Tag: imf

HH:MM:SS:FF expressions in IMSC1

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" and ttp:frameRateMultiplier="1000 1001"
  • 2 + 22 / (30 * 1000/1001) = 2.734066… seconds if ttp:frameRate="30" and ttp: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.

Edit Rate in IMF Composition

The IMF Composition Playlist defines Edit Rate values for the Composition itself (/CompositionPlaylist/EditRate) as well as for individual Resources (//Resource/EditRate).

These values define the granularity of time-related parameters specified in the Composition Playlist, including:

  • the duration of the Composition itself since the duration of individual segments is a multiple of /CompositionPlaylist/EditRate; and
  • the EntryPoint and SourceDuration parameters used to specify the Playable Region of a Resource.

These values do not however specify the sampling rate or playback speed of the essence, which is set by metadata in the underlying Track File, e.g. the Sample Rate item of the File Descriptor for MXF Track Files.