Forklaring på 24p Judder

Casablanca

   Min nabo fandt den her på nettet til info.
The Big Judder Problem and the Overhyping of 24p
Evan Powell, November 11, 2008
ProjectorCentral.com

Your brand new 1080p home theater rig is finally installed and ready to go. You're all set to sit back and watch your favorite classic, Casablanca, like you've never seen it before, with stunning clarity and contrast. Your Blu-ray player outputs 1080p/24, the signal format everyone is raving about. You've got the Blu-ray edition of Casablanca. Your 1080p projector displays the Blu-ray signal in native 24 fps format--everything is as pure and pristine as it can get. 
So, you hit the play button. The Warner Bros logo splashes onto the screen. The globe rotates slowly. The map of the Mediterranean is rendered in breathtaking sharp detail. You smile in deep satisfaction with your new system. And then it happens. At two minutes and 3 seconds into the film, the camera holds the skyline for a moment, then pans slowly down to street level. You recoil in horror as the picture comes completely unhinged. It stutters and shakes like a delirious madman. The buildings are seemingly in the throes of a bizarre earthquake. It hurts to watch it. You blink repeatedly in disbelief. How could your brand new state-of-the-art 1080p projection system with pure, native 24p transmission come so dramatically unglued? 
Welcome to 24p. What you just experienced was motion judder, an extremely annoying artifact that derives from the fact that movies are filmed at 24 frames per second (fps). The 24 fps sampling rate was adopted as a de facto standard in 1926 when the budding film industry recognized they needed a sampling rate fast enough to support a coherent audio track. (The first talkie, The Jazz Singer, was released in 1927). Prior to audio-enabled movies, they were filmed at even slower speeds, in part to save film, and in part because film exposure speeds were a lot slower back then. 
The industry standard 24 fps film rate is an albatross that we've been stuck with ever since. As it turns out, it is way too slow to resolve camera panning motion cleanly. So when a movie camera pans at an unfortunate speed, you get motion judder. Sometimes you get it in spades. The sad fact is, your high resolution 1080p/24 system is simply showing you the picture as encoded on the Blu-ray disc in its authentic naked form. We just never saw it in our homes quite as naked before the advent of Blu-ray and HD DVD. 
Actually, we've never seen 24 fps film quite this naked even in a commercial movie theater since the double shuttering action of the movie theater's projection system reduces the experience of judder and flicker. You can see some judder in the movie theater, but it is not as pronounced as it is on a digital home theater projector playing Blu-ray or HD DVD at 24p. 
But wait, wait, wait, you say...... "I thought these judder problems were related to this 3:2 pulldown thing, and once we went to 24p, we'd have a clean picture." Well, a lot of people anticipated that, because all we've seen in the NTSC world until recently is 24 fps film converted to 30 fps display. That conversion from 24 fps to 30 fps (typically referred to as 3:2 pulldown) does indeed introduce a slightly different kind of judder, as well as some blur, when the camera pans. So it is perfectly natural to assume that a native signal that hasn't been compromised by this nasty 3:2 pulldown conversion process would look better. 
As it turns out, the opposite is often the case. The motion judder in native 24p can be atrocious. You can test it yourself if you have the equipment to do it. We'll assume that if you have a Blu-ray player, you are more likely to have a copy of Casino Royale than Casablanca. If you do, find a messy panning scene in Casino Royale. There are lots of them, but there's a real beauty in the 9th chapter, starting at 1 hour, 11 minutes and 13 seconds. The dealer is dealing, and the camera pans slowly around the table. 
In 24p playback, this scene is a pure, unmitigated disaster. The people seated at the table come apart at the seams, the tuxes flash and strobe, the Casino Royale logo on the card table blinks like a neon sign. Once you've replayed this travesty a few times, switch your Blu-ray player to 60p output and run it again. Yes, it is still a mess. But look at it closely ... the juddering effect is actually reduced. That is because the 3:2 pulldown is blurring and masking some of the latent motion judder in the film. There is certainly a separate conversion judder that is added to the visual stew with 3:2 pulldown, but oddly enough it works in contravention of the latent 24p judder. The net effect is that the image is a bit blurred, and the overall judder is noticeably reduced. Scenes like this do not look great in 60p, but they look worse in 24p. After all the hype over 24p (the benefits of which we eagerly anticipated as much as anyone), it must be admitted that 60p playback can, in the final analysis, be less distracting for many people. 
Having been told that 3:2 pulldown judder is the scourge of humanity, it may be shocking to hear that it is not the worst fate that can befall the home theater enthusiast. But the fact is, the antiquated 24 frames per second sampling rate is the more onerous problem. And this is no secret. Professional cinematographers are acutely aware of the limitations of 24 fps capture rates. They go to great lengths to control the camera in such a way that juddering effects are minimized, because directors don't like it any more than we do. One common technique is to put the camera on a track and move the camera at the same pace as a moving subject, so that the subject remains stationary in the frame. If the subject does not move across the screen, the subject does not judder. However, if the background is moving behind the subject, the background will indeed judder. 
If you've still got your Casino Royale disc in the Blu-ray player, you can see a good example of this. Go to chapter 9 again, and to 1 hour, 9 min, 6 seconds. Here Bond is walking through the hotel. Notice that the cinematographer places Bond in the right half of the frame, and the camera retreats as Bond approaches to keep him in a stationary position while the background pans. Bond turns to his right, and the cinematographer continues to hold him in the right half of the frame. When this scene is played in 24p, Bond remains stable and in focus while the background judders like crazy. When you play it back in 60p, the juddering effect in the background is still there, but it is reduced--it is easier to live with. 
Notice further that the background in this scene is somewhat out of focus. In situations like this, cinematographers can use larger apertures to minimize the camera's optical depth of field. By doing so, they can focus on the foreground subject and intentionally throw the background out of focus. This causes the motion judder in the background to become less apparent to the viewer. In this particular scene they did not fully accomplish the objective, but it helps. 
