It’s not possible with current technology to manufacture color-accurate OLED displays without PWM, because the color shifts with the amperage, so the only color-accurate way to regulate brightness is by varying the pulse width while keeping the same amplitude. Increasing the frequency would help those who are sensitive to it, but that would also increase power consumption.
Yeah, this is the root of the problem. The color changes with intensity, so the only way to change intensity with consistent color is to drive it at full power for a little then turn it off for a little, an rely on human persistence of vision to average that out.
But PWM is precisely how dimmers for incandescent bulbs worked too except they run on AC. Dimmer switches truncated the sine waves in much the same way that PWM chops of portions of the DC (see image) which is why the drivers of LED and CFL replacement bulbs struggle with dimmers.
Using a (variable) resistor to drop the current wastes energy as heat and doesn't make practical sense in a device like a smartphone or tablet. PWM is extremely power-efficient and has been used for both LEDs and OLEDs for as long as there's been flat panel displays.
DC and AC just refer to whether the waveform is constant or if it has non-zero frequency content. A PWM is AC, it's a square wave of varying duty cycle. It's easier and more efficient to create than a sine wave because it's basically a switch (on, off).
DC dimming that people are discussing is creating a current source of controllable but quasi-constant current. A LED is a diode (the D in LED) so it has a nearly constant voltage across it (there's a really steep knee in the V-I curve). Power is controlled by the amount of current passed through it. One way to control the current is to put a resistor in series and drive with different voltages across the pair. Higher voltage means higher current, and more voltage drop (and power loss) in the resistor. But there's other ways to do it that are more efficient, but still less efficient than PWM. Series inductor comes to mind.
It’s clear that how PWM works causes some people to have eye strain or even headache. Why LCD screen dimming doesn’t have PWM issue Unlike LED and OLED?
It used to. If people with sensitivities don't see it with more modern LCDs, which have LED backlights, it's likely because the white backlights are phosphor coated. There are no "white" LEDs, the light comes from a phosphor that's energized by a blue or UV LED. The phosphor probably does some of the averaging that OLED relies on the eye to do. LEDs turn on and off almost instantly, but if you remember the days of CRTs, the phosphors take time to release the last of their energy.
Also, since the phosphor is responsible for the color, not the LED itself, color shifts in the underlying LED are probably less of an issue so they may not hit it with such a harsh PWM waveform.
