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.

Comments

Re:Is it only the monitor?

[ZorinLynx]

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:Seizure disorder

[KitFox]

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:MOD PARENT DOWN

[Palmateer]

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

[Chirs]

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.

Re:Seizure disorder

[dentin]

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.

Re:first world problems

[AmiMoJo]

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.