Parallel playlist

The SMIL parallel playlist is a list of media objects that start playback simultaneously.

Children of the parallel playlist can be specified to start at a specific "wallclock" time defined by the player's real-time clock. See the section on Wallclock for details.

Basic parallel playback

The following example will play both <seq> playlists at the same time. Each <seq> is playing in its respective region.

<par>
    <seq repeatCount="indefinite">
        <img src="pic1.jpg" dur="5s" region="main" />
        <img src="pic2.jpg" dur="5s" region="main" />
        <img src="pic3.jpg" dur="5s" region="main" />
    </seq>

    <seq repeatCount="indefinite">
        <img src="side1.jpg" dur="5s" region="side" />
        <img src="side2.jpg" dur="5s" region="side" />
        <img src="side3.jpg" dur="5s" region="side" />
    </seq>
</par>

Slide Show with Background Music

<par>

  <seq repeatCount="indefinite">
    <img src="pic1.jpg" dur="5s" />
    <img src="pic2.jpg" dur="5s" />
    <img src="pic3.jpg" dur="5s" />
  </seq>

  <audio src="music.mp3" repeatCount="indefinite" />

</par>

The parallel schedule has two children: a sequential playlist containing 3 still images, and a single audio media object. The sequential playlist and the audio object start simultaneously, achieving the effect of a slide show of 3 photos while music plays in the background. The sequential playlist is the simplest form of playlists in SMIL.

In a sequential playlist, media objects are played in the order they are listed in the SMIL playlist. One media object starts playing after the proceeding one ends. 

Source: a-smil.org