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PWM Flickering

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So I just got a new monitor and apparently it uses PWM flickering even at 100% brightness. I noticed right away it strains my eyes after a few hours of use. Is this something that takes time getting use to, or will I experience this forever using the display? I'm coming from an older samsung that was a CCFL and didn't have PWM flickering, so I wondering if it's something my eyes are adjusting to?
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post #2 of 2
    Most LCD monitors use PWM to dim the backlight, and yes, this invisible flickering can cause even more eyestrain than a CRT monitor at a decent refresh rate.  This is because the whole LCD screen is strobing on and off (usually around 200 Hz) in contrast to CRT screen, where some part of the screen always lit because the electron beam is continually scanning from the top to the bottom of the screen.  Unless you know otherwise, there is a chance that your old screen also used PWM.  The eyestrain is worse on many of the new LED backlit monitors because the LEDs are so bright and have extremely fast switching times, while by comparison, the CCFL bulbs are dim and fade in and out instead of switching abruptly on and off.

    Will you get used to it?  I can't say for sure, but the flickering would not be good for your eyes, even if, at some point, you no longer notice the irritation.  Most people also get "used to" the strong RF signals that come from wireless networking devices and cellphones and no longer notice the headaches that they initially cause.  I guess it is a matter of getting used to a decreased feeling of wellness by forgetting what "better" felt like. redface.gif

    Why is PWM used in LCD backlighting?  There are a number of reasons, including:
  1. Ghosting artifacts on moving objects and response time can be dramatically improved if the backlight is flashed, because the LEDs can be switched on after the LCD panel has "faded" to the next frame, and then switched off while the LCD panel is "fading" to the next frame.  If the backlight was on continually, this "fading" would be visible, thus increasing the LCD's "response time" specification.  LCDs that do this aggressively use very bright backlights so that they can continue using PWM even at maximum brightness.
  2. CCFL bulbs tend to glow unevenly (brighter on one side) when dimmed.  This phenomenon is also quite temperature sensitive.  However, when they are flashed on and off at full intensity, the light is even from side to side regardless of the temperature.
  3. For the digital processor in your monitor, it is easier and cheaper to output a digital PWM signal than to output a linear analog voltage.
  4. The brightness range will be very even and linear across its whole range due to the brightness being a factor of "on" time, rather than some backlight's non-linear (arbitrary) brightness at a certain power level.
  5. For dynamic or localized dimming, it is much cheaper to control individual LEDs directly from I/O pins on the monitor's processor IC using PWM, than to build a proper buck-regulator for each backlight LED being controlled.

    Unless you need this particular monitor (high refresh rate, or fast response time), I recommend that you replace it with a monitor that doesn't use PWM on the backlight.  Since I don't have a list of model numbers, and since new ones are coming out all the time, what I do is build a simple circuit that converts pulsing light to sound, and then I walk from screen to screen in the local MicroCenter store looking for screens that don't make an audible tone come from the circuit.  When I find a quiet one, I make sure that the brightness isn't at 100%.  This narrows down the choices very quickly, but it is well worth it for the comfort of the eyes during long duration computer use.


    Common causes of eye strain from LCD monitors, roughly in order of significance:
  1. PWM backlighting
    • Get a different monitor
    • If the monitor doesn't use PWM with brightness at 100%, run the monitor there and wear sunglasses.  I am not kidding.
    • Since the eyestrain from PWM is usually caused by the iris muscle twitching slightly trying to adjust to the flickering light level, try placing a bright lamp or two (CFL bulbs are best) behind the monitor, possibly to the side of it to provide a source of non-flickering light to dilute the flickering so that your irises will get a steady signal to close down.
    • If you're an electronics hobbyist with good understanding of electronics, you can open the LCD monitor and possibly disable the PWM effect by changing the backlight circuit.  I've successfully done this with several LCD monitors (all CCFL, though).
  2. Display too bright
    • Turn down the brightness.  Unfortunately, due to the idiots that sometimes end up building LCD screens, the screen may not only dim the backlight, but crunch the darker colors to black as well (sometimes terribly) when you turn the brightness control down.  You may have to use some display calibration program to restore the full range of the LCD screen.
  3. Display too close
    • Move the display back, further away from you.  1 yard (3 feet) seems to be good for me.
  4. Text too small, or screen slightly blurry
    • If you have a high resolution screen, it is possible that the text is too tiny because your screen has an above average DPI (dots per inch).  To fix this, you need to increase the DPI setting in Windows.
        —In Windows XP, right-click the desktop, click Properties, go to the Settings tab, click [Advanced], go to the General tab, and select a higher DPI number.
        —In Windows Vista, right-click the desktop, click Personalize, and click "Adjust font size (DPI)" in the left-hand pane.
        —In Windows 7, right-click the desktop, click Screen resolution, and click "Make text or other items larger or smaller."  After changing the size, click "Set custom text size (DPI)" in the left-sidebar, and make sure that "Use XP style scaling" is enabled.  Otherwise, some programs will be enlarged and blurry, making the problem worse.
    • If your monitor uses an analog signal, run the auto adjust feature.
    • If you're running Windows 7, try running the ClearType calibration tool.  By default, it tends to make text too gray; but this can be adjusted to make text easier to read.  To do that, right-click the desktop, click Screen resolution, click "Make text or other items larger or smaller", and click "Adjust ClearType text" in the left-sidebar.
    • Some video cards (again, idiots designing things) have ridiculously large inductors in series with the analog VGA output (and sometimes the analog output on the DVI port too).  This causes the screen to be blurred horizontally, making text fuzzy and hard to read.  It can also fool the eyes into thinking that they're not focused.  If you have identified this problem, and you're very adept with a soldering iron, you can remove those inductors entirely and replace them with a solder short.  I have done this successfully and to great relief with an nVidia 8400 GS video card, and I'm certain that the problem exists on other video cards as well.

    I hope this is clear enough and helpful to you! thumb.gif
 
Edited by Techie007 - 10/30/13 at 7:08am
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