@OQ3: I cannot for the life of me imagine how disabling FRC/Dithering could be harmful to the monitor. A bigger issue, it seems to me, is that there is hardware FRC and software FRC. In other words, the graphics card can do its own FRC separate from what the monitor does, and when the monitor sees the pixels it won't know that they were derived from a dithering algorithm. The Intel GPU has special instructions for dithering of various kinds, and I know that other GPUs (Nvidia, Radeon) have similar capabilities.
BTW some Linux users have managed to increase PWM frequencies of their monitors through software hacks. _That_ is something that could potentially be dangerous e.g. by creating excess heat.
That's not how brightness of your screen works. The LED stays at a constant brightness. The only thing that adjusts your screen's brightness is how much voltage is applied to the liquid crystals. The orientation of the liquid crystals determines how much of the LED's light permeates through.
With that said, if you're seeing flickers, it could be that the LEDs are faulty, but they are not designed to flicker.
Otherwise, it could be a problem elsewhere.
jongmini, this is very interesting to me. If I understand stand correctly your stating LED's are solid state lighting in computer monitors? I notice the term "solid state" used in regards to LED lights in household/business lighting and yet if you film them with slow motion video they flicker intensely, I would like to know how you are certain computer LED's do not pulse/flicker? Are you a hardware engineer or lighting expert? I am genuinely asking this question, not being sarcastic, I would like to know what qualifies your statement as fact. I have read many papers on LED lighting and the facts as I understand it are that many LED technologies are designed to flicker either for dimming or power savings, maybe this is not the case in some computer monitors? I cannot be certain of that, however here are a few links including various content referencing exactly what I am talking about and hopefully explains why I am confused by your statement that LED's do not flicker:
I also have some links to papers written by PHD's in light, I actually have hundreds of links to information confirming flicker as a part of both fluorescent and LED backlighting....I am responding to your post because it is very important that our factual statements here are accurate or we chase our tales far too much:)
Thank you, Jesse
Jesse, just to clarify: LEDs (light emitting diodes) _are_ solid-state devices. They are made out of semiconductors (at least, standard LEDs are). Like all diodes, they have a current/voltage relation which makes them pass current very easily in one direction and very little in the other direction. Unlike non-LED diodes, they also emit light at particular frequencies when current is passing in the forward direction. They are actually analog devices: the more current you pass through them, the brighter they get. However, they are usually used with a fixed forward voltage and thus a fixed forward current. Increasing the voltage would increase the current exponentially, which is not good for all kinds of reasons. If LEDs had a nice linear voltage/current relation in the forward direction it would be relatively easy to control their intensity using changes in voltage. However, since this relationship is exponential, a very small increase in current leads to a very large increase in voltage and thus brightness. That's why most LED backlights use PWM instead of current control for dimming. With PWM, the LED is either off (no voltage, no current and no light) or on (fixed voltage, fixed current and a lot of light (the maximum ever given off by the display)). If you switch back and forth between off and on repeatedly, your eye perceives light which is in between full on and full off. The longer you leave the LED off vs on, the dimmer the apparent light. Unfortunately, unless the PWM frequency is very fast (probably in the kilohertz range), some people will see the flickering, even if only subconsciously, leading to eyestrain. Most PWM operates at much lower frequencies, typically around 220 Hz. And here we are. Current-controlled LED dimming _is_ possible, but it is tricky and probably requires much greater quality control over the LED fabrication process, which is why monitors that use this are rare and tend to be expensive.
Interestingly, most LED light bulbs also flicker, but for a completely different reason. They flicker because the power source is 120 V AC which has a frequency of 60 Hz (in North America), whereas a diode can only pass current in one direction. So if you connected an LED to a 60 Hz current source it would be on only half the time and would flicker at 60 Hz. There is an easy way to improve this called "full wave rectification" which results in the diode flickering at 120 Hz, and that's what most LED light bulbs flicker at. To get rid of flicker either requires a constant current source like DC (which isn't available in house power systems) or smoothing the full wave rectified waveform using e.g. a capacitor (this doesn't get rid of flicker but it can reduce it to negligible levels). That means extra circuitry, extra bulk, and extra price, so most manufacturers don't do it (at least, not the last time I checked). The Phillips LG bulb didn't flicker when I tested it, but I didn't like the light quality. The Cree bulbs (much cheaper) flickered horribly and I couldn't use them for more than a few minutes. A lot of car lights which use LEDs also flicker from what I've heard. And, of course, even if you have an LED bulb that doesn't flicker by itself, if you want to be able to dim it, that may introduce more flicker depending on how the dimming is done.
mvanier, I agree with everything you said based on my reading and I was trying to make the same points but I believe I wasn't completely clear in my response. LED's themselves are just light bulbs essentially however they flicker due to dimming technology or power delivery. So that being said the conclusion is that the back light flickers whether it is supposed to be solid state or not, whether this is the bigger issue or spectrum depends on your point of view but together they seem to be multiplied in their negative effects.
