Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

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Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by Chief Blur Buster » 27 Mar 2020, 20:23

Educational article, about people confused about PWM dimming versus motion blur reduction...

Properly Choosing Poisons In Pick Poison

PWM is usually bad. But sometimes some people have MORE headaches from motion blur (this was a big problem in VR until VR headsets started strobing). But this can still affect high-motion material, like scrolling / panning / turning, especially if you're eye tracking in nauseating motion blur. In this case, strobing (PWM backlight at ONE FLASH per frame only) becomes the "better of pick poison".

Did You Know? Motion Blur Reduction Strobe backlights are simply one-flash-per-refresh PWM. A "good kind of PWM".

First, to understand how PWM interacts with motion blur, see this simple diagram.

Image

That's why unsynchronized PWM is so bad:

Image

With PWM dimming, motion look serrated, creating eyestrain that is sometimes also unrelated to the PWM flicker. To some people, they are so sensitive, that it almost feels like a serrated knife that stabs the eyes. This is why PWM dimming is hated by many. However, single-strobe PWM (blur reduction PWM) has far less eyestrain than multi-strobe PWM (PWM dimming strobe).

That's why some people don't get eyestrain (or less strain) from blur reduction modes (at fps=Hz), but gets far more eyestrain from unsynchronized PWM dimming. Which is awful, and should not exist in a monitor.

Sure, we wish we could eliminate motion blur without PWM, but that currently requires higher frame rates at higher refresh rates. Sometimes even 240fps at 240Hz doesn't yet reduce enough motion blur (without strobing).

Some people still get eyestrain from both, but usually, blur-reduction PWM is far by the lesser of evil compared to multi-strobe PWM dimming. It's far worse than CRT 30fps @ 60Hz for some human eyes. Not everyone gets the same kind of eyestrain/fatigue from the same causes from the same display. Focus eyestrain. Motion sickness. Motion blur eyestrain. Brightness strain. Blue light. Color blindness. Etc. Everybody sees differently.

Finite-refresh-rate displays are inherently imperfect compromises, especially for some people sensitive to the artifacts that cannot be solved by most displays. You might be one of the 90%+ that is not bothered by this or that, but the other 10% or 1% are. That's one reason why some of us like strobe backlights modes (while others hate them). The imperfectness of displays is why Blur Busters continues to successfully exist. :D

References:
-- LCD Motion Artifacts 101
-- Blur Busters Law: The Amazing Journey To Future 1000Hz Monitors
(there's several flicker paragraphs halfway down. Please read this page carefully)
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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by Chief Blur Buster » 27 Mar 2020, 20:24

All strobe backlights, are automatically a PWM mode.
(A good kind of PWM)

PWM-free monitors still re-enable (a better kind of) PWM for motion blur reduction.

BFI is PWM.
Strobe is PWM.
ULMB is PWM.
LightBoost is PWM.
Aim Stabilizer is PWM
PureXP+ is PWM
DyAc is PWM

PWM stands for Pulse Width Modulation. Flashing a backlight on and off. A strobe backlight is also flashing. Except it's properly synchronized to the refresh rate.

It's only PWM-free in non-strobed mode. There is less motion blur with PWM because PWM dimming provides "accidental blur reduction" benefits. Unfortunately, other people get more pain from PWM dimming than from blur reduction benefits.

Open up a monitor menu on a GSYNC monitor that includes ULMB, and you'll see "ULMB Pulse Width", so you can adjust the size of the pulse to improve your MPRT. LightBoost 10% vs 100% is a Pulse Width Adjustment! So if you were a user of LightBoost 10%, you simply adjusted your LightBoost PWM setting.

These settings adjust the size of the pulse width in PWM:
  • NVIDIA "LIghtBoost" % setting
  • NVIDIA "ULMB Pulse Width" setting
  • BenQ "Strobe Duty" setting
  • ViewSonic "PureXP+ Levels" (Light|Normal|Extreme|Ultra)
  • Acer "VRB" Normal/Extreme
The industry simply moved to PWM-free for non-strobed modes, and healthy one-pulse-per-refresh PWM for strobed modes.

