Why stuttering in 24p content is something we'll have to live with for a long time

[matbezlima]

First, I need to establish some things.

The lack of a consistent meaning to the terms "stutter" and "judder" across all industries is very frustating. As a ground rule, I think it is better to define "judder" as uneven frame pacing (such as a 3:2 pulldown to fit 24fps in a 60Hz refresh rate, which means that some frames will stay on screen for longer than others), and "stutter" as the issue I will actually be talking about here in sample-and-hold displays.

The definitions I am using for stutter and judder here are how RTINGS, a site of specialists in reviewing TVs, defines them. But be aware that in many contexts, it's common to see the term "judder" being used with the meaning I set here for "stutter", and vice-versa.

Now I ask you guys to please watch the video below before proceeding to read the rest of my post. It explains in great detail what is "eye-persistence motion blur" (and keep in mind that whenever I say just "blur", I mean "eye-persistence motion blur"). It explains the concept far better than I ever could, with very educative and helpful graphs, so please watch it before reading the rest of my post. The video is overall great aside from three small technical hiccups. There is one instance when he says "microsseconds" when it should be miliseconds. There is a moment when he says "smooth", thought he meant "less blurry", at 14:27. And, at 15:30, what he calls "24fps judder" is actually 24fps sample-and-hold stutter, as we have defined the terms previously.

YouTube

Now, to understand the stuttering issue, we need to keep in mind what is a sample-and-hold display, and also that all our modern monitors/TVs, such as LCD, OLED and QLED, are sample-and-hold. Such displays hold the entire frame, from top to bottom, completely static in your screen till it transitions to the next frame. The faster the TV's gray-to-gray pixel response time is, the longer the whole frame stays completely static on the screen. Gray-to-gray is how long it takes for the pixels to transition from one color to the next.

For such displays, there is a stutter-to-blur continuum, and the frame rates at which stutter transitions to blur gets higher the better the display's grey-to-grey response becomes. The stutter of any object we track on the display follows the same law of eye-persistence motion blur, as the basic principle is the same. Don't forget the stutter-to-blur continuum. With slower grey-to-grey response, it adds more blur, and it makes the frames blend more into each other, lessening stutter. Gaming monitors and high-end TVs all have very, very fast grey-to-grey response time. Hell, even the mid-tier TVs nowadays have good response times, which maximizes stutter.

If you want to really did deeper on the technical side of this, check out this Blur Busters article, about the journey to 1000Hz sample-and-hold displays.

https://blurbusters.com/blur-busters...mple-and-hold/

Such displays will be the Holy Graal of display technology. They will be able to display any frame rate perfectly. You would be able to simulate the look of a three-bladed shutter film projector. Actually, you can do this in a 144Hz display (though HDR and overall image brightness will take a HUGE hit). How? Spread the 24 frames every second across a refresh rate of 144hertz. But instead of displaying each frame for 6 refresh cycles, do it like this:

Frame 1, black frame, frame 1, black frame, frame 1, black frame.

The same will be done to frames 2, 3 4, and so on. You will see that it will fit neatly with a 120Hz refresh cycle of a sample-and-hold monitor! By doing this, you will also get the motion artefacts of such projection: as the flickering refresh rate (96Hz) is three times higher than the frame rate (24fps), you will get triple images. Remember the double image artefacts in CRTs when displaying 30FPS?

In a 1000Hz display, you could simulate a panel with an extremely slow grey-to-grey response time whenever displaying low frame rate content. By doing this, the extra blur added will make the frames blend into each other far better, dramatically reducing stutter.

Of course, you will always want fast gray-to-gray response times for high frame rate content, such as videogames. You will want as little blur as possible. And remember that the stutter of low frame rates becomes blur in high frame rates. But even if you had a sample-and-hold display with instant gray-to-gray response time, you will still see far more blur in 60FPS being displayed in it than you will in a CRT. The phospor doesn't glow for long, so CRTs have very low persistence. But you will have understood all of this already if you saw the video and read the article.

Last, but not least, I wonder if 120Hz refresh rates of modern TVs aren't already enough to simulate a slow gray-to-gray response time panel. I wonder if we really need 1000Hz for that. If 120Hz can simulate a very slow gray-to-gray response time panel, then why haven't TVs done that already rather than traditional motion interpolation, which introduces the dreaded soap opera effect?

[eChopper]

partly for this reason i have not abandoned my pioneer kuro plasma . for blu rays . motions resolution trumps form factor and apps

[oddbox83]

Thing is, the CX/GX series did a pretty good BFI mode by all accounts. They dropped it, yet it was highly praised. This puzzles me.

I don't have to live with stutter - Sony thankfully has some rather nice, relatively artefact free motion smoothing on it's lowest active setting. Just a tiny amount of motion smoothing but no SOE nicely counteracts the stutter. Something I could never do with the LG B8 as even on it's lowest setting, the smoothing artefacts annoyed me more than the stutter. I have been told LG has improved much since then though, so their "cinematic motion" may well be equal to my Sony setting.

