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Media Source API: Automatically Ensure Seamless Playback of Media Segments in Append Order

The HTML audio and video elements enable you to load, decode and play media, simply by providing a src URL:

<video src='foo.webm'></video>

That works well in simple use cases, but for techniques such as adaptive streaming, the Media Source Extensions API (MSE) provides more control. MSE enables streams to be built in JavaScript from segments of audio or video.

You can try out MSE at simpl.info/mse:

Screenshot of video played back using the MSE API

The code below is from that example.

A MediaSource represents a source of media for an audio or video element. Once a MediaSource object is instantiated and its open event has fired, SourceBuffers can be added to it. These act as buffers for media segments:

var mediaSource = new MediaSource();
video.src = window.URL.createObjectURL(mediaSource);
mediaSource.addEventListener('sourceopen', function() {
  var sourceBuffer =
      mediaSource.addSourceBuffer('video/webm; codecs="vorbis,vp8"');
  // Get video segments and append them to sourceBuffer.
}

Media segments are 'streamed' to an audio or video element by adding each segment to a SourceBuffer with appendBuffer(). In this example, video is fetched from the server then stored using the File APIs:

reader.onload = function (e) {
  sourceBuffer.appendBuffer(new Uint8Array(e.target.result));
  if (i === NUM_CHUNKS - 1) {
    mediaSource.endOfStream();
  } else {
    if (video.paused) {
      // start playing after first chunk is appended