(They just don't market strobe as PWM. But strobe is same thing as PWM).

The industry never stopped using PWM
Because PWM is still mandatory for motion blur reduction -- at least until we can do strobeless blur reduction via ultrahigh refresh rates instead. They just switched from nasty multi-strobe PWM to good single-strobe PWM.

Image
  • PWM dimming uses top rows (360Hz or 480Hz PWM on a 120Hz monitor produces that)
  • PWM blur reduction uses bottom row (beautiful single pulse PWM during fps=Hz).
Strobing is PWM. PWM is stobing. It's the same thing, artifacts-wise. Whether be PWM dimming (120fps at 360Hz PWM dimming) or multi-strobing (e.g. 60fps at 120Hz LightBoost). Except blur reduction strobing is simply strobing synchronized to refresh rate. Or PWM synchronized to refresh rate. It's also why you want fps=Hz to sync your strobe rate to the frame rate, to maximize motion pleasure and minimize eyestrain from artifacts (such as duplicate images).

Marketing simply changed.

Good PWM is now labelled "motion blur reduction" by it various brand names (LightBoost, ULMB, DyAc, ELMB, PureXP, etc). So you see, a PWM-free monitor with motion blur reduction mode, STILL has PWM. Just PWM made optional.

Hope this helps people understand what has happend to PWM -- the best attribute of PWM (blur reduction) is kept, and made optional. Typically as easy as turning on/off a monitor setting.
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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by thatoneguy » 05 Mar 2021, 14:38

Just came across this again and I started to wonder...
As far as I know PWM works by showing duplicate frames right? And a mismatch refresh rate and framerate can be harsh on the eyes.
As I understand it the higher the PWM Refresh Rate the more duplicate frames produced and those duplicate frames create a sort of merging effect the higher the refresh rate is.
My question would be, how high of a refresh rate would you need before you wouldn't notice any duplicate frames?

Like I can imagine something like a 6khz PWM Display which would be a perfect match for 60hz multiplication(and for other odd refresh rates maybe variable PWM(something like G-Sync but for PWM instead)) and maybe that rate would be high enough to where you don't notice the duplicate frames?
In that case you wouldn't need to use strobing or motion interpolation.

Although I'm aware a lot of people are sensitive to PWM and I've heard of PWM having a bunch of artifacts but I'm not an expert on the subject.

On that subject, since PWM and Strobing are kinda the same...would a future display strobing at something like 6khz solve the same issues for old 30/60fps content?
Since we know you get double image effect at 60fps@120hz or 30fps@60hz but at say strobed 60fps@6000hz you would get 100x multiplication and at strobed 30fps@6000hz you would get 200x, so I wonder how much you would need before you start to not notice the multiplication image effect anymore? I imagine image resolution would play a role in this as well.
Or maybe perhaps you would need much higher refresh rate(like a million hertz) to the point where it's not practical at all with current tech.

I don't believe I've seen this discussed anywhere before...but theoretically(at least the way I'm thinking about it) as long as the display generates enough duplicate images to fool the human eyes/mind it should be sufficient. But maybe I'm wrong and there might be unavoidable artifacts.
Of course this still wouldn't be as good as sample-and-hold 1000fps+ for VR and other futuristic content like that but if it works the way I think it might work it could solve the issue with low framerate content.
-No more stutter due to very fast pixel response time ala OLED Sample-and-Hold at 24fps
-No persistence blur
-No interpolation needed(24fps Film remains as blurry and juddery as when shot but without any additive blur incurred by the display i.e zero persistence 24fps Film with zero noticeable multiple image effect with only the blur from the camera itself)
-No noticeable flicker

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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by MCLV » 05 Mar 2021, 16:24

It does not help if you strobe the same frame multiple times, see FAQ for Motion Blur Reduction.

However, increasing number of strobes per frame will approximate motion blur with increasing accuracy. Hence, 60 fps on 6000 Hz strobed display would look basically identical to 60 fps on 60 Hz sample and hold display.

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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by Chief Blur Buster » 05 Mar 2021, 19:55

MCLV wrote:
05 Mar 2021, 16:24
It does not help if you strobe the same frame multiple times, see FAQ for Motion Blur Reduction.

