Why Google Limits 90Hz on the Pixel 4 to High Brightnesses

Just recently, a keen Reddit user figured out that the 90Hz “Smooth Display” on the Google Pixel 4 only stays at 90Hz during high brightness levels. Many early Pixel 4 adopters were able to notice that the display did not seem to stick to 90Hz very often, and this system behavior seems to have been the reason why. Naturally, some users were outraged about the matter, and some were curious about why Google would implement one of their products’ biggest features in this peculiar way. Many people believed the reason had to do with battery life, but that didn’t really make any sense — why would 90Hz be limited to higher brightness, when battery consumption is more of an issue? While digging through the updated Android 10 source code release coinciding with Pixel 4 launch day, XDA’s Editor-in-Chief Mishaal Rahman found a commit that explains the official reason why Google enabled this behavior:

 

Google says the reason they limit 90Hz to higher brightnesses on the Pixel 4 is that screen flicker may be visible when the display and ambient brightness are low, due to hardware limitation. This hardware limitation is related to the panels’ pulse-width modulation (PWM), which is how display brightness is primarily controlled for OLED displays. Google expands that their displays have different gamma curves for different refresh rates (which is true for most displays) and that it is difficult to match the brightness between the two calibrations. Since the display brightness is controlled by PWM, which flickers the screen on-and-off in very fast succession, the different calibration curves could ever-so-slightly alter the PWM “pattern,” and that could introduce a noticeable “flicker” upon switching between 90Hz and 60Hz.

Google’s Smooth Display system keeps the display at 60Hz whenever the screen is idle or displaying animation/video that is 60fps or less, and immediately switches back to 90Hz as soon as you touch the display or an on-screen animation (like a notification) occurs, so it switches between 90Hz and 60Hz very often — a slight flicker upon every switch may indeed become annoying. This seems to only be an issue at lower brightness because that is when the PWM pulses are shorter and more prone to its flicker being noticeable. No flicker issues are present with the “Force 90Hz” option in the developer options since that does not switch between the two refresh rates.

Furthermore, Google claims that human eyes are more sensitive to the flicker in darker viewing environments, which is suggested through some studies. This is the reason why Google allows the display to switch to 90Hz at lower display brightness levels, even though flicker may be visible, because the flicker may not be as apparent in a brighter environment.

Google released a statement to The Verge that the company “constantly assess whether these parameters lead to the best overall user experience,” and that they have “previously planned updates that [they’ll] roll out in the coming weeks that include enabling 90hz in more brightness conditions.” Hopefully, Google can improve the tuning, and perhaps provide an option to completely disable their brightness-conditioned throttling so we can still enjoy some of the battery savings from their “smart” switching.

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