HVX 24 fps or 23.976?

D

Deadzone

Active member
I was having a discussion with my coworker about frame rates. Our show shoots on HD cameras at 24fps. We get dailies on VHS tapes that have TC which reads 00 to 29. However, we set up our avid projects as 24 fps projects and I notice that after the footage is digitized certain TC numbers are skipped to account for the fact that the footage is actually 24fps instead of 30fps.

To my understanding, six frames have to be "added" to the 24fps during the downconvert to get to the 30fps that VHS runs at. But actually 23.976fps is fitting into a 29.97 fps, correct? So here are my questions.

1. Do HD cameras actually shoot 23.976 fps, or does it shoot 24fps then slowed down during the downconvert to fit 29.97? Will the HVX shoot actual 24fps or 23.976 since the footage will not have to be down converted to some other format.

2. My coworker said that film never really runs at 24fps. It runs at 23.976. Is this correct, or do people only run film at 23.976 fps to fit it into 29.97fps for the transfer to video. I always thought projected motion pictures ran at actual 24fps.

3. Why does the avid skip a TC frame number instead of adding a TC frame number when 30fps footage is digitized into a 24fps project? The VHS downconvert actually has too many frames compared to the original footage. You would think that the avid would repeat certain numbers rather than skip them.

I did a search on this site and couldn't find the answers. If somebody could point me in the right direction or provide and explanation, that would be great.
 
Film transferred to video usually has two number sequences imprinted on it: film KeyKode, and NTSC Timecode. Timecode goes 0-29, KeyKode goes 0-23.

NTSC video is 29.97 "frames" per second (which is a misnomer; it's actually 59.94 "fields" per second). Film gets transferred to video using 3:2 pulldown, which turns every two film frames into five video fields, or 4 film frames into 5 video "frames".

The reason you're seeing missing numbers in the timecode is totally unrelated. That's "drop frame" timecode, which was invented to correct for the difference between 29.97 and 30fps. Timecode is nominally based around the concept of there being 30 frames per second, but there *aren't* 30 frames per second, there's 29.97. That's .1% different. So "drop frame" timecode skips a frame or two every minute and every 10 minutes in order to keep the timecode "running time" accurate. If you use "non drop frame" timecode, every frame will be numbered individually, no numbers are skipped, but by the time you get to timecode 01:00:00:00, more than one hour will have gone by. With "drop frame" timecode, 01:00:00:00 occurs exactly one hour into the program -- it counts time accurately, but doesn't count frames individually and sequentially. Non Drop Frame (NDF) timecode numbers each frame individually and sequentially, but doesn't count time accurately.
 
That's the strange thing. The VHS has NDF Timecode (we inched through it frame by frame and saw everyone being counted). However, when I play back the same footage digitized in the Avid in a 24fps project, it starts skipping a timecode frame here and there (every fifth frame I believe). The DF issue should only come up if you cut the footage into a sequence and turn the sequence into a drop frame sequence, or if the original footage is shot in DF timecode. But the source footage timecode should not change from NDF to DF. (Weird)

I'm still wondering though if HD cams that shoot in 24fps progressive actually shoots in 23.976 so that they can transfer the footage to tape easier. And does film actually run at 23.976 or 24fps. Any ideas?
 
Deadzone said:
1. Do HD cameras actually shoot 23.976 fps, or does it shoot 24fps then slowed down during the downconvert to fit 29.97? Will the HVX shoot actual 24fps or 23.976 since the footage will not have to be down converted to some other format.

2. My coworker said that film never really runs at 24fps. It runs at 23.976. Is this correct, or do people only run film at 23.976 fps to fit it into 29.97fps for the transfer to video. I always thought projected motion pictures ran at actual 24fps.
Click to expand...

1.) The DVX/HVX will shoot only 23.976. You RARELY see true 24fps used in HD, even for projects requiring film outs. The CineAlta will allow either 23.976 or 24fps for true film emulation, useful for downconverts that exactly emulate the route a 24fps film would take in doing video dailies/offlines.

2.) Film in the theatre does indeed run at true 24fps, and motion picture cameras shooting theatrical projects generally shoot pure 24fps. When transferring 24fps film to tape, the telecine plays it at 23.976 when recording to NTSC frame rates. If you were to record double system sound with an un-locked mini-disc or DAT, and post sync, you would discover that sync will drift (if I remember right) a frame every 30 seconds because of this difference.

Now you CAN shoot film at 23.976 if you want, which would be the "better" frame rate for going to NTSC or NTSC frame rate HD., but the cinematographer that has shot pure 24fps has not commited a grave sin.

Regards,

Jim Arthurs
 
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