However, increasing number of strobes per frame will approximate motion blur with increasing accuracy. Hence, 60 fps on 6000 Hz strobed display would look basically identical to 60 fps on 60 Hz sample and hold display.
Correct.

Here's 360 Hz PWM at 60 Hz:

Image

Notice how it starts to resemble motion blur again? Bingo.

Now if you have enough PWM, it just fills in to a continuous blur, and you've got a blurry mess again instead of zero-blur.
TL;DR: Ultra-high-Hz PWM just becomes sample and hold to human eyes.

Image

Scientific Explanation:
The display is flipbooking through a series of static images. Your analog eyes are moving all the time. As you track moving objects, your eyes are in a different position at the end of a refresh cycle than at the beginning of a refresh cycle. The multiple repeat flashes of PWM is like a stamp that offsets bit-by-bit. Eventually there's enough stamps of PWM that it just looks like a continuous blur. The humankind invention of using a series of static images is the problem. The concept of "frame rate" is a useful, albet artificial humankind invention, because we don't have any method of doing an analog-motion framerateless display, and will always be guaranteed to have blur and/or stroboscopic effects at low frame rates.
thatoneguy wrote:
05 Mar 2021, 14:38
-No more stutter due to very fast pixel response time ala OLED Sample-and-Hold at 24fps
-No persistence blur
-No interpolation needed
Display motion blur physics doesn't work that way.

Unfortunately, ultra-high-Hz PWM replicates sample and hold of the underlying frame rate (e.g. 60fps 60Hz)

Also, there are two causes of eyestrain from PWM

1. People who are PWM-insensitive (doesn't mind flicker/strobe/PWM of any kind, even unsynchronized)
2. People who are PWM-sensitive only to framerates mismatching stroberate (aka "motion blur reduction" PWM)
3. People who are PWM-sensitive to any kind of PWM

I fall in the category of #2.

The bottom line is that some people get eyestrain from PWM artifacts, instead of eyestrain from PWM flicker.

These are essentially two separate causes of PWM strain, and different people are affected.

Image

Everybody sees differently. We respect that everybody has different vision sensitivities. Focus distance? Brightness? Display Motion Blur? Flicker? Tearing? Stutter? Some of them hurts people's eyes / create nausea / symptoms sommore than others. One person may be brightness sensitive (hate monitors that are too bright or too dim). Another may get nausea from stutter. And so on. Displays are inherently imperfect emulations of real life.

Even back in the CRT days -- some people were insanely bothered by CRT 30fps at 60Hz, while others were not at all.

Also, thanks to the Vicious Cycle Effect where higher resolutions, bigger displays, wider FOV, and faster motions amplfiy PWM artifacts. They become easier to see on 8K displays than 4K displays than 1080p displays.

The visibility of PWM artifacts increases with bigger resolutions & bigger FOV & faster motion. Where the problem of (framerate not equal stroberate) become more visible again at ever higher PWM frequencies. This is part of why I'm a big fan of retina frame rates at retina refresh rates for many use cases.

In fact, many people are bothered by PWM artifacts (#2) when the display is big enough - such as virtual reality. Headaches of display artifacts (PWM artifacts, motion blur artifacts, etc) are biggest with a massive IMAX display strapped to your eyes. So, that's how VR headsets work to minimize the percentage of nausea and headaches in the human population, their PWM frequency is matched to Hz to fix headaches from duplicate images.

From an Average Population Sample, Lesser Evil of Pick-Your-Poison Display Artifacts

VR researchers discovered people got a lot of nausea/headaches with things like display motion blur and PWM artifacts, so they (A) chose to strobe, and (B) they chose to strobe at framerate=Hz.

Strobing / blur reduction / BFI / "framerate=Hz PWM" are essentially the same thing -- synonyms from a display point of view. We don't normally equate "PWM" with "motion blur reduction" but motion blur reduction on LCD is essentially flashing a backlight, and that's the scientific definition of PWM (an ON-OFF square wave).

