What is DC Dimming and why is it important for AMOLED smartphones?

OLED panels have many advantages. They have better color accuracy and contrasts (including better blacks) and are thinner and brighter than traditional LCD panels. Therefore, it is not surprising that smartphone manufacturers are switching to OLED panels for their smartphones.

Even medium ones (for example, Mi 9 SE / Galaxy J6 Plus) started getting these relatively expensive display panels. But there is one drawback associated with OLED displays, and it relates to how the panel handles low levels of brightness.

Oneplus 6 comes with AMOLED screen

Most OLED panels use a technique called pulse width modulation (PWM) to reduce brightness. This is different from the dimming DC, which was used by liquid crystal panels to control the brightness levels. Most users will not be able to distinguish dimming from PWM from dimming on direct current, however, there is a very small part of the population affected by the previous technique.

As OLEDs reach average levels, and sometimes budget smartphones, their user base is growing rapidly. With a significant increase in AMOLED consumption in the past few years, you will often see similar comments on the Internet.

To understand what DC dimming is, we first need to learn about PWM dimming and how this can make some users uncomfortable.

How does PWM Dimming work?

PWM stands for pulse width modulation. As the name implies, in this technique you change the width of the light pulse to control the brightness of the display panel. Simply put, you play with the amount of time that the screen remains on or off. When the brightness decreases, the screen remains longer than it remains, which is perceived as lower brightness.

After reading this, you must be wondering - wait a second, will the screen turn off at low brightness? I never noticed it!

This is because the frequency of the pulses is very high (usually more than 200 Hz), so the human eye will not be able to notice individual pulses. Instead, the human eye averages the amount of light from these pulses. Thus, the perceived brightness is determined by the ratio of the on and off state of the panel.

The following diagram shows how the PWM brightness adjustment works.

With 100% brightness, you can see that the light is always on. When the brightness decreases to 50%, the panel switches between on and off at equal intervals. However, with a further decrease in brightness, the screen remains off longer than it remains. Thus, at extremely low levels of brightness, there is a significant flicker effect.

What is so bad about PWM Dimming and its flicker?

Usually, even at low brightness levels, the average human eye will not be able to see the flicker from darkening PWM. But this does not mean that it will not affect you in any way. Some people are more sensitive to this flicker. And in these people, PWM darkening can lead to eye strain, headaches and migraines with prolonged exposure. This is especially true in low light.

What is DC Dimming?

DC blackout essentially controls the brightness by varying the power supplied to the circuit. Since power = voltage x current, an increase or decrease in any of these inputs will change the power supplied to the display panel, and therefore its brightness.

Sounds pretty simple right? So why aren't manufacturers using DC dimming in your AMOLED smartphone?

Well, smartphone makers have been using DC blackout on LCDs for several years now, but only recently they started adding it to AMOLED smartphones. This is because DC dimming has a big disadvantage. At low brightness levels, colors often appear skewed on OLED panels. In OLED panels, a change in voltage can change the emitted colors.

Thus, at lower voltage levels, the perceived quality of a smartphone's display can be significantly reduced.

So what's the solution?

Smartphone manufacturers are increasingly aware of possible health problems associated with dimming of the PWM brightness. However, given that the flicker of PWM actually affects only a certain group of people, and the disadvantage of dimming DC on AMOLED panels, the latest smartphone makers prefer to provide it as an extra feature.

Oneplus has already confirmed that he tests dimming DC on their smartphones. This feature may be added in a future update. In fact, Chinese smartphone makers seem ready for this feature. Xiaomi, Vivo, OPPO and even Meizu came forward and confirmed the decrease in DC brightness on their existing or future smartphones.

Vivo iQOO Hands On: Halo Lights, virtual buttons and headphone jack highlight it!
Vivo iQOO comes with AMOLED panel and will have DC Dimming function via update

Despite the fact that blackout PWM may cause discomfort to some users, most people will not feel the difference. Therefore, it is very important to know that your sensitivity to the flicker effect decides how it will affect you.

Black Shark 2 front side
Black Shark 2 comes with DC dimming. (our practical classes here)

Therefore, if you have too much eye strain or a headache due to the fact that you caught on the smartphone screen for too long, do not blame the PWM blackout yet. Try to create good habits for smartphones, which include adjusting the display brightness to comfortable levels (instead of 100% at any time) and maintaining a decent distance between you and the screen of your smartphone.

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