What causes motion judder? Is it 1080p/60?

Film is typically shot at 24 frames per second. Any time that film is displayed at a frame rate that is not an even multiple of 24, judder will result. This is what happens when film goes through the telecine process and comes out at frame-rates of 30 or 60.

To eliminate judder, the following things are required:

1) A display that will accept 1080p/24 input from HD-DVD/Blu-Ray and that will display it at an even multiple of 24.

and/or

2) A display that will deinterlace film based content and display it at an even multiple of 24.

Below is a good description of the telecine process that Gremmy is talking about which takes film at 24p and converts it to 60i. 60i creates a 3-2 cadence (3 film frames shown followed by 2 film frames) in order to make up for the fact that 60i (30p) isn't an even multiple of the native 24p film rate. 60hz interlaced comes from the ntsc standard which we are stuck with for broadcast sources but as Gremmy points out we can avoid if a projector has a 24p or 48p inpput. A person also needs a source to support 24p or 48p output or a video processor that will take 60i and convert it to 24p or 48p like the DVDO Iscan's do.

http://www.hometheaterhifi.com/volum...e-10-2000.html

I was just thinking it would be great to have a running list of PJ's that meet these requirements and maybe how it was confirmed (judder test, test patterns, mfg specs, etc)

This feature is must for my next projector. Maybe a new thread is in order.

Can the upcoming Mits 5000 do 1 of these 2 things?

it can do both

Nice

"Film is typically shot at 24 frames per second. Any time that film is displayed at a frame rate that is not an even multiple of 24, judder will result."

Likely not terribly practical from a processing standpoint but couldn't 24hz film be frame rate converted to 120hz cleanly (5x frame rate) then frame rate converted back to 60hz. (1/2 frame rate)?

Sort of like how with digital audio you can do sampling rate conversion cleanly between 44.1kHz and say 96kHz by first oversampling to a common multiple of 44.1 and 96kz then downsampling to 96kHz.

Shawn

I don't think so. At 120hz, each frame is displayed 5 times per second. Slicing that in half (to 60hz) results in each frame being displayed 2.5 times per second, which sounds to me like another 3:2 cadence and resulting judder.

Maybe someone else has a better idea of how this could be possible, but I'm not so sure.

Correct, you must have even multiples: 24, 48, 72, 96, or 120

Having said that I would like to see displays with a 120Hz (internal) refresh rate. This would work great with 24fps film and 60Hz video. It would also have to do 100Hz for PAL.

Also to be clear, usually when people talk about judder they do mean 3:2 cadence or other problems which indeed can be pretty unpleasant.

But also remember that even with film-rate content at an even multiple of 24hz you still do have the fact that the source is 24fps which is not that high. That "judder" still is present, and you can see that sometimes on really fast pans even when you're at the theater watching film.

Oh Chris you're always such a spoil sport. As a kid at the Saturday matinee I used to wonder how they got the stage coach wheels to turn backward as it slowed down. Remember, 24fps, it's a feature not a bug!

Excellent point. 24fps is inherently a pokey technology that creates a lot of annoying blur. I had an interesting discussion with Wm about this topic and I hope he doesn't mind me saying that he has viewed some 60hz motion compensated HD material that really looks a lot better than native 24fps shown at 48hz. The only problem that I see with it is that it increases storage capacity by 2.5x. This could be the killer app for BD-50 though.

Mark: there is also a featuer on some video cards that interpolate new frames basically increasing the FPS of the source content. I forget what it's called, it's like trivision or dimension or DNM TNM or TVM or something like that. Someone will know what I'm talking about and then I'll look foolish, but....

24 fps doesn't create blur -- film creates blur. When the shutter opens while the film is moving, this creates a blur on the film exposure itself. This can actually help create somewhat natural looking motion. The weasel cam routine from the battles in Gladiator and Saving Private Ryan achieves that effect by using a very short exposure, which avoids bluring with fast camera motion.

That said, the illusion of motion created by 24 fps is very visible, even at a multiple of 24. That said, I think the eye does a better job tricking the brain into believing in the motion of a regular cadence. The 3:2 cadence is harder for the brain to believe in, and is less pleasing to the eye.

60i video has the best illusion of motion currently available (without 'lab' technologies). Unfortunately, shooting film at higher that 24 fps is very expensive, and would require industry wide changes. I believe that this was tried in the 70's and didn't go very far.

I would be very surprised if the film moved while the shutter was open. This would create a vertical-only blur. I always assumed that the film is stationary and any blur is due to exposure time of each frame (approaching 1/24 sec). Seems like this could easily obliterate any advantage of 1080p over 720p.

BTW, not that it matters for the purposes of this discussion but NTSC frame rate is actually 0.1 % less than 30 Hz (ie: 29.97 Hz) with corresponding field rate of 59.94 Hz). During telecine conversion, the film frame rate has to be slowed as well (to 23.976 Hz). Not sure if this carries over into HDTV standards.

Brent

While it has been a slower processes than with other mediums we will probably see the end of most use of film in the next decade. The unfortunate problem at this point is that Hollywood is sticking by the legacy 24fps format. They really need to move to at least 30fps in the post film era.