For people matching #2, the fix is to lower refresh rate and raise frame rates, until they converge to framerate=refreshrate=stroberate. When this happens, you get beautiful CRT motion clarity, zero blur, zero duplicates, etc. You still have flicker, but there can be a point where flicker is the lesser evil (as long as flicker frequency is high enough).

When screens are gigantic to eyes enough (like VR), it becomes a problem bigger and more visible than CRT 30fps at 60Hz.

VR researchers found that the fewest % of headaches occured with VR PWM at framerate=Hz. You do need a proper triple match to eliminate the maximum amount of VR nausea for the maximum population though: refreshrate == framerate == stroberate PWM, and the frequency of this to be beyond flicker fusion threshold (i.e. 120Hz instead of 60Hz).

It is the lesser of evil of a pick-your-poison problem of displays that can't yet perfectly match real life.

That's why if you want this for your desktop games, you must have the triple match, framerate = refreshrate = stroberate.

That's why blur busting is so difficult in many games at these non-retina frame rates. You need technologies similar to VSYNC ON, but with lower lag, to keep frame rate synchronized. You need GPU powerful enough to run framerates equalling refresh rates that are strobing high enough not to flicker. You need high-quality strobing without ghosting or crosstalk. Etc. So, 120fps, 120Hz PWM, 120Hz display refresh -- very tough to do with many games.

(BTW, this is also partially why RTSS Scanline Sync was invented as a substitute to laggy VSYNC ON -- I helped Guru3D create RTSS Scanline Sync -- which is essentially a low-lag VSYNC ON alternative.)

But what if you hate sample-and-hold blur *and* flicker?

The only way to have cake and eat it to is higher frame rates at higher refresh rates -- aka 1000fps at 1000Hz. For now, a good compromise could be 240fps at 240Hz strobing, provided you can hit the magic match framerate = refreshrate = stroberate

This is precisely why I wrote Blur Busters Law: The Amazing Journey To Future 1000 Hz Displays.

Motion blur is frame visibility time, from first visibility time to last visibility time of unique frame.

The blur can be distorted (ghosting, coronas, duplicates, rainbow artifacts, etc).

But it's still a blur/smear/artifact of a sustained long frame visibility time -- whether caused by slow MPRT, slow GtG, multiple strobes (multi strobe from PWM or low frame rate similar to CRT 30fps@60Hz strobe). This otherwise increases time between the first time a specific unique frame is visible for, to the end of the specific unique frame visibility time. The whole time length always guarantees artifacts of some form, it's as immutable as speed of light, regardless of what tricks the displays does!

You can push artifacts below human thresholds for various use cases -- like maybe 240Hz or 480Hz is enough for a smartphone while you might need 10,000Hz for a 180-degree retina VR headset. The exact threshold to retina refresh rates (displays look like real life motion) varies on a lot of variables of the Vicious Cycle Effect -- bigger displays / higher resolutions amplify Hz limitations.

Only way to emulate analog motion is analog motion, but you can get close with utlrahigh framerates at ultrahigh refresh rates (aka 1000fps+ at 1000Hz+) so that you don't have extra blur/artifacts/ghost/duplicates/whatever on a display above-and-beyond real life.
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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by thatoneguy » 05 Mar 2021, 22:56

MCLV wrote:
05 Mar 2021, 16:24

However, increasing number of strobes per frame will approximate motion blur with increasing accuracy. Hence, 60 fps on 6000 Hz strobed display would look basically identical to 60 fps on 60 Hz sample and hold display.
That doesn't make any goddamn sense though. At 60fps@6000hz you would get 100x multiple image effect. It absolutely wouldn't look the same as 60fps@60hz Sample-and-Hold just like 60fps@120hz on Strobed Display/CRT looks nothing like it.
60fps@120hz strobed...double-image effect or not looks a shit-ton smoother than full persistence 60fps@60hz.
Like are you telling me that you'll get 16.67ms of blur by strobing at an extremely high refresh rate? Sorry but I REFUSE to believe this.