Therefore, there are two basic conclusions we can draw. First, motion judder is a natural byproduct of the 24 fps film rate. You will see it if you play a Blu-ray or HD DVD movie in native 24p transmission. How much of it you see will be directly related to how much moderate speed camera panning there is in the movie. Second, 3:2 pulldown conversions are a secondary source of judder. However, they tend to blur and soften the more aggressive instances of motion judder that you'd see in native 24p display. Leaving conventional wisdom aside, if you have the option to play your Blu-ray movies in either 24p or 60p, don't be surprised if you prefer the relative stability of 60p. 
So, is there any way to get rid of judder? 
Absolutely. One solution to the problem is called frame interpolation. But before we get to that, let's get clear on the root problem: the reason the picture judders when the camera pans is because the standard sampling rate of 24 frames per second is not fast enough to fully resolve the motion. If we captured movies at 60 fps and played them back at 60p, juddering artifacts would be pretty much nonexistent. 
Frame interpolation is a process by which the projector (or video processor) approximates what a film would have looked like if it had been captured at a much faster sampling rate to begin with. What it does it this: It buffers two or more sequential frames of the film, and evaluates the motion shifts between them. Then it uses this information to create interim frames that are partial steps in the motion sequence between each real frame. For example, the Panasonic AE3000's "Frame Creation" system will look at either two or three prior frames (depending on the mode you select), and create three interim frames that are each 25% incremental steps in the motion between two real frames. These are fed out at the rate of 96 Hz each. So in reality, in each 1/24 second, what you are actually seeing on the screen is one original frame from the Blu-ray disc, and three subsequent "motion adjusted" interim frames, all four of which are being displayed in sequence at 96 Hz. To put it another way, when using the AE3000's Frame Creation mode, a total of 75% of the image information on the screen is not on the Blu-ray disc at all, but rather is being created by the projector! 
The result is this: When you watch Casablanca, and you get to the point where the camera pans down from the skyline to street level, the picture is smooth, stable, and clear with no hint of motion judder. After seeing conventional 24p playback side by side with this, it is no contest. One delivers a visual nightmare, the other gives you a smooth, clear image. 
Panasonic is not the only vendor to have included frame interpolation on its home theater projectors this fall. Several others will have it also. These include the Sanyo PLV-Z3000, and several of the Epson home theater models. Each vendor has a different implementation of frame interpolation. But they all address the basic problem of film judder by creating interim frames that represent small steps in the motion sequence. One way or another, they will enable you to experience the film pretty much as if it had been captured at 60 fps to begin with. 
As a caveat, we should point out that frame interpolation is a brand new feature on home theater projectors. As such, it might introduce artifacts of its own until the wrinkles are ironed out. The only model we've seen to date with this feature is the Panasonic AE3000, and we have not seen any artifacts that could be attributed to the Frame Creation system. However, frame interpolation does increase video delay on the AE3000, and we expect it will do so on every model. So users will want to incorporate an audio delay into the system in order to resolve lip-synch issues. 
Now, before we conclude, it is important to acknowledge that many videophiles today prefer the judder of the 24 fps film experience. To some film buffs, tampering with the natural judder is a sin, because it just doesn't look like authentic film if it lacks the inherent instability of the low sampling rate. And indeed, they are right, it doesn't. Frame interpolation techniques can make the picture look too clear, too stable, to the point that it can be unnerving if you are not used to it. (For this reason, the AE3000 gives it to you as an option, and you can turn it on or off as you see fit.) 
But think about this. Back in the early days of silent films, there was no standard frame exposure rate. The cinematographer manually turned the crank on the camera and exposed film at anywhere from 12 to 24 frames per second or more. The ideal goal of the professional projectionist in a movie theater was to play back that film at the rate at which it had been exposed, in order to make motion look natural. But all too often, theater management wanted it played back faster than it had been exposed. After all, time was money, even back then. And truth be known, some of those films did in fact have better entertainment value when they were showed at modestly accelerated projection speeds. Artificially rapid motion was an aesthetic that many movie viewers came to enjoy. Sometimes, when action and comedy movies were projected at the rate they were filmed, people would complain that they were too slow. Romantic films, on the other hand, looked absurd when played back faster than the rate they were filmed. But the point is that people back then had their own ideas about what film should look like. 
Today, the 24p sampling rate defines our own aesthetic reality about what a film should look like, judder and all. However, we are in a period of rapid technological transformation. Someday, in the not too distant future, films will be made at 60 fps. The generations to follow us will experience films captured at 60 fps as the aesthetic norm. They will be accustomed to the crystal clear motion that high resolution 60 fps capture will deliver (and that frame interpolation can give us a glimpse of today). When they look back at the 20th century and early 21st century films produced in the old 24 fps format, they will think it quaint that we could have lived with the juddering limitations of our technology. That is worth bearing in mind as we think back with bemused nostalgia on the era of the silent films. 

Redigeret af SVP03 - 26-November-2008 kl. 08:19


Skift forum	Forum-tilladelser Du kan ikke oprette nye emner i dette forum Du kan ikke besvare indlæg i dette forum Du kan ikke slette dine indlæg i dette forum Du kan ikke redigere dine indlæg i dette forum Du kan ikke oprette afstemninger i dette forum Du kan ikke stemme i dette forum