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Shirasaki

macrumors P6
May 16, 2015
15,709
11,007
Come to think about it, maybe LED just doesn’t have much choice in terms of dimming technology if PWM is not being used. There were rumoured DC dimming and LED can be powered by DC but that one didn’t realise for the time being.

With that being said, it is also possible that PWM issue is too minor for Apple to bother with. Maybe it’s time to plan the exit strategy.
 
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Sheepish-Lord

macrumors 68020
Oct 13, 2021
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I would be curious how many are truly affected by PWM. Additionally, there’s different thresholds people can tolerate so while I may have PWM, you may not necessarily be sensitive.

A YouTuber, Zollotech, has always claimed to suffer from PWM back in the day but over the years he literally uses every new Apple product and doesn’t complain so maybe the frequencies are fine for him.
 
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headlessmike

macrumors 65816
May 16, 2017
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Come to think about it, maybe LED just doesn’t have much choice in terms of dimming technology if PWM is not being used. There were rumoured DC dimming and LED can be powered by DC but that one didn’t realise for the time being.

With that being said, it is also possible that PWM issue is too minor for Apple to bother with. Maybe it’s time to plan the exit strategy.
PWM is DC dimming and (O)LEDs are always powered by DC power.
 
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Shirasaki

macrumors P6
May 16, 2015
15,709
11,007
PWM is DC dimming and (O)LEDs are always powered by DC power.
Not the DC dimming I am talking about (which is similar to dimming incandescent light bulb in old days). PWM utilises the duty cycle to reduce the brightness of an LED by rapidly turning on and off LED in predetermined intervals. That’s where eye strain comes from as some people’s eyes are sensitive to such rapid turning on and off.
 

headlessmike

macrumors 65816
May 16, 2017
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Not the DC dimming I am talking about (which is similar to dimming incandescent light bulb in old days). PWM utilises the duty cycle to reduce the brightness of an LED by rapidly turning on and off LED in predetermined intervals. That’s where eye strain comes from as some people’s eyes are sensitive to such rapid turning on and off.
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.
eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL2RpbW1lci1zd2l0Y2gtZGlhZ3JhbS02LmdpZiIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19
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.
 
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Shirasaki

macrumors P6
May 16, 2015
15,709
11,007
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.
eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL2RpbW1lci1zd2l0Y2gtZGlhZ3JhbS02LmdpZiIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19
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?
 
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headlessmike

macrumors 65816
May 16, 2017
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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?
There could be a multitude of reasons, but my point is that PWM is nothing new or unique to OLEDs and to say that just because a device uses PWM means that people will get headaches from it is just plain wrong.
 

Shirasaki

macrumors P6
May 16, 2015
15,709
11,007
There could be a multitude of reasons, but my point is that PWM is nothing new or unique to OLEDs and to say that just because a device uses PWM means that people will get headaches from it is just plain wrong.
To me this correlation is simple to understand: PWM causes rapid flicker. Some with sensitive eyes (for example those suffering epilepsy) can feel it and cause discomfort. The underlying cause of such sensitivity is still up to debate, but multiple people stating using LCD screens don’t cause them the same discomfort as using LED/OLED does put doubt on the PWM OLED is using.
 

klasma

macrumors 603
Jun 8, 2017
5,968
16,766
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.
 

headlessmike

macrumors 65816
May 16, 2017
1,271
2,565
To me this correlation is simple to understand: PWM causes rapid flicker. Some with sensitive eyes (for example those suffering epilepsy) can feel it and cause discomfort. The underlying cause of such sensitivity is still up to debate, but multiple people stating using LCD screens don’t cause them the same discomfort as using LED/OLED does put doubt on the PWM OLED is using.
I've read (actual) research that indicates that under controlled conditions test subjects report equal or less eye strain from OLEDs vs LCDs in blind tests with otherwise identical conditions. One thing that is known to cause eye strain is dark modes where white text is shown on a dark background, with eye fatigue increasing the higher the contrast is between light and dark. And since OLEDs work best with darker interfaces that could very well be a reason why people feel discomfort. There are standards that manufactures need to comply with when it comes to flickering light sources and OLEDs fall well into what is considered safe, e.g. for people suffering from photosensitive seizures.
 

AgeOfSpiracles

macrumors 6502
May 29, 2020
435
820
This is a small subset of people self-diagnosing themselves. The last thread here on the topic was pages and pages of people asking for studies that demonstrate that PWM is the culprit, and there were none forthcoming. I'm not saying the sufferers are making it up, but I am far from convinced that they know what's actually happening.
 

Analog Kid

macrumors G3
Mar 4, 2003
8,979
11,735
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.
eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL2RpbW1lci1zd2l0Y2gtZGlhZ3JhbS02LmdpZiIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6Mjg1fX19
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.
 

Cirillo Gherardo

macrumors member
May 9, 2024
49
67
Come to think about it, maybe LED just doesn’t have much choice in terms of dimming technology if PWM is not being used. There were rumoured DC dimming and LED can be powered by DC but that one didn’t realise for the time being.

With that being said, it is also possible that PWM issue is too minor for Apple to bother with. Maybe it’s time to plan the exit strategy.
I'd say the $176 billion in OLED iPhone sales per year suggests it's more than just possible.
 

NT1440

macrumors G5
May 18, 2008
14,723
21,358
This is a small subset of people self-diagnosing themselves. The last thread here on the topic was pages and pages of people asking for studies that demonstrate that PWM is the culprit, and there were none forthcoming. I'm not saying the sufferers are making it up, but I am far from convinced that they know what's actually happening.
I’ve come around on it since the talk started years ago. Some humans are just more sensitive to certain things than others. I myself can hear electronic buzzing more distinctly than others. Growing up my friends were always amazed when I would tell them the internet was going to drop because the tone of the buzzing the router made had changed pitched to my ears.

PWM sensitive people, however, also seem to have no interest in getting into the nitty gritty of specifics and assume all PWM is the same. To take its existence on a product as confirmation that they’ll be sensitive to it is just nonsense. There’s a huge range of frequencies that it can operate at and that seems to have an impact on whether it actually hurts their eyes. The higher ranges seem to alleviate the effect that *some* people have.
 

headlessmike

macrumors 65816
May 16, 2017
1,271
2,565
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).
It’s semantics but since the PWM signal never switches polarity in my experience it’s still referred to as DC, albeit a noisy DC.

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.
A similar smoothing effect should be achievable by adding a capacitor or inductor to the pulsed circuit. That’s effectively how switched mode regulators work after all. I would imagine that’s already the case though.
 
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DaveOP

macrumors 68000
May 29, 2011
1,580
2,331
Portland, OR
I would be curious how many are truly affected by PMW. Additionally, there’s different thresholds people can tolerate so while I may have PWM, you may not necessarily be sensitive.

A YouTuber, Zollotech, has always claimed to suffer from PMW back in the day but over the years he literally uses every new Apple product and doesn’t complain so maybe the frequencies are fine for him.
This is exactly how it was for me. PWM a few years ago really bothered and fatigued my eyes, but lately I don't notice it at all. I don't know if its high refresh rate helping or what, but I don't even have to think about it anymore.
 
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