I remain of the opinion that if we're ever able to strobe at Megahertz refresh rate(or higher) then framerates will become irrelevant and the "double-image effect" will be history.
The only problem with strobing at framerate multiples is this stupid effect and the whole "oh the only way you can eliminate this effect is by running refresh rate the same as your framerate" mindset has always bothered me.
Sure, right now I don't contest it, right now it's the only way to do it. But I'm convinced that won't be the case in the far future and I'm 100% certain sure that strobing at ridiculously high refresh rates is another way you can get rid of it. After all, there is only so many duplicate images you can create before the human eye can distinguish them.
I'm sure at least if you strobe at something crazy like at subpixel-level refresh rates(~6.2 Megahertz refresh rate for 1080p) it would be a complete non-issue, but it might not even need that much.

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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by MCLV » 06 Mar 2021, 04:03

Yes, you will get 16.67 ms of blur (at 60 fp)s by strobing at extremely high rate since multiple images will eventually blend into the motion blur of sample and hold display. At 60fps@6000hz this would happen for movement speeds below 6000 pixels/s since the movement between strobes would be less than one pixel.

Look at these simulations for movement speed of 32 pixels/frame:

1 flash per frame = 60fps@60Hz strobed
motion_blur1.png
motion_blur1.png (22.2 KiB) Viewed 13841 times
2 flashes per frame = 60fps@120Hz strobed
motion_blur2.png
motion_blur2.png (36.23 KiB) Viewed 13841 times
4 flashes per frame = 60fps@240Hz strobed
motion_blur3.png
motion_blur3.png (51.19 KiB) Viewed 13841 times

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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by MCLV » 06 Mar 2021, 04:08

8 flashes per frame = 60fps@480Hz strobed
motion_blur4.png
motion_blur4.png (63.77 KiB) Viewed 13840 times
16 flashes per frame = 60fps@960Hz strobed
motion_blur5.png
motion_blur5.png (72.73 KiB) Viewed 13840 times
32 pixel motion blur of 60 Hz sample and hold display = equivalent to 32 flashes per frame or 60fps@1920Hz strobed for this movement speed
motion_blur6.png
motion_blur6.png (54.74 KiB) Viewed 13840 times
As you can see, you get closer and closer to motion blur of sample and hold display if you increase strobing rate.

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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by Chief Blur Buster » 06 Mar 2021, 05:19

MCLV wrote:
06 Mar 2021, 04:08
[Whacks my point with one hammer hit]
Fantastic answer, MCLV.
I'm impressed how quick a study you are.

I'd add some minor nuances — you need slight overkill sampling to "retina it out" (subpixels, nyquist factors, motion jitter, scaling, odd divisors, etc) -- so a simple ~3x-4x oversampling of strobe will tend to retina-out PWM. However, eye tracking speed ability is also a limiting factor in ability to detect PWM. 6000 pixels/sec eye tracking is too fast for most humans on a 1080p display, and you'd need to eyetrack faster than that to detect 6000 Hz PWM separation effects on a 24" 1080p display.

Either way, you don't need a PWM oversampling factor for ultrafast motion since most humans usually lose ability to track faster than approximately 1 to 2 screenwidths per second movement -- between 2000 to 4000 pixels/sec.

So 6000 Hz PWM is already mathematically a good oversampling factor necessary to become a sample-and-hold equivalence for 24” 1080p since your eye tracked only one subpixel (0.33 pixel) at 2000 pixels/sec tracking in 1/6000sec, so you can’t see the PWM separation effect anymore for any nonsuperhuman eye-trackable motion speeds at normal arm’s length viewing distance.
thatoneguy wrote:
05 Mar 2021, 22:56
That doesn't make any goddamn sense though. At 60fps@6000hz you would get 100x multiple image effect. It absolutely wouldn't look the same as 60fps@60hz Sample-and-Hold just like 60fps@120hz on Strobed Display/CRT looks nothing like it.
An example of 100 flashes over an 8 pixel step would mean 0.08 pixel distance (just a few dozens micrometers apart) between duplicate images -- the separation distance becomes much smaller than pixels. At 6000Hz PWM, the PWM is essentially "retina-resolution'd" out to a sample-and-hold equivalence for all reasonable human eye-tracking abilities.

