timeValue, timeScale and SMPTE – III

Joe van Tunen’s explanation

3) Glenn said “a QuickTime movie doesn’t need to have a consistent frame
rate”. This means that the timeValue does not need to increment by the same
amount all the time (timeValue increment / timeScale is the duration of the
frame). Even for movies that are supposed to have a consistent frame rate,
there could be times when the timeValue increment is not consistent (dropped
frames or rounding error or latency during movie capture). For example, an
NTSC movie with a timescale of 30000 should have frames at exact timeValues
0, 1001, 2002, 3003, etc. A dropped frame would be represented as an
increment of 2002. If the timeScale were less than 30000 then the timeValue
could not be incremented exactly. For example, if the timeScale were 2997
then the timeValue increment should be 99.9999 but will appear in the
QuickTime movie as 99 or 100. Latency could increase variability.


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