Developer Documentation

QuickTime 4 API Documentation

QuickTime Movie File Format Specification, May 1996

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Creating Video Tracks at 29.97 Frames-per-second

NTSC color video is not 30 frames-per-second (fps), but actually 29.97 fps. The previous example showed how the media time scale and the duration of the frames specify the video's frame rate. By setting the media's time scale to 2997 units per second and setting the frame durations to 100 units each, the effective rate is 29.97 fps exactly.

In this situation, it is also a good idea to set the movie time scale to 2997 in order to avoid movie edits that don't land on frame boundaries. The movie's time scale is stored in the movie header atom.

With a time scale of 2997 in the media header atom, the time-to-sample atom would contain the following data values:

Atom size

24

Atom type

'stts'

Version/Flags

0

Number of entries

1

Sample count

n

Sample duration

100
© 1997 Apple Computer, Inc.

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