At 100 duplicate images, the separation distance is 1/100th the distance (of the movement step in the underlying frame rate). You would need superfast motion (6000 pixels/sec -- faster than this TestUFO -- can you eyetrack this?) to detect 6000 Hz PWM. But most motion on a screen is far less than 6000 pixels/sec.

The duplicate image is essentially retina'd-out out below human visibility noisefloor, and looks just like a continuous blur.

Please refer to MCLV's fantastic example.

The example only goes up to a mere 16 duplicate, and it's already beginning to look almost sample-and-hold motionblur.

Now, if you have superfast motion objects, like 8000 pixels/sec, then the 100 strobes could start to separate (133 pixel step between frames at 60 Hz), but 8000 pixels/sec is one screenwidth in 1/4th second on a typical 1080p 60Hz display.

_____

How High a PWM Frequency Becomes Equivalent To Non-PWM?

Higher resolution, faster motion, means higher PWM frequencies is needed before it retinas out. But, real life is practically as sharp as you can get, and humans still couldn't really tell flicker from a fluorescent tube at this frequency as seen in the chart below -- about 10 Khz.

Image

You definitely need higher PWM frequency to retina-out the PWM. The lighting industry have determined approximately ~10 KHz to be indistinguishable from steady state lighting (euivalent to sample and hold from a display POV).

From this lighting industry paper room-lighting tests of AC-powered fluorescent lights, it led to the standardization of 20 kilohertz ballasts for fluorescent lights to retina-out the PWM -- the point where rolling eyes around in a flickering room, doesn't cause the room to look stroboscopic to human vision. That said, this is from softer sinewave phosphor flicker, rather than LED squarewave...However, this would only raise detection threshold somewhat, by less than an order of magnitude.

These are actual scientific tests that also confirm my tests too, in a different context (AC/PWM-powered room lighting). People stopped seeing PWM in room lighting (it looked analog) after about 10,000 Hz. And PWM on a 24" display is harder to see than PWM in room lighting.

Sure, sure. tweak the variables and one can begin to see 1 MHz PWM. If you have a rocket sled with a tiny 1MHz flashing LED, going 1 kilometer per second (1,000,000 millimeters per second) across your face. That 1 megahertz PWM would still be barely have a dotted path with 1mm gaps if you stared stationary through the rocket path, only if you managed to be close enough to see 1MHz PWM dotted path. But in real life, you won't be able to eye-eyetrack 1 kilometer per second motionspeeds on a desktop display.

I can confirm I can see 864 Hz PWM in a display, but in tests, I could not tell 10 Khz PWM in the backlight of a 24" display. So, my visual tests are quite consistent with existing research. If you are in the same city as I am (Hamilton/Toronto) I am happy to show you.
thatoneguy wrote:
05 Mar 2021, 22:56
Like are you telling me that you'll get 16.67ms of blur by strobing at an extremely high refresh rate? Sorry but I REFUSE to believe this.
Ah, but therein lies the rub -- you can't tell me 2+2=5. ;)

Firstly, my reputation precedes me. I'm hired to train classrooms at manufacturers already about display science & physics, and I'm already cited mentioned in over 20 peer reviewed research papers including by NVIDIA too.

Secondly, may I humbly suggest, that perhaps, there may just simply be terminological misunderstandings or an accidental napkin math error (like maybe you forgot that duplicate step-distance halves for the same motion speed at double PWM frequency?). Perhaps MCLV's diagrams shall convince you? This is just simple Motion Blur Physics 101.
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Re: Strobed Motion Blur Reduction Is Same As PWM (but good kind of PWM)

Post by du57in » 16 Feb 2023, 16:03

I have a question about this (please feel free to delete or move if appropriate).

The new iPhone 14 is measured by some reviewers as having a 60Hz PWM rate with a 60Hz refresh rate. When those reviews were published, PWM sensitive users were furious that Apple went with such a low PWM refresh rate.

If I understand this correctly, this should actually be much better for PWM sensitive users as opposed to the non-synced rates of previous iPhones.

Am I missing something?

Thank you!

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