Quicktime File Format (2012-08-14).pdf

Some Useful Examples and ScenariosCreating Video Tracks at 29.97 Frames per SecondThe movie time scale is independent of the media time scale. Since you want to avoid movie edits that don’tland on frame boundaries, it is a good idea to keep the movie time scale and the media time scale the same,or the movie time scale should be an even multiple of the media time scale. The movie time scale is stored inthe movie header atom.With a time scale of 600 in the media header atom, the time-to-sample atom would contain the data valueslisted in Table 6-1 (page 350).Table 6-1 Example data values for time scale of 600FieldAtom sizeAtom typeVersion/FlagsNumber of entriesSample countSample durationValue24'stts'01n20Creating Video Tracks at 29.97 Frames per SecondNTSC color video is not 30 frames per second (fps), but actually 29.97 fps. The previous example showed howthe media time scale and the duration of the frames specify the video’s frame rate. By setting the media’s timescale to 2997 units per second and setting the frame durations to 100 units each, the effective rate is 29.97 fpsexactly.In this situation, it is also a good idea to set the movie time scale to 2997 in order to avoid movie edits thatdon’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 data valueslisted in Table 6-2 (page 350).Table 6-2 Example data values for time scale of 2997FieldAtom sizeAtom typeValue24'stts'2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.350