Frame Rate

Frame Rate

Format Description Created by

Film 23.976 (24) fps, progressive, frame-based film cameras, progressive video cameras
True Interlaced 29.97 fps, interlaced, field-based video cameras
Telecined Film 29.97 fps, interlaced, frame-based telecining process

Frame-based (or “progressive-frame”) video was recorded to film first–an entire frame was captured to a square of film, and then scanned into a computer frame by frame when it came time to put the film onto a DVD. Progressive video can also be made by a progressive-frame video camera, but these are only just being adopted by mainstream. Frame rate is exactly 24 fps.

Interlaced video, sometimes called “true-interlaced” to make up for the bozos who have dilluted the term by incorrectly calling telecined film “interlaced,” is video which was recorded in fields. A video camera records the odd lines of video onto a tape, then it records the even lines, and continues this pattern of odd-even, odd-even. Each of these sets of lines is called a “field” and the process of weaving the odd and even fields together to make frames is called Interlacing. Presumably, there was motion from the time where the odd field was recorded to the time when even field was recorded, and so you will see interlacing lines (also called “combing” due to the comb-like appearance of the artifacts) when there is motion. When played back on a TV, this doesn’t matter since standard TV only displays interlaced content anyway. You won’t notice it, at least as far as you don’t notice the fact that all television video is interlaced. The “Framerate” is exactly 29.97 fps though it’s technically more correct to speak of a “field rate” of twice that (since there are two fields per frame).
Telecined Film

Finally, there is telecining… Movies shot on film are shot at 24 frames per second and then, at the movie theater, the film is played back at 24 frames per second from a film projector. However, once the movie is released on vhs or dvd or whatever, it must somehow be adjusted to play on a television, which, as mentioned, only plays 29.97 fps video. This conversion process is called Telecining. In this case, you will have have frame-based (though it will have been mangled by the telecining operation) video at 29.97 fps. See Telecining for more info on this process.
Regarding DVD’s

Though Interlaced video and Telecined video come from quite different sources, the TV sees no difference. As long as it is receiving 29.97 frames per second, it is happy. DVD’s may have any of the three above formats stored on them. If the video is stored as 24 fps, the DVD player telecines it before sending it out to the television. If the video has been telecined already or if it was already 29.97 when it was recorded to the dvd, it requires no telecining and is sent to the TV as is.
A few final notes on framerate… It used to be that American television was broadcast at an exact 60 fields per second (60 being convenient because the power grid also runs at 60 Hz–still does, for that matter). When color TV signals came along, both the audio and the video were slowed down a tiny bit to make room for the color information. Hence the change from 60 to 59.94 fields per second. This is why TV is 29.97 (59.94 divided by 2) frames per second rather than a nice, even 30.
This is also why you may have seen the number 23.976 floating around. To convert film to TV, not only is the dreaded telecine operation required, but a very slight slow-down is also required. 24 fps film is slowed to 23.976 fps… then it is telecined, which adds 1 extra frame for every 4, for a ratio of 1.25 to 1. 23.976 * 1.25 is exactly 29.97. Again, see the notes on telecining for more info on this process.