Interesting, thanks for the heads-up. I hadn't thought that something this sophisticated could be done in real-time. I'm sure more processing helps so it coulld be a YMMV case as far as the technology goes.


I too would be surprised if the film was moving while the shutter was open. It's been years since I last played around with an Ariflex but I thought that the shutter and film were time synchronized and locked together. The film doesn't necessarily have to stop moving for this to happen. If they weren't synhronized then increasing the fps would only add to the vertical blur and not improve it.


Good point Brent and yes the same 59.94hz and 23.98hz carries over into HDTV standards including things like the HDMI spec. Interestingly 1080p24 timing is actually spec'ed at 1080p @ 23.98hz so the reduction at 24hz doesn't apply just to film. These timings are all available in the CEA-861-D or later spec.

Mark: I decided to stop being lazy and google it, what I was thinking of was Trimension DNM, and has been around for quite some time. The HTPC forum will likely have info about it as some have tried it out and stated it was very very different some like it I guess, others didn't.

Getting a little off topic, but it's worth pointing out that at least one film director (James Cameron) appears to be trying to get studios and theaters to finally break the 24Hz barrier, and 120Hz seems a likely target.

While I applaud any effort in that direction, if/when it finally begins to become a reality, I'll be anticipating an upsurge of motion sickness in the theater.

Interesting, I did a search on the HTPC forum and you're right, it pops up everywhere. I haven't been using my HTPC much since I installed my HD-A1 so it's probably getting behind the times as far as the technology in htpc goes. At any rate, this trimension stuff is really interesting and as you mention pretty controversial. Here is also some stuff about it from Philips that includes some downloadable demos.

http://www.trimension.philips.com/in...page=home.html


Even 30hz would be a big improvement and many film cameras support this fps. At 30hz a normal cadence could be used which would improve both motion blur and telecine judder. Plus only 25% more film is consumed.

Don't get me wrong though I'm all for 120hz and I would think that the reduced blurring would result in added realism. Bring on the motion sickness bags!

I agree with Mark that 30fps would likely give a noticable increase in smootheness to the eye, and have minimal impact in the amount of film consumed.

Adding to that, it then fits inside one of the standard broadcast HD containers (60i) with minimal fuss, and eliminates 2:3 judder to boot!.

I've always wondered about film stock and processing costs. I mean for a movie with a double-digit-millions budget, is 25% more film really that big of an impact? I assume the telecine, storage, and FX costs would go up too, but still how much of the overall budget goes to such things?

Film speed will not be changed because of all the legacy equipment. No one is about to spend large amounts of capital on replacing all the film equipment with the digital change over looming. If this were the case I would also like to ses Vista Vision for constant height in film. The problem is going to get the industry to change the standards to a higher rate for digital "film". I would like to see something like the following to become the standards.


A 4K plus system with cameras and projectors
3840x2160 for 1.78 (drop 1.85)
5080x2160 for 2.35 scope (no lenses or matting)
36 to 42 bit color depth
60fps or 120fps

While 120Hz may be excessive there are a couple of good reasons to use it.

60Hz sounds to common and a consumer standard plus the most important reason 120Hz gives you backwards compatibility with 24fps film.

Crap -- that was a typo. The film is held motionless. I meant the camera moving while the film was still. The exposure of the film shows this motion as blur.

Edit: you can have registration issues between the shutter and the film frame, and that could cause issues, but that's the exception.

Is there any real advantage to the "multiple of 24" idea with a digital display? Won't the device simply repeat the frame X number of times? I understand that with a CRT this would be a problem, because you would get flicker from the slow refresh, but with digital devices the entire frame is displayed at once. What benefit would you get from 72fps over 24fps?

24 works fine, the point is it just should be a multiple of 24. If it's not an even multiple (including a multiple of 1, which is 24) then you get judder introduced.

Okay, yes, I get that. Unfortunately the NTSC/ATSC standards are set and there's nothing we can do about that. I was addressing the fact that when it comes to disc based film source material, that some people are obsessed with 72fps or 120fps. I just wanted to know if there was an advantage to it considering those specific circumstances. I appreciate your reply. Question answered.

24fps is a old Hollywood standard and is not supported by NTSC. It does have provisions in the ATSC standard but I don't think it has been used yet.

If a digital display refresh rate is at only 24Hz it will flicker the same as film (unless the pane has a super slow decay rate). So you need to up the refresh rate to at least 48Hz to avoid flicker. I can even tell when my display is in 48Hz mode by looking and focusing just off screen. I can detect flicker that I can't see in 60Hz mode.

There's no inherrent benefit to one multiple over another in terms of flicker, as long as the displays themselves are equal. If you had a "zero decay" display that could instantaneously refresh, it would look exactly the same with 24 hz film content at 24 hz as at 120, or 12000 hz...

(William, I think you're in agreement, I'm just stating htis for other folks on the thread).

24fps film flickers at 48Hz. With a bowtie shutter, each frame is flashed twice before the next frame is advanced into the gate. (A 2:2:2 cadence)

Dave