Analysis: DC Dimming Vs. PWM – Can You Dim AMOLED Displays ...

What is PWM?

The smartphone has become a constant companion for many people. Studies say that on average we turn on the smartphone 88 times per day and look at it for 3.25 hours per day. This means that the screen we are looking at should be as easy on our eyes and body as possible.

Unfortunately, there is a problem: On one hand, the display brightness should be set at only 30 - 40% to protect the eyes, but on the other hand, many users perceive a flickering of the screen at reduced brightness, which causes them headaches.

The reason is called Pulse Width Modulation, or PWM, and we have already discussed in a fairly detailed article what PWM means, why it is absolutely necessary for modern devices, and what problems it causes.

In brief, digital signals can only really be turned on or off, and Pulse Width Modulation is the attempt to achieve values that are in between, as it is possible with analog signals. In the case of brightness, we want to be able to adjust it to 30%, for example.

However, since this would only be possible with a relatively large and expensive analog controller, PWM is used instead. The signal is simply turned on and off very quickly, so at 30% brightness, the screen is turned off 70% of the time.

To prevent the user from perceiving this as a visible flicker, very high speeds are necessary (usually more than 200 times per second). Unfortunately, not all devices achieve such speeds, and also every person reacts differently to various PWM frequencies. So even if the flickering is not visible, it may cause headaches.

Devices with AMOLED displays are particularly affected, since the individual diodes themselves are illuminated and there is no background illumination as in LCD displays.

DC Dimming – The method to prevent flickering?

Our PWM article still says that there is currently no alternative, since analog parts for smartphones are too large and power-hungry.

However, there has been a new (but really already fairly old) term making its rounds on the Internet and press releases from manufacturers for a while: DC dimming.

The term is not new, since this technology has been used in LCD screens for years. The light source of the background illumination is simply fed less voltage and shines less brightly. This is also the reason why most LCD displays are only affected by PWM at low brightness levels, if at all.

In this regard, AMOLED displays have a strong disadvantage. If you feed less voltage to the organic diodes, not only do they limit their brightness, but their color also changes, so that there might suddenly be visible differences in the color reproduction.

Yet, manufacturers such as Xiaomi and OnePlus decided to offer DC dimming in their devices with AMOLED displays on a trial basis. We have already examined the effects of the new technology (called Flickering Protection by Xiaomi) in the Xiaomi Black Shark 2 and are now looking at it in more detail again in the OnePlus 7 Pro.