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Ask Slashdot: Does LED Backlight PWM Drive You Crazy?
jones_supa writes "I would like to raise some discussion about a hardware issue that has increasingly started to bug me: backlight flicker, from which many LED-backlit monitors suffer. As you might know, the backlight and its dimming is driven by a pulse width modulated square wave, essentially flicking the LEDs on and off rapidly. Back in the CRT days a 100Hz picture was deluxe, due to the long afterglow of the display phosphor. LEDs, however, shut off immediately and my watering eyes and headache tell that we should be using frequencies in multiple kHz there. Unfortunately we too often fall behind that. As one spark of hope, the display review site PRAD has already started to include backlight signal captures to help assessing the problem. However with laptops and various mobile gadgets, finding this kind of information is practically impossible. This issue sort of lingers in the background but likely impacts the well-being of many, and certainly deserves more attention."
So do LEDs bother your eyes? I think CRTs gave me headaches far more often than has any form of flat panel display, at least partly because of the whining noise that CRTs emit.
Your comment gave me a headache. Also, are you allergic to Wi-FI?
Might be a first world problem but that doesn't make any less real.
It will never change unless someone starts the conversation.
It's just you.
If you're sensitive to them, don't buy them.
Please don't make every LED / LCD on the planet more expensive because of a tiny minority of people who blame things like PWM for their symptoms (correctly or not).
Like with flourescent lamps, and people who can't be in an air-conditioned room, and people who have to play games with altered FOV's because it makes them sick. You're a tiny minority, or else half the world would feel ill all the time. Please find another way to cope with it (i.e. glasses, double-blind tests to convince yourself it's placebo, or whatever).
Some high-end displays flicker like a movie projector, only turning on LEDs during the refresh interval when the entire image is cached by the TFT so there's no tearing at all, not even as much as a CRT. If this is true you might find those expensive displays especially annoying.
If it's PWM annoying you, shouldn't setting the display to max brightness entirely fix the problem? I wonder if you're picking a scapegoat for your headaches.
So do LEDs bother your eyes? I think CRTs gave me headaches far more often than has any form of flat panel display, at least partly because of the whining noise that CRTs emit.
No. You're imagining things.
But, that being said, you're not alone. I heard somebody walk into the retail establishment that I work at and said, "I'm disappointed that you guys installed automatic doors that emit so much radiation, but I'm glad that at least you don't have horrible fluorescent lights that would make me unable to shop here." Of course, she was saying that standing under about 500 CFL's that she assumed weren't fluorescent because of their size, shape, and color.
I don't respond to AC's.
One thing I do know was/is a problem with monitors and eyestrain has to do with fluorescent lighting in the room. We can't see it with the naked eye, but the fluorescents are also flickering at 60hz and I've had it happen in the past that if the CRTs I was using were out of sync, (running at 75hz or similar) after a while I'd get weird eye strain from something we can't consciously perceive but our eyes still try to correct for. I usually solved the problem by either setting the CRT sync rate as high as it would go or syncing it to 60hz, or preferably getting rid of the fluorescent lighting completely in my workspace when possible. Maybe a similar effect is at work here?
No. You're imagining things.
Happened to me once that I got upgraded to a larger monitor, and when I turned it on it was like being physically smacked in the face. It's a long time ago so I can't remember exactly what happened then, but I didn't use that monitor.
http://en.wikipedia.org/wiki/Saccades
The average (quality) CRT is perfectly fine for most people. They do not emit any high-frequency noises, nor do they have major flickering or geometry issues. To suggest that all CRTs are crappy is doing them a total disservice.
:)
That said, there are plenty of CCFL-using LCDs which have given me dry eyes and a funky feeling after staring at them for a while, possibly due to the polarized light. Or perhaps just because they were low-quality pieces of junk.
If you want to check if there's any significant flickering that'd annoy you, check the display from the corner of your eyes. The peripheral vision of the eye is far more sensitive to motion than the central part you generally focus on. If you can't see flickering with your peripheral vision, it's just not there for you.
Thanks for the whine story, though. Would you care for some cheese with that?
Site & blog: http://www.mayaposch.com
We have a user here who got a new laptop last summer, it had a LED backlit LCD. Within 20 minutes she was calling saying it was making her feel sick/headache. We tried adjusting refresh rate, brightness, no help. Put a CFL backlit LED laptop in front of her and she was fine. Tried LED standalone monitor, it also bugged her though not as much. So, we had to find a laptop that had a CFL backlit screen, wasn't junk,and met our other requirements (docking connector mostly). Ended up getting a previous year model Toshiba Tecra with a Core2Duo.All the rest of the laptops we bought had i5's in them by that point.
Add a 10pF cap, no more flicker. But seriously, i have had ZERO experience with this. None of the 20+ laptop/desktop screens we have at work, or my screens at home (TV/computer) flicker at all. They are all cheap crap LEDLCD's. A few are the newer LEDIPS. Zero flicker. Even when filmed with 60hz cameras, no flicker at all.
