The Big Judder Problem and the Overhyping of 24p
November 11, 2008,
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.
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