View Single Post
Old 26th March 2008
  #17
Super Moderator
 
Remoteness's Avatar
Again, another awesome post that needed to be added to this guideline thread.
Jacob Farron's post was so cool, I had to copy it to this thread.

I trust Jacob Farron will not mind the extra bandwidth in this forum.

Thank you and remember to have fun today!


Quote:
Originally Posted by jacobfarron View Post
Let me start this by saying I have Pro Tools for video, film, and multimedia / Dialog Editing for Motion Pictures / Sound for Digital Video, and many other books. NONE have completely described why 23.98 lines up with 29.97, and the methodology behind it. And if they do, I have yet to comprehend it. I've read all the posts here, and the chapters dealing with timecode over and over and could not get it. Now I believe I do, it's all about the time reference! So, here's my entire explanation, compiled from various books and my own figurin':

Film speed is 60Hz – That is the length of one second for Film.
Film is shot at 24fps referenced to 60Hz.
24fps / 60Hz

Originally, black and white video was 30fps, referenced to 60Hz. Since it’s interlaced, each frame is composed of 2 fields, for a total of 60 fields.
30fps / 60Hz

NTSC color video was changed to 29.97fps, so there must be a new reference.
30fps / 60hz = 29.97fps / X (solve for X)
X = 59.94Hz
So, the new reference for NTSC is 59.94 HZ – This is the length of one second for NTSC, as well as the amount of fields per second.

Now, most video monitors will only display 29.97fps. To allow film to be played at this rate, first it must be referenced to video time instead of film time (NONE of my books described it in this way)
24fps / 60Hz = X / 59.94Hz (solve for X)
x = 23.976 (This is where the 23.98 number comes from)
This change in reference of time (60Hz verses 59.94Hz) is where the .1% slowdown comes from, NOT from a 2:3:2 pulldown.

Now, 23.976fps (aka 23.98fps) can be edited on a 23.98fps timeline with 23.98 timecode in Final Cut Pro, and edited in Pro Tools with a 23.98 session Time Code rate. HD video is often done this way as HD monitors can display 23.98fps, though I read film is usually edited in 29.97fps. So, film takes an additional step. After the .1% slowdown due to reference change, Film is generally pulled down via the 2:3:2 method which maps the first film frame to 2 fields of video, the next frame of film to 3 fields of video, and so on. Thereby fitting 23.98 progressive film frames on 29.97 interlaced video frames. There are duplicate fields when this happens. But, this allows Film to be displayed on a standard NTSC monitor.

Now, if an HD video camera wants the “film look”, they shoot 24P (which, unless it’s the Varicam or the like) is 23.98, and no speed adjustment is necessary before pulldown. If the camera shoots a true 24fps just like film, it must be slowed by .1% to simulate the effect of changing reference from 60hz to 59.94hz, to arrive at 23.98. This 23.98fps can either be left alone and played on HD monitors or computer monitors, or pulled down to 29.97fps, and played on any monitor – the pulldown only changes the frame count, not the speed. 29.97fps pulled from 23.98 has duplicate fields, whereas 23.98fps does not.

How audio is concerned:
When shooting HD 23.98fps, audio recorded separately (if not on the camera) is 48k at 29.97fps, because there is never a change in reference (59.94Hz). Thus, a .1% slowdown never needs to occur, even if the 23.98fps video is pulled down to 29.97fps – again, only the frame count changes.

For a film shoot, or TRUE 24fps, production audio is recorded at 48k and 30fps (60Hz reference aka pilot tone), when it’s slowed .1% it becomes 47,952Hz and it’s reference is now 59.94Hz, which matches NTSC–and will sync to both 23.98 AND 29.97 – because there is no time change between the two. Production audio can also be recorded at 48,048 and at 30fps, so that when pulled down .1% to a 59.94 reference, it plays back at 48k.


Therefore, audio can be interchanged from 23.98fps and 29.97fps video (keeping in mind audio must be slowed down if the 23.98 originated at 24fps film) because both are referenced to 59.94Hz. However, one may run into trouble having to autoassemble from production audio tapes that are in a different time code than the Pro Tools session.


So, if I've said something in error, please help me fix my understanding. If I've simply repeated what's already been said, show me where. And, if everything is wrong, Mod's just delete this post so everyone isn't as confused as I!