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Comment by _2d30 | Hacker News Reader

I’m not familiar with this. I have a X and haven’t noticed any flickering of the screen and really love it generally. Can you expand on this?

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The OLED display on the X uses pulse-width modulation, which is basically where the OLED panel flickers at different frequencies to adjust its brightness. Most people (like me) don't really notice it, but for some it can cause headaches over prolonged use, and in some extreme cases really bad migraines.

Ah interesting. I definitely don’t notice it at all.

Nit pick. Pulse Width Modulation holds the frequency steady and varies the duty cycle, that is the ratio of on time to off time.

See here for more info: https://en.wikipedia.org/wiki/Pulse-width_modulation

Does this mean that the R, G and B values of any pixel is either 0 or 1 at any given instant? And how is it different from FRC: https://en.wikipedia.org/wiki/Frame_rate_control

Basically yes. But the frequency used for PWM is usually _way_ higher than what should be realistically visible to the human eye - If it was me, I'd probably use a couple of kilohertz.

Not sure how specifically it's done for OLED though.

Is PWM for varying the brightness of each pixel separately, or for adjusting the brightness of the entire screen? How are these two implemented generally on modern screens? Are PWM and FRC complementary or does one supersede the other? You can tell me about LCDs since you know them better.

At least on my Samsung Galaxy S3, turning the brightness to 100% stops the PWM flicker. If the current to each individual sub-pixel is also PWMed, it's done at a much higher frequency than the global brightness control PWM frequency.

I thought I was imagining it until I saw your comment. Interesting that some see it, while others don’t though.

I found it noticeable at first, it left my eyes feeling slightly tired and “scratchy” and the flicker was particularly noticeable in dark environments with low brightness (which probably makes sense, I guess the “off” part of the duty cycle is longer at lower brightness). I don’t notice it at all now, though I can still spot it if I look for it in a dark environment.

Can you cite any evidence that PWM results in either poorer image quality or a poorer experience?

I don't believe it exists, and I place this in the same category of people who think that wifi makes them sick.

Image quality is improved by PWM brightness, because the alternative is running at 100% duty cycle and changing brightness in software, which causes color banding because you're throwing away bit depth. Additionally, even slow (200Hz approx) PWM is well above the flicker fusion threshold for all humans, so no flicker is visible when your eyes are still.

The problem is phantom array effect artifacts when you move your eyes. These appear and disappear in response to eye movement, so they can be perceived as motion, which is distracting to some people. I personally find PWM very obvious and annoying, but I suspect I have unusually weak saccadic masking.

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