What is PWM?
Why is Samsung AMOLED using PWM?
What is the problem with PWM?
Oscilloscope measurements
Kernel PWM comparison
PWM visualized in slow motion
Disadvantages
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- Pulse Width modulation (PWM) is a way to control display brightness.
- When you lower the brightness, the light intensity of the display is not reduced by voltage, but instead quickly turned off and on by the electronics with a fixed frequency.
- If this frequency is high enough (>4000hz), it is indistinguishable by the human eye.
Why is Samsung AMOLED using PWM?
- AMOLED can't be dimmed low enough (<40cd/m2) by just reducing voltage without noticeable image deterioration.
- This deterioration affects grayscale linearity, color temperature and shows another display issue called mura defects.
- Grayscale linearity and color temperature can be corrected to some extent by the display driver.
- But mura defects are basically small irregularities / deviations caused by the OLED production process and will become more visible with lesser voltage applied.
- PWM can prevent those issues by dimming the brightness with short on/off intervals instead of voltage dimming, thus the applied voltage can be high enough (~100cd/m2) to avoid optical deterioration and we still perceive a lower brightness.
What is the problem with PWM?
- PWM is not bad per se, it is the very low frequency of 240hz that Samsung chooses to implement.
- Some people that are very susceptible to flickering (often without knowing it), may notice eye strain, headache or even nausea (comparable with motion sickness).
- This is also the reason why almost all modern computer displays are assembled with a flickerfree backlight.
Oscilloscope measurements
- Default kernel PWM behaviour with 240hz (10% on-time and 90% off-time ratio).
- Flickerfree kernel with a more consistent amplitude (note: the 60hz refresh interval is intended).
Kernel PWM comparison
- Default kernel.

- Flickerfree kernel.

PWM visualized in slow motion
- Both videos are starting at 100% brightness -> 60% -> 20% -> 5%.
- Left video: stock kernel, right video: flickerfree kernel (the pink stripe is the refresh interval).

Disadvantages
- If the brightness is below 40cd/m2 the visibility of the above mentioned mura defects will increase.

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