Track Selection

Some media streams offer multiple audio or video tracks. In MPEG-DASH this is done by placing the different tracks in separate Adaptation Sets. dash.js allows the application to define an initial track at startup and switch between tracks at runtime.

Initial track selection

dash.js offers multiple ways to control the initial track selection as described below.

Initial media settings

To select an initial track prior to the start of the playback based on specific media settings use the setInitialMediaSettingsFor() function. The function takes an object as input allowing you to define initial values such as the target language. For a detailed description of this method checkout our API documentation.

Example

An example how to set the initial audio track by specifying the target language is shown below:

player.initialize(videoElement, url, true);
player.setInitialMediaSettingsFor('audio', {
    lang: 'et-ET'
});

A working example can be found in our sample section.

Custom track selection function

You can also define your own custom track selection function. This function will be called by the player to determine which track to select.

Example

 var getTrackWithLowestBitrate = function (trackArr) {
    let min = Infinity;
    let result = [];
    let tmp;

    trackArr.forEach(function (track) {
        tmp = Math.min.apply(Math, track.bitrateList.map(function (obj) {
            return obj.bandwidth;
        }));

        if (tmp < min) {
            min = tmp;
            result = [track];
        }
    });

    return result;
}

player.setCustomInitialTrackSelectionFunction(getTrackWithLowestBitrate);

A working example can be found in our sample section

Changing the default track selection logic

dash.js offers various predefined approaches to select the initial track. By default, the selectionPriority attribute from the MPD is used to determine which track to select. This logic can be disabled by adjusting the corresponding settings flag:

player.updateSettings({
    streaming: {
        ignoreSelectionPriority: true
    }
})

The default track selection mode can be changed using the selectionModeForInitialTrack setting. The following modes are supported:

Mode	Description
highestBitrate	This mode makes the player select the track with a highest bitrate.
firstTrack	This mode makes the player select the first track found in the manifest
highestEfficiency (default)	This mode makes the player select the track with the lowest bitrate per pixel average.
widestRange	This mode makes the player select the track with a widest range of bitrates.

Example

player.updateSettings({
    streaming: {
        selectionModeForInitialTrack: 'highestBitrate'
    }
})

Track selection at runtime

To switch to a different track at runtime use the setCurrentTrack(track) method. You need to provide a valid track as the input to this function. A list of all available tracks can be obtained by calling getTracksFor().

Example

const targetIndex = 1;
const availableTracks = player.getTracksFor('audio');
const targetTrack = availableTracks[targetIndex];
player.setCurrentTrack(targetTrack);