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Ask Slashdot: Does LED Backlight PWM Drive You Crazy? 532

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.
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Ask Slashdot: Does LED Backlight PWM Drive You Crazy?

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  • by Anonymous Coward on Thursday June 20, 2013 @01:43PM (#44062455)

    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.

  • by ZorinLynx ( 31751 ) on Thursday June 20, 2013 @02:07PM (#44062775) Homepage

    NEWER ballasts run in the 10-40kHz range.

    Older fixtures still use magnetic ballasts. No solid state switching circuitry. Big, heavy, tar-filled ballsts that hum and drive the lamps at a good ol' 60Hz.

    Sure, they last 10-15 years or so, and the drop-in replacements can be solid-state... But the magnetic ballasts are pervasive, manufactured by the millions, and are still sitting new-in-box in supply closets all over the world.

  • Re:No (Score:2, Informative)

    by DRJlaw ( 946416 ) on Thursday June 20, 2013 @02:11PM (#44062849)

    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.

    No, he's not. That whining noise is well known, objectively measurable, and explainable by actual physics.

    Unless IBM was imagining things. []. Or you deny the existence of eddy currents [] in what is essentially a high frequency electromagnet (see material under "stray losses").

    But, that being said, once you've blown out your hearing above 15 kHz you have no reason to believe that high frequency sounds exist -- dog whistles, ultrasound machines, animal ecolocation are all a giant conspiracy to fool you, not features of the real world.

  • Re:Seizure disorder (Score:5, Informative)

    by KitFox ( 712780 ) on Thursday June 20, 2013 @02:15PM (#44062907)

    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.

  • Re:No (Score:4, Informative)

    by X0563511 ( 793323 ) on Thursday June 20, 2013 @02:30PM (#44063161) Homepage Journal

    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.

  • Re:MOD PARENT DOWN (Score:4, Informative)

    by Palmateer ( 1533975 ) on Thursday June 20, 2013 @02:53PM (#44063431)
    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.
  • some as low as 90Hz (Score:5, Informative)

    by Chirs ( 87576 ) on Thursday June 20, 2013 @03:27PM (#44063869)

    According to this article ( LED backlights generally pulse between 90-420Hz, not in the KHz or MHz range.

  • Re:Seizure disorder (Score:5, Informative)

    by dentin ( 2175 ) on Thursday June 20, 2013 @04:14PM (#44064339) Homepage

    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.

  • by AmiMoJo ( 196126 ) * <mojo@wo[ ] ['rld' in gap]> on Thursday June 20, 2013 @04:14PM (#44064345) Homepage Journal

    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.

  • Re:Sigh (Score:4, Informative)

    by AmiMoJo ( 196126 ) * <mojo@wo[ ] ['rld' in gap]> on Thursday June 20, 2013 @04:22PM (#44064437) Homepage Journal

    No, it's real. Some monitors flicker their backlights at 60Hz, the same as the screen refresh rate, to improve motion reproduction.

    It turns out that even if the transition time of LCD pixels was zero motion would still look a bit blurred compared to CRT. The slight flicker that CRTs have actually makes objects in motion look sharper and easier for the eye to pick out. Some LCD monitors flicker their backlights to mimic this effect.

    IIRC the first manufacturer to do it was Benq with a CCFL backlight. In fact I have the non-flicker version of that monitor, the FPW-24Z or something like that.

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