Idiot. You are very likely in the majority that cannot see that flicker. There is a minority that can, and for them it is a very serious problem.
Using higher PWM frequencies is not an issue at all, it just has to be done.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
You are just another asshole saying hurtful things without even understanding the problem. There is a minority in the population that can see lower-frequency flicker. You are very likely not one of them, or you would not say such incredible stupid things.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
For over thirty years now I've been working with various display devices of a wide variety of design, manufacture, size and refresh rates. About sixteen years ago I started having the symptoms you describe -- headaches, watering eyes, etc. The internet back then isn't what it is now, so my first reaction was NOT to post something on a tech forum and open myself up to a lot of ridicule and abuse. Instead, I made an appointment with an ophthalmologist. After a thorough examination and some tests he advised me to take occasional breaks from the monitor throughout the day and rest my eyes. He also gave me some techniques to use for this. I took his advice and my symptoms went away virtually overnight. I have not had any problems since.
You should go see an ophthalmologist -- not an optometrist -- but a real eye expert. You might be surprised to learn that your problem has nothing to do with refresh rates or anything of the sort.
Proverbs 21:19
Waa my computer is too flickery, someone call the waambulance.
For people with photosensitive epilepsy, it might more more like "someone call the ambulance."
Generally triggers between 3-30Hz with some rare cases up to 60Hz (who can't do much under indoor lighting). 100Hz for a backlight is not an issue and if it is, make a few thousand bucks selling yourself to science.
@Whee
I suspect author is also bothered by wifi signals emanating from his router.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
Sig Battery depleted. Reverting to safe mode.
The cycle rate for triggering seizures isn't typically that high. Most LED backlights cycle at 120hz (they should be driven ). Older LCD flatscreens had crappy tubes and had a very visible flicker.
http://en.wikipedia.org/wiki/Photosensitive_epilepsy
When functioning correctly, mains-powered fluorescent lighting has a flicker rate sufficiently high (twice the mains frequency, typically 100 Hz or 120 Hz) to reduce the occurrence of problems. However, a faulty fluorescent lamp can flicker at a much lower rate and trigger seizures.[4] Newer high-efficiency compact fluorescent lamps (CFL) with electronic ballast circuits operate at much higher frequencies (10–20 kHz) not normally perceivable by the human eye, though defective lights can still cause problems.
Am I the only one who has an issue with this definition? I realize that English is an evolving language, but it seems like this became a new definition because too many people were too fucking stupid to understand the actual meaning of the word. Similar to how the word "epic" no longer has the same impact it once did. Perhaps we can redefine "figuratively" to have the classic meaning of "literally".
Am I the only one who has an issue with this definition?
Yes, you are literally the only one.
I can actually see the flicker effect he talks about - on some rare models. I can only see it near the edge of my vision (where your eyes are primarily motion/change sensitive). 70hz seems to be about the point where this stops, but anything lower than that is perceivable to me.
Just like I can hear that irritating whine he talks about (though likewise, not all of them do that)
The inverter on my LCD at home actually makes all sorts of horrible whines if it's in standby mode - so I either leave it on or unplug the thing.
For large sets, this will be our guide even unto death, for the LORD will work for each type of data it is applied to...
I will agree with the Cadillac taillight issue. It doesn't give me a headache, but it does have a Persistence of Vision effect which disorients me. As the vehicle moves either subtly in it's lane, or changes lanes, it leaves a trail of ghost images which occupy a significant part of my field of vision. It's a good thing these are (for now) in the minority. I don't understand how these could have been approved from a safety perspective.
I suspect author is also bothered by wifi signals emanating from his router.
If my eyes are watering after a long session its because my screen is too bright, which is exactly the opposite of what he postulates as the problem (on off cycles of LEDs). Brighter requires longer "on" cycles, which in turn are less perceptible. Yet for most people overly bright screens are the source of complaints.
Ah, the good old "it doesn't happen to me, he's a liar" reasoning.
PWM lighting is annoying if the frequency isn't high enough. Rates that that drive me crazy don't drive everyone else crazy. I perviously didn't know why some displays made me slightly nauseous and others didn't until I started to dabble in electronics and learned what PWM actually is and built a circuit that gave me headaches.
I don't understand how the carrier frequency is chosen in consumer goods, but it seems in times past it was based around whatever clock source was conveniently available, and those sources are generally completely arbitrary. I found is rather funny how one arbitrary number can make me hate your product if it wasn't high enough.
To be fair, things are a lot better for me now than they used to be. Probably because the conveniently available clock sources are faster now, or maybe some switched to adjusting the current directly? Maybe also that VFDs and LED displays have given way to LCD displays. And nothing was worse to me than a CRT with phosphors that decayed faster than the retrace. Yetch.
More Twoson than Cupertino
This goes for LED brake lights, LED Christmas lights, and LED traffic light, and roadside LED signage.
