoyances, has no luxuries at all (usually), but dammit, they _always_ work, and are reasonably priced.

Asus: The "high-end" motherboard. Just like cars, where more money gets you some bells and whistles, but not always more reliability, Asus motherboards are bought by people trying to show their box is "awesome" because it cost more. If you look past the pricing, quite a good board. Lots of support, too.

ABIT: The ricers mobo. ;-) Designed for overclockers, with the stability overclockers (not sysadmins) expect. Usually the higher cost for these boards nullifies overclocking benefits, but just like people who add "Type-R" stickers to their cars, the people buying these boards don't care.

PC Chips (aka any weird Chinese name you can think of): When cheap-enough (Soyo) isn't. Zero support, stolen/fake parts, and a high failure rate. But look at those prices! Often found in low-end Brand Name machines.

ECS: PC Chips "top-notch" line. A well supported stolen/fake parts brand motherboard.

A-Open: Overall, pretty good stuff. Good in most categories (price, support, quality, performance) but fell out of favour with after providing me with a broken BIOS for an old board, ruining it (didn't have an EEPROM burner at the time). Definately not an overclocker's board.

Shuttle: Haven't had enough experience with their product. Boards I have seen were reasonable.

Tyan: Haven't seen too many of these boards, but people I know tend to regard them as a good for a frankenserver board.

There's others (gigabyte, biostar [pc chips?], intel, DTK, etc) but I simply don't see these boards in operation much anymore.

So that's why they sold you MSI. You came in telling them your board was causing you hell, so they gave you the bulletproof one. I'd have reccomended you to stick with it and buy a PCI sound card (heck, if it were my store, I'd probably just give you a used SB PCI128), but hey, that's just me. Then again, I'd have replaced your board with an MSI with the Nforce2 chipset, so you'd have decent sound to start with.

Wow!!!!

[FreeLinux]

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

The noise you hear is actually fairly simple to explain. First of all, people should realize that this is not RF noise coming through the speaker, as you tried to explain. This is a noise generated by the vibration of a system component.

Your graphics card is the culprit. Remember that your hardware is full of clocks(vibrating crystals) and switches(transistors). These microscopic components move or vibrate at very high frequencies. Vibration creates noise, as we all know. But, the vibrations(or frequencies) change when the image on the screen changes. Certain colors and certain movements on the screen create frequecies that are perceptible to human hearing and you hear a slight buzz or high pitched whine form your video card.

If you want to test my answer, try changing the frequencies for your display and you will hear the sound come and go. You will also notice the pitch will change when different frequency setting are used.

Some hardware is less prone to this because of thicker cladding or more secure mountings but, they all do it. It's just that some equipment is louder than others.

Re:Static electricity?

[unitron]

That high pitched whine isn't the CRT itself, it's the audible result of the laminations of a transformer core expanding and contracting under the influence of the current, which is varying at the horizontal sweep frequency, in the transformer windings. This expansion and contraction is called magnetostriction.

IT's RF crosstalk, my iMac has it too

[Stuntmonkey]

Since I haven't seen a cogent explanation posted yet, I'll take a swing.

Fact 1. 60 Hz transformer hum is unrelated to what you're hearing. (That would be at 120 Hz and relatively independent of screen refresh, which doesn't match your symptoms.) As a sidebar, 60 Hz hum is caused by the Lorentz force between the electric current in a transformer coil and the magnetic field that the current induces, which produces an radial outward force on the coil. (This effect is what limits the size of magnetic fields we can create in the laboratory -- no one wants an exploding electromagnet.) As the 60 Hz alternating current runs back and forth through the transformer, a 120 Hz mechanical vibration is induced.

Fact 2. It's also not directly related to "CRT whine". We can tell this because: (a) CRT whine is independent of whether screen contents are changing, and (b) CRT whine is a directly audible mechanical vibration, not a crosstalk into your audio out line. CRT whine is caused by the electronics that drive the electron gun's horizontal deflector. For example, if you scan 500 lines 60 times a second, the signal on the horizontal deflection plates is at 30 kHz, which some people can hear. Most modern computers have enough scan lines and a high enough refresh rate that the signal frequency is too high for anyone to hear, making this not a common problem with newer computers.

Fact 3. What you're hearing is caused by capacitive coupling between signal lines (wires) inside your computer. Because of the electric repulsion between electrons, high-frequency signals can "conduct" across the air between separate wires, especially if the wires are close together. In your case, it's crosstalk between the display and audio circuitry. This crosstalk interference can be reduced with grounded metal RF shielding, but it adds cost/bulk/weight and so manufacturers try to minimize the amount they use. An audio company would shield the DAC and preamp components carefully to bring the noise below a perceptible level; a typical computer manufacturer will just make it sound ok for ordinary use.

My guess... ...is that you have an active-matrix LCD screen, not a passive LCD or CRT. The reason is that you only hear the noise when the screen content changes. Unlike the other two, an active-matrix screen has transistors at each pixel that remember their state. Thus there is a drive signal to the screen only when pixels are changing. Only if relatively large portions of the LCD are being continually rewritten will the duty cycle of this drive signal be substantial, and therefore the crosstalk be audible.

Conclusion: It's annoying, but there's not much you can do about it without buying a higher-quality (i.e., better-shielded) audio card.

yep, it sucks, but there's a solution:

[Anonymous Coward]

my mitsubishi display sometimes starts making that noise and its *REALLY* annoying, ... like some of u guys explaining this in this page, my friends look at me like im crazy when i try to explain them.... of course, they dont hear anything.... : S

The thing that sometimes works with that is to change the colordepth from 32 to 24 or 16 bit, or the other that works always -at least with my computer- is to change the refresh rate from emm, "optimum" to 75hz or 68 or so.