Playback scheduling via WallClock

The wallclock module of SMIL supports using ISO-8601 date/time specification as part of its event triggers.

For digital signage, wallclock is a crucial component that allows precise scheduling of media playback.

Wallclock provides an absolute and precise synchronization mechanism. While one may specify time offset values in the dur and begin attributes, such offsets are relative and errors easily accumulate to prevent using time offsets as a precise synchronization mechanism.

Due to its absolute timing, elements using wallclock events must be direct children of <par> and <excl> playlists.

Description in the table below is tied to the usage of the wallclock value in the begin attribute.

WallClock Value	Playback start description
2020-01-01	Starts on midnight of January 1, 2020, it keeps playing in the sequence from that time on.
2020-01-01T09:00:00.000	Starts at 9:00 AM of January 1, 2020, it keeps playing in the sequence from that time on.
2020-01-01+w1	Starts on the next upcoming Monday (w1 — ISO weekday, 1=Monday … 7=Sunday).
2020-01-01-w7	Starts on the last Sunday (w7) before (-) January 1, 2020 (equivalent to December 29, 2019), it keeps playing in the sequence from that time on.
R/2020-01-01T09:00/P1D	Starts at 9:00 AM and repeats (R) every day (P1D). The date gives the reference starting point and can be any date in the past.
R/2020-01-01+w3T09:00/P1D	Starts at 9:00 AM every Wednesday (+w3): combine the weekday designator with the daily repeat period (P1D) for weekly scheduling.

Supported repeat period: the only supported repetition period is P1D (daily). For weekly schedules, combine P1D with a weekday designator (+wN), as in the last row above. Other ISO-8601 periods (P1W, P2D, PT1H, …) and the bounded-repeat prefix (R6/…) are not supported — a schedule written with them will not repeat as intended and may never play. Use one wallclock-scheduled block per required window, or conditional expressions for weekday/time-based rules.

Examples with beginning and end

Play at specific time each day

The following code starts a playlist between 9 AM and 12 PM each day.

What is specific for this use case is that Wallclock() is using the date inside begin as a reference point for starting the playback. The date in the end attribute has to be the same if the playlist should be repeated (R) in the set time every day (P1D).

<par dur="indefinite">
    <seq begin="wallclock(R/2020-01-01T09:00/P1D)" end="wallclock(R/2020-01-01T12:00/P1D)">
        ...
    </seq>
</par>

A scheduled playlist MUST BE enclosed within a section. If the schedule is valid forever, set the duration of the playlist to "indefinite."

The date 2020-01-01 gives a reference to a starting point in time, and can be any date in the past. "P1D" is the ISO-8601 designation for a repeat period of 1 day.

Play at a given day of week

Play between 9 AM and 12 noon on each Wednesday morning:

<par dur="indefinite">
    <seq begin="wallclock(R/2020-01-01+w3T09:00/P1D)" end="wallclock(R/2020-01-01+w3T12:00/P1D)">
        ...
    </seq>
</par>

+w3 designates the 3rd ISO weekday, or Wednesday. Note the repeat period stays P1D — the weekday designator is what restricts playback to Wednesdays (P1W is not supported).

Play at a specific time

Play once on January 1, 2010, between 9 AM and 12 noon.

<par dur="indefinite">
    <seq begin="wallclock(2010-01-01T09:00)" end="wallclock(2010-01-01T12:00)">
        ...
    </seq>
</par>

Note that the R repeat designator is removed.