Based on materials from Android central
A display with a high refresh rate is a definite boon. Whether it's a monitor, TV or smartphone, a high refresh rate produces a smoother and generally more attractive picture to our eyes. This is why the sudden switch from 90Hz to 60Hz that happens on the Pixel 4 looks so abrupt and unpleasant. This is a flaw that Google should address, but it appears to have been intentional.
If you watch the Google Pixel 4 demo starting at 44:21, you'll hear Google VP of Product Sabrina Ellis mention that Google has 'added smart settings that allow the refresh rate to tweak itself based on what you do'. talking about the highly acclaimed Pixel 4 display.
What Ellis is talking about is true. Yes, everything looks bright and beautiful, the 90Hz display is excellent, and Google also introduced Ambient EQ in the Nest Hub for the Pixel that works really well. But further confirmation of Google's blunder was found by XDA Editor-in-Chief Mishaal Rahman in the phone's source code. The Pixel 4 display is set to lower its refresh rate to 60Hz when the display is below 75% brightness. But that's not all! Jeff Springer researched the subject and found that if the ambient light sensor detects bright light, the display switches to 90Hz regardless of the screen brightness.
It looks like Google's 'advanced smart settings' isn't very smart either. This whole high-tech feast cannot guarantee that the screen will always look its best. And if you dig deeper, you get the feeling that Google deliberately forced the display to change the refresh rate in this way, based on physical principles or even medical hypothesis.
The goal of any smartphone manufacturer is to offer the user a cool screen. This means it must be properly color-calibrated and covered with something that provides a large viewing angle while being fingerprint-resistant. Of course, this makes sense if the panel itself is of high quality. It is the hardware capabilities that determine how the display dims between frames. Apparently, Google is using a hardware solution that differs from other devices with screens with a refresh rate of 90 Hz from manufacturers such as OnePlus or Xiaomi. Reddit user Lojcs explains it this way:
'I think this is because the' pixels' use PWM (Pulse Width Modulation), while other phones that have OLED high refresh rate screens like OnePlus, Xiaomi and Asus use DC dimming. With DC dimming (aka anti-flicker), pixels are darkened by applying less voltage to them. '
There are good reasons for using PWM in a display, the most important of which is burn-in prevention, which was a weak point in the Pixel 2 XL. The disadvantage of the technology is that a high refresh rate won't matter at a lower brightness, because the screen is dimmed for a longer period of time. At least on paper. While biology and mathematics say our eyes won't tell the difference, users say otherwise. And user feedback is the most important thing if you want to sell something. Ironically, OnePlus solves the problem by using both PWM and DC dimming in its 90Hz displays.
But perhaps PWM has nothing to do with it. Google is well versed in biology and mathematics, and how to create safer products for your health. And here we come to the hardest part – Google can use what is called Ferry-Porter's law, which states that the critical flicker frequency increases in proportion to the brightness of the stimulus.
Flicker Critical Frequency is a medical term related to how our eyes perceive flickering lights and the health consequences of them. This is the minimum frequency of flashes of light, which is accompanied by the feeling of a continuous uniform glow. It deals with three key issues: lighting, screen refresh rate, and frame rate. There are many special resources on the Internet where you can learn more about the phenomenon, but in short, the point is that when our eyes see flickering light, we experience severe headaches. Many of us know this firsthand. And since we spend a lot of time buried in our phones, preventing this is very important.
In theory, Google is not wasting its use of PWM, adjusting refresh rate based on screen brightness and adjusting refresh rate based on ambient brightness. This makes the Pixel 4 screen burn-out and suitable for people who suffer from flicker-related headaches. But if so, someone in the company misunderstood the task. Calibrating the color and lowering the refresh rate requires 'extra smart features' that are smart enough to get the result you want with any combination of frequency and brightness. Maybe it would be worthwhile to test these functions a little longer or to abandon the idea altogether.
Or maybe the explanation of the phenomenon generally lies somewhere else. In any case, it was interesting to dig into this question.