That being said I want to once again bring my unanswered question into this discussion, if power delivery flickers anywhere from 60-120Hz will an LED monitor plugged into your household power without a capacitor still flicker even if it is a flicker free monitor explaining why no one is having luck with "Flicker free" monitors? Do these monitors have e power capacitors built into them...?
Another interesting topic is the Phillips LED bulb you mentioned, was it the one with 3 yellow covers? I also tried this bulb. There is a restaurant close to me that used them in their chandeliers so imagine 6 of them in close quarters in each chandelier. I can tell you that environment is so weird looking to me at night, it is as if the spectrum appears like incandescent 3000K but everything appears to be moving or spotty. I wonder if the power delivery is causing them to flicker and perhaps they are not even flickering in sinc causing an overall much lower ambient Hz rate of flicker? What are your thoughts? Those Phillips bulbs are some of the worst for me, soooo amazingly bad within seconds!
Thank you for your response,
I have one additional thought on this as well and that is about heat, has everyone noticed how many of these LED household lights are now looking like alien space ships with all the fins and such to dissipate heat? I wonder what else they are doing to deal with heat and maybe it is some other form of pulse than PWM we do not know about yet? It is interesting, I spoke with a general manager at a restaurant who explained to me his LED light bulbs are not lasting any longer than incandescent light bulbs, they are burning out before the 25 year promise in less than a year!
Just my Opinion but I think this technology in general has some serious growth to do before it is working up to the calculations that have been made.
First, some qualifications. I do not have a degree in electrical engineering. However, I do work for a display company and have learned quite a few things through work.
Typically, you will notice that even when you lower the brightess of the screen all the way to "0", it does not look like a powered off display. This is because the LED is still functioning as it was at full brightness. To summarize.. the brightness level of the LED does not change.
What changes then? You were right in suspecting there is a change in voltage, but it's not to the LEDs, but rather to the liquid crystals themselves. Run voltage to the thin film transistors that run under the liquid crystals, and they will change their orientation, letting light from the LEDs pass through to the colored filter.
If there's flicker, it's most likely from the panel itself, and not from the LEDs.
I have a high speed camera (1000fps) and I can see about every 3 frames a black screen (for the displays using pwm, of course)
Also for example Eizo has published a study on this too http://www.eizo.com/global/library/basics/eyestrain/
So please don't confuse people here.
I read indeed about the procedure you described but it is for sure not used in every computer screen, maybe only your company uses...
The black screen you are seeing is the panel's refresh rate.
As I've previously iterated multiple times, the LED does not flicker.
The black screen you "see about every 3 frames" is the limitation set by the graphic card's refresh rate.
Read this article, and you'll understand what I mean: http://hardforum.com/showthread.php?t=928593
As for the screens my company uses.. well, let's put it this way: everyone on these forums uses those screens.
Ok, thank you explaining the educated position you are coming from so we can respectfully understand your knowledge. I believe you are saying Apple displays do not use PWM? We have all heard this before and that is definitely part of the question about flicker however perhaps they still flicker due to the refresh rate then that is similar to PWM?
Thank you for your response,
Interesting link there Jongmini. The other thing that is interesting is that regardless of how it is happening if there is a black screen every 3 frames essentially the LED back light is flickering at us due to FPS and refresh rate.
It's like saying the sun isnt flickering its just the fence your driving past blocking the sunlight every other fence board. It doesnt matter if its the sun or the fence, the flicker is happening to our eyes, there is an on/off light quality, correct?
Jesse, I wouldn't worry about the effect of the 60 Hz power supply on the monitor. For one thing, most of us use laptops regularly that aren't even plugged in, and we still get eyestrain symptoms from this. For another, computers all have power supplies that take in whatever power source is available and kick out DC power of whatever voltage is needed to power the system. It is not theoretically impossible for a monitor to have some interference from a 60 Hz power source but this is something the industry is well aware of and has dealt with over the course of decades.
As for people "not having had success with flicker-free monitors", I wasn't aware of that. The last report was from someone who ordered a flicker-free monitor but got one with PWM by mistake. I would very much like to see reports from people using documented flicker-free monitors e.g. the BenQ ones, the Eizo ones, and the Dell U2413 and cousins.
Jongmini, I disagree with your statement. According to what you said, the display is being dimmed by adjusting the pixel values to darker values. This is certainly possible, but it makes no sense to do it this way as you will be drastically hurting your ability to render colors accurately. And the use of PWM to dim backlights is well-established in the industry, so your statement that the LEDs do not flicker makes no sense. PWM is just that: flickering the LED on and off around 200 times a second (or more, if you're lucky). Also, the link you posted is from 2005; things have changed since then. That is not to say that there cannot be pixel-level flickering; temporal dithering is just that and we've covered that in some detail. Flicker can come from multiple sources.