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Clarifications for Time Expression Semantics

  • The following is a Change Proposal for Issue 199
  • Editor: Glenn Adams.
  • Date: January 28, 2013.


TTML defines the media time base as follows [1]:

If the time base is designated as media, then a time expression denotes a coordinate in some media object's time line, where the media object may be an external media object with which the content of a document instance is to be synchronized, or it may be the content of a document instance itself in a case where the timed text content is intended to establish an independent time line.

The media time base is related to local real time in accordance to the related media play rate and the related media real start time (i.e., the real time when the related media playback started), parameters not modeled by TTML itself. The relationship between media time (M) and local real time (R) is as follows:

R = playRate * M + realStartTime


M ∈ ℜ | 0 ≤ M < ∞ | M in seconds
playRate ∈ ℜ | −∞ < playRate < ∞ | playRate is unit-less
realStartTime ∈ ℜ | 0 ≤ realStartTime < ∞ | realStartTime in seconds, with 0 being start of epoch

Without loss of generality, we will assume playRate is 1 (one) and realStartTime is 0 for the remainder of this document, which simplifies this relationship to R = M.