We've been told for so long all motion smoothing is bad, we need to unlearn that a little. If only to counteract what annoys some of us a great deal, this stutter on high response rate sample and hold tech. A tiny amount of high quality smoothing is very effective on sample and hold displays like OLED. Just enough to lessen stutter, but not to make any smoothing obvious and certainly not add obvious artefacts around fast moving objects that lesser quality or set too high processing introduces.

[matbezlima]

Originally Posted by oddbox83

Thing is, the CX/GX series did a pretty good BFI mode by all accounts. They dropped it, yet it was highly praised. This puzzles me.

I don't have to live with stutter - Sony thankfully has some rather nice, relatively artefact free motion smoothing on it's lowest active setting. Just a tiny amount of motion smoothing but no SOE nicely counteracts the stutter. Something I could never do with the LG B8 as even on it's lowest setting, the smoothing artefacts annoyed me more than the stutter. I have been told LG has improved much since then though, so their "cinematic motion" may well be equal to my Sony setting.

We've been told for so long all motion smoothing is bad, we need to unlearn that a little. If only to counteract what annoys some of us a great deal, this stutter on high response rate sample and hold tech. A tiny amount of high quality smoothing is very effective on sample and hold displays like OLED. Just enough to lessen stutter, but not to make any smoothing obvious and certainly not add obvious artefacts around fast moving objects that lesser quality or set too high processing introduces.

I've heard great things about the BFI in not only those LG models, but also in the Clearness setting of Sony TVs. Sadly, they are all dropping their 120Hz BFI, which I can't understand! 60Hz BFI noticeable flickers and makes the image MUCH darker. Not to mention that current consumer TVs can't get bright enough to have BFI while not killing some of the HDR grade to some extent. As the image is not on screen all the time, the pixels need to get far brighter to fully make up for that. No consumer TV has done it so yet.

About motion smoothing, do your settings create artefacts in 12FPS content? Stop-motion and hand-drawn animation often hold the same image or drawing for two frames, if not more. That creates a horrible mess for any interpolation tool. It destroys the timing of the original animation, these tools obviously don't know the artistic intent. Even in the lowest motion smoothing settings on my TV (1 in a scale of 1 to 10, the 1 setting is so low that it pretty much doesn't make any difference in mitigating stutter), I can see weirdness popping up in hand-drawn animation, weirdness that I can't see at all in live-action.

With all of that said, I advocate for a different kind of motion smoothing: one where the TV would simulate a slow response time panel. I wonder why TV brands haven't tried that yet. Maybe it would already be possible in 120Hz TVs, but such TVs are far from the rule yet outside top models, so maybe the brands are lazy. Or maybe 120Hz is not enough to properly simulate such slow response time panels without creating its own set of weirdness.

[Sessame]

120hz BFI was dropped likely because of this.



For movie content, getting rid of persistence blur is tricky because displays would need to offer a 24hz native BFI mode but the problem with that is that the flickering would make it nearly unwatchable. If you think 60hz BFI is harsh on OLED or LCD then you have no idea how intense it can get when Hz is dropped even lower. It's why I suspect manufacturers haven't done it.

Might be worth mentioning but you could add black frames between each frame to a 24hz source and encode it with FFMPEG to play it back on a 120hz OLED display, which produces perfect black levels, to simulate what 24hz BFI would look like. (It won't work on anything but OLED, as other displays don't produce perfect blacks so the flicker doesn't remove the persistence blur)

But again keep in mind this produces isanely harsh flicker and the brightness drops like rock.

OLEDs have far harsher flicker with BFI enabled than CRT's ever had due to the way the display tech draws the frame. So while BFI is a cool trick to make sample and Hold displays achieve better motion clarity, BFI doesn't come without its issues.

[matbezlima]

When you do BFI in lower frame-rates, it's not about the stutter-to-blur continuum anymore. That is for sample-and-hold with no BFI. For BFI, or any sort of flickering, the "stuttering" will be the strobing, and the resulting double image artefact as the flickering is at a higher refresh rate than the frame rate (if the flickering refresh rate is double the frame rate, it's a double image artefact, and if the flickering refresh rate is triple the frame rate, it's a triple image artefact). The only way to eliminate such artefacts would be flickering at 24Hz, but that would be unwatchable. 30fps games in CRTs have a double image artefact.

But many people treasure the look of a three-bladed shutter projector. And you can simulate it with a 144Hz OLED display, as I explained in my OP.

In a sample-and-hold display with fast pixel response time and no BFI, the eye persistence blur of 24fps will actually look like stutter. There is a stutter-to-blur continuum as the frame rate gets higher, as explained in the Blur Busters article that I linked to in my OP.

[matbezlima]

Also, blur is not an enemy in 24fps content. All movies are shot with built-in motion blur, capturing half of all blur due to the shutter being open all the time.

While I advocate for sample-and-hold displays that simulate very slow pixel response time sample-and-hold displays, I also think we need to careful in how we should fine-tune such simulation. We don't want to risk adding blur in way that it starts looking like a movie shot with a 360 shutter. That is just as bad as soap opera effect.