IMSC1 requires the use of ttp:timeBase="media"
. As a result, time expressions of the form:
hours ":" minutes ":" seconds ":" frames
are neither SMPTE ST 12 time labels nor related to any timecode or frame rate of the related video object.
Instead they express an offset in seconds, given by the following formula (see Annex N.2 in TTML):
3600 * hours + 60 * minutes + seconds + (frames / (ttp:frameRate * ttp:frameRateMultipler))
Note that the offset depends on the value of ttp:frameRate
and ttp:frameRateMultipler
, which are not necessarily equal to the Composition Edit Rate.
For example , the time expression 00:00:02:22
corresponds to an offset of:
- 2 + 22 / (24 * 1000/1001) = 2.9175833… seconds if
ttp:frameRate="24"
andttp:frameRateMultiplier="1000 1001"
- 2 + 22 / (30 * 1000/1001) = 2.734066… seconds if
ttp:frameRate="30"
andttp:frameRateMultiplier="1000 1001"
To avoid ambiguities, one can always use offset-time, e.g. 2.9175833s
, or clock-time with fraction , e.g. 00:00:02.9175833
, neither of which use frames.