23.976 fps vs. 24 fps

bakerbud9 wrote on 7/22/2006, 11:44 PM

I know that DVDA only accepts 23.976 fps MPEG-2 files and will recompress a true 24 fps file. I also know that ATSC lists both 23.976 fps and 24 fps as acceptable frame rates. But I also know that tube televisions can't really display true 24 fps, but with some 2-3 pulldown trickery they can display 23.976 fps just fine.

So perhaps my question is a bit off-topic, but: why the heck is true 24 fps even an option with ATSC? I mean, it seems no one supports true 24 fps.... except a real, old-fashioned mechanical movie porjector. But in the digital realm, true 24 fps seems to be a fairy-tale or wishful thinking. So what's the point of making it part of the standard? Where is it ever used?

Comments

Brazilian wrote on 7/24/2006, 10:21 PM

23.976 is 24fps at "video rate", for the same reason NTSC is really 29.97 and not exactly 30fps which I won't go into here.

All of the true 24p formats are more 'production formats' than transmission formats, more useful for things like digital intermediate applications (scanning film digitially, compositing/processing/etc it, and then printing it back to film).

There's nothing stopping you from shooting and doing post work at true 24p, but for almost all video distribution 59.94 or 23.98 rates are going to be used, in which case you'd have to slow down your audio in order for it to remain in sync with the video (24fps being played back slower at 23.98). This is what happens to theatrical films when put on DVD - technically speaking when you watch it at home, it's running a hair slower (and thus longer) than it was shot in real life :)

95% of the time you're going to be working with 23.98 material, not true 24p. ATSC just includes pretty much every possible resolution and framerate combination to be tollerant.

bakerbud9 wrote on 7/25/2006, 8:56 AM

I guess what I'm wondering is: since ATSC is a standard for digital broadcast and transmission, why include all the other framerates? I mean, even the new HD digital broadcasts are at 29.97 fps. Features shot on film are likewise transmitted at 23.976 fps with the 2-3 pulldown so that the transmitter and/or DVD player can instert the redundant fields "on the fly" to achive true 29.97. So the reality is that nothing has really changed from NTSC in the arena of broadcast and consumer video, I mean, there's still only "one" supported framerate: 29.97.

This isn't to say that all the other framerates aren't useful. I know they are. Like you said, Hollywood films are generally finished during the post-production phase at true 24 fps and then slowed down by .1% for release on DVD.

But that gets to the heart of my point: by the time you want to distribute your media content over the airwaves, DVD or video, then it all has to be converted ultimately to plain-old 29.97 fps. So it seems really wishful thinking for ATSC to specify all these other framerates, when broadcast and DVD will apparently never support anything other than 29.97. I mean, keep in mind that ATSC is standard for *digital broadcast,* not an encyclopedia of suggestions for "useful framerates."

I guess it makes me wonder: did the ATSC group anticipate that at some point in the future 29.97 would "go away" and digital transmissions and DVDs would actually be able to support the other framerates, as well?