For Telecine & Audio
I can think of no other subject in audio post that causes more confusion than 3/2 pulldown. Here’s a conceptual explanation that should help.
The films that you see in the theatre and the films that you watch at home run at 2 different speeds. Although it’s imperceptible to the eye, it’s true. The video copy runs at a slower rate than the original film—23.976 frames per second, to be exact. Since film and video do run at these 2 different speeds, and 24fps doesn’t mathematically correspond to 29.97fps (video speed, not 30fps), you will need to duplicate some extra frames of your film when transferring it to video to compensate for the different frame rates, and slightly alter the playback speed of the film. This is the pulldown, meaning “pulling down” extra frames.
You must be aware that because your film was slowed down during its transfer to tape (telecine), your audio must be slowed down as well, or they will play back at different speeds and consequently become out of sync. If you’ve transferred your film to tape without the audio, when you digitize/assemble your audio you must pull it down either during or after digitizing/assembling before beginning to cut. The audio will be brought back up to its original rate when it’s married to your print.
If your film-to-tape transfer includes audio, and you plan to digitize your picture and sound into your Avid/FCP, then your audio has already been pulled down (slowed down) by the post house that did your telecine.
If you’ve shot on HD and not Film at 23.98 fps, then you have no audio pull-down issue to worry about as your film is already at video speed not film speed (not 24 fps).
Telecine Summary (Thanks to Phil Benson)
Film telecines (in the U.S.) should create videotapes with 29.97 fps non- drop frame timecode, (not 30 fps). This is for an NTSC videotape. Going from a 24 fps film image to a 29.97 fps videotape requires 2 steps, which happen simultaneously inside the telecine machine:
- For every second, 24 film frames become 60 video fields (3 video fields are created for every ‘odd’ film frame and 2 video fields are created for every ‘even’ film frame-hence the name 3/2 pull-down, also known as 2/3 pull-down).
- The entire system is slowed down by 1/10th of a percent (0.1%). This means that 23.96 fps corresponds exactly to 59.94 fps; basically, the videotape image runs 0.1% slower than the correct “film rate” of the original film.
When transferring a 24 fps film to 29.97fps NTSC videotape:
- The telecine process slows the film play speed by 0.1% to 23.976 fps.
- The telecine process creates extra pulldown fields to compensate for the 4:5 ratio of film to video frames.
- If audio is synced to the picture, the audio is also slowed down by 0.1%.