Differences in analog and digital media speed

Factual Questions
1

I tried Googling this but I can’t find anything.

Why is it that:

  1. When NTSC video is digitized the digital video is usually 29.97fps instead of the original 30?

  2. When film is digitized the digital speed is usually 23.976fps instead of 24?

  3. But when PAL is digitized it stays at 25fps?

2

  1. NTSC black and white video was exactly 30 fps. However, when they added color, they also changed it to 29.97 fps. This was due to phasing problems between the color signals and the audio carrier, which caused visible distortion to the signal on the screen. By tweaking the video rate slightly, this put the color signals out of phase with the audio carrier and greatly reduced the visible distortion. The change had nothing to do with digitizing and pre-dates digital signals by quite a bit.

  2. This is related to the NTSC fps rate. When you convert film to TV, there is automated equipment that basically converts the frames from the film rate to the TV rate. If the film is exactly at 24 fps, then the audio slowly goes out of sync with the video. By matching the fps rate to a simple ratio of the NTSC rate, films can be easily converted to TV format without the audio going out of sync.

  3. PAL was introduced after NTSC had color, and was intentionally designed to avoid some of NTSC’s issues like color and sound phasing. PAL stands for Phase Alternating Line and the way that the signals are encoded tends to naturally filter out phasing problems related to color signals.

3

Thank you very much. I appreciate the explanations.