I find the PWM flicker of LED brake lights _VERY_ disorienting.
Monitors I can aviod.
According to this article (http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm) LED backlights generally pulse between 90-420Hz, not in the KHz or MHz range.
There is a real component to it, particularly if you're a migraine headache sufferer. Migraineurs tend to be sensitive to certain frequencies of flicker. I find that fluorescent lights are uncomfortable and tiring whereas LED bulbs and incandescent bulbs are fine. Then again, I've never had a problem with LED/LCD, fluorescent LCD and CRT monitors because the flicker rates seem to be at rates that don't bother me. (Staring at highway markings close to the car at highway speed drives me absolutely bonkers, though. Good thing I don't really need to do that. :)
As someone who experiences this issue, I can confirm it exists. I imagine most people are sensitive to it at some frequency but it may not be at frequencies that are ordinarily an issue. Get a strobe light, play with it and chances are you'll find a frequency that bothers you.
Many of the backlight control boards are only 400Hz carrier for efficiency. If you are dimming to 10% that is 20Hz pulses, which is the range for problems. Changing the current changes the color temperature of the backlight, distorting colors. Adjustable backlight color temperature therefore further complicates matters, since they need part of the dimming range to compensate for the current change.
It can be improved, but it is a cost/efficiency/performance trade off
...or maybe his problem with eye strain have something to do with staring, wide-eyed, at a single object, in a florescent light, dry, air-conditioned environment for 8 hours a day while on a steady diet of diuretics like sugary caffeinated substances.
I did not see anything in the summary to indicate that jones_supa had positively identified the LED backlight as the source of his problems to the exclusion of all else.
For the WIFI people there are strong indications it is psychological only. For the flicker-sensitives, it has been known to exist as effect since movies exist. The original cinematic 24 pictures/second was selected because most people cannot see that flicker or are not bothered, but it is known that some can and may even get headaches, etc. The problem is really not new, just the place it turns up in is. So, we can say scientifically that some LED backlights may have that effect on some people. Of course, this would require rather low PWM frequencies, for example because an old CFL design was just adapted, where the slow CFL PWM is used for LEDs. CFL inverters run somewhere in the 50-100Hz range and their PWMs are synchronized to that. If you use the same PWM for LEDs, some people will see flicker. CFLs are pretty sluggish, leaving only minimal flicker, while LEDs are fast.
Of course, if you design for LEDs, you can run the PWM at > 1kHz, and there nobody should see any flicker. Ideally, you can run it at > 50kHz, then nobody can hear anything either.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
I'm pretty sure you're confused about this. PWM is 'pulse width modulation', not 'pulse removal modulation'. If you are dimming to ten percent, you'd expect the pulses to be ten percent as wide as at 100%, with the pulse rate unchanged.
Alter Aeon Multiclass MUD - http://www.alteraeon.com
I don't understand how the carrier frequency is chosen in consumer goods
I do. I write firmware that does backlight PWM for a living (among other things).
Everyone used to do high frequency flicker-free PWM, in the kilohertz range. Then they noticed that you can improve the motion handling capability of an LCD panel but flickering the backlight at the same frequency as the screen refresh. It's kind of like how a CRT's phosphors fade and thus flicker at the refresh rate. Turns out it stops LCDs blurring with motion too.
In practice most monitors do both. They use high frequency PWM to set brightness and then switch that on and off at a low frequency like 60Hz. This is what causes the annoying flicker, but hay, at least the crappy review sites can say motion reproduction is better than the competition.
const int one = 65536; (Silvermoon, Texture.cs)
SJW, n: "Someone I don't like, and by the way I'm a fuckwit" - AC
Ah, the good old "it doesn't happen to me, he's a liar" reasoning.
There are enough examples of the placebo effect that this reasoning is a fair place to start. If you claim to observe an effect that most people cannot, you need to produce data that shows you can discriminate the effect under blind conditions. Otherwise we have no reason to believe you are any different from EM hypersensitives, etc.
Give me Classic Slashdot or give me death!
Actually some people might notice things that others don't. Most LED tail-lights drive me absolutely crazy, but my dad doesn't even notice them. Same for sounds, the old TV in our bedroom has a high whine that only I can hear, my girlfriend can't hear it, and thinks I'm crazy since I unplug the TV before going to bed. Same for flourencent lights, some people can see the flicker from crappy ballasts, some are oblivious. People have different sensitivity to frequencies at the edge of perception, some people won't notice it, and some will. Welcome to normal human variation.
I recently went shopping for decent IPS displays, and most of the LED ones do noticeably flicker at low backlight levels. Some, cheaper ones, were tested with noticeable flickers at all levels. I picked one that still used tubes, since I generally work with low brightness levels (for print work), and even decent LED monitors started flickering there (and its hard to get a good, wide gamut, monitor with LEDs and not break the bank).
A patriot must always be ready to defend his country against his government. -edward abbey