23.976 or 24p...I'm confused...

Yoyodyne wrote on 12/5/2005, 12:32 PM
Just a quick question - I think I'm a bit confused about the 23.976 or 24p thing. I'm guessing 23.976 is a pulldown thing with fields and 24p is just frames. Trying to come up with a "24p" workflow and wondering what the story is on these two - Is there a good white paper or something that explains this?

Thanks a bunch for the help.

Comments

farss wrote on 12/5/2005, 12:47 PM
23.976 is I think the correct frame rate for when pulldown is being applied. As it's so close to 24 I don't think anyone bothers to mention the difference, same goes for 30i, it's actually some number to 3 decimal places.
Ah, the joys of PAL and Simple Minds!

Bob.
GlennChan wrote on 12/5/2005, 9:51 PM
NTSC actually runs at 29.97 frames per second, or 59.94 fields per second. It's actually 30 times 1000 divided by 1001, but people round it off because that little tidbit on the end really doesn't matter (I believe this is true). The engineers came up with that frame rate so that color television would be backwards compatible with B&W television. Color television has the color subcarrier superimposed on top of the luma signal... when they had the frame rate at 30fps, they found that B&W sets picked up the color subcarrier. By changing the frame rate slightly they fixed that problem.

If you add 3:2 pulldown to 24.000fps material, you get 30.000fps. This won't play back on 29.97fps equipment.
If you add 3:2 pulldown to 23.976fps material, you get 29.97fps which does play back.
Grazie wrote on 12/5/2005, 11:27 PM
Glenn, thank you. I'm PAL, but you'ev cleared up - SIMPLY - what it means. G
Chienworks wrote on 12/6/2005, 3:33 AM
The actual frame rate is a repeating decimal, 29.97002997002997002997.... Which is almost exactly the same as 29.97. However, if your material runs at exactly 29.97 it will be off by one frame ever 9 hours, 16 minutes, 6 seconds, and 20 frames or so. True, this may not seem like a huge deal; who makes 9 hour videos anyway? The problem is that Vegas slips the first frame just a few seconds in from the beginning. So, yes, it does matter, but not much. Interestingly enough, Vegas is one of the few NLEs that gets this right.
John_Cline wrote on 12/6/2005, 6:47 AM
Yes, if you place any 29.97 material that has been generated or processed with anything other than Vegas, at exactly 2 seconds 28 frames into the clip, Vegas will repeat that frame for one frame and then continue. In other words, the frames at 2:28 and 2:29 will be identical. This is a very consistent behavior in Vegas and you might not notice it's happening unless you look closely. If you capture and work in Vegas exclusively, it will never be an issue.

However, here is a command line utility that will "conform" a 29.97 clip to 29.97002997002997002997 by simply rewriting a couple of values in the header of the .AVI file. (ie: 2997/100 to 30000/1001)

SpeedMangler.zip

John
Coursedesign wrote on 12/6/2005, 7:17 AM
23.976 or 24p has screwed up many a sound recordist (who chose the wrong setting on their time code recorders).

It's a small difference in the picture, but a big difference in the audio when you're trying to lipsync.
Yoyodyne wrote on 12/7/2005, 12:17 AM
Thanks for the info folks!

"the frames at 2:28 and 2:29 will be identical."

This has driven me nuts on a couple of occasions working with After Effects...
Chienworks wrote on 12/7/2005, 4:32 AM
It is easy enough to split the clip at that point and chop out the extra frame. Annoying yes, but still easy once you know what's going on.
John_Cline wrote on 12/7/2005, 5:52 AM
"It is easy enough to split the clip at that point and chop out the extra frame. Annoying yes, but still easy once you know what's going on."

Or just run SpeedMangler on the clip. I have it set up in my "Send To" section of Windows Explorer. I just highlight the clip, right-click on it, go to "Send To" and select "SpeedMangler." I can post the simple instructions on how to set this up if anyone is interested.

John