Time in QuickTime Movie File Tracks
Each track in a QuickTime movie file has its own playback rate and duration definition.
Usually, the rate and duration of each track are the same, or related, because the
various tracks (such as audio and video) are intended to play back in sync.
Because each track has its own independent definitions of time, you can just as easily
assign a video track a frame rate of 29.97 fps and the timecode track to be 23.98 fps.
One example of how this is useful is when you are editing film (24 fps) transferred to
NTSC video (29.97 fps).