[Nexus 9 Q&A, Help & Troubleshooting] PhilipL:
Hi

The flickering issue is a known problem in the industry with these types of display. Backlights are controlled by PWM (pulse width modulation), where the LEDs are very quickly turned on and off thousands of times a second. We can't see this on and off happening, it's too fast, we just perceived the LEDs as being dimmer or brighter. The dimmer the display the more the LEDs are off compared to on in a second.

Over the top of this switching is a grid of pixels that also refresh and flicker independently, again to fast for us to notice, but sometimes the frequency of changes on the LCD panel line up in such a way with the frequency of the backlights that cause us to see a brief change in brightness.

The best example many times slower of what is happening is something we've probably all seen, a blinking LED that we look at or see from the corner of our eye that appears to miss out a flash. This is because at the moment the LED flashes we blinked and missed the flash, we can with a bit of concentration deliberately blink at the right time to make it seem like the LED is constantly off. If the off blink period is very quick with a longer on period, we could do the opposite and make it seem like the LED is constantly on by blinking during it's off period. This is what is happening on these panels, for a brief period things line up so that the panel is flickering opposite to the backlight and it appears to change in brightness or flicker.

This is why the problem is often worse at lower brightnesses, because the off period of the backlight is much longer, and we are in a dimmer environment so any flicker is immediately more apparent.

As components vary in tolerances different devices might exhibit the problem at different brightnesses or maybe not much at all, or one person seems to tune into this brief flicker where for another person they don't notice it.

The HTC One X had large complaints of LCD flicker, this was partly resolved by a firmware update. The update simply applied different algorithms to the pulse width modulation to dim the backlight at different frequencies to minimise the effects, probably pushing the problem to a different brightness setting where it was not so noticable. Indeed you can read reports of people that had no flickering problems before, only to start to suffer it after the fix was applied.

Mobile devices like tablets are difficult to eliminate these problems completely because they need to support a big range of brightness and are starred at intently. They have very bright LEDs for outdoor use, but these then need a long duty cycle of being off in order to make them run at a much lower brightness. Also the LCD display with it's natural flicker which interferes with the backlight will see that frequency of that flicker change as it ages, when the device is hot or cold, or when the battery voltage is high or low. Adaptive brightness adds another set of variables to the mix, as does a fast CPU that will cause very fast voltage fluctuations which can then contribute to the problem.

If you look at the display while it flickers from an acute angle, which means you are looking past the LCD display into the backlight directly, hence the washed out colours (not so easy on an IPS display with it's wide viewing angles) you will likely see no flickering, with the flickering returning only when you are looking directly into the screen and so looking through the LCD panel against the back light. If you could take the LCD panel and hold up against a window while it was flickering, so removing the backlight from the equation, again you would see no flicker, the problem is when they come together.

So it isn't a fault, it doesn't indicate that the back light is about to fail, and may be reduced or eliminated by a firmware update in the future. An exchange might be lucky for you, might be worse, or exactly the same. When other people say they have a perfect display with no flicker, it may just be because they have different settings applied or just don't notice it.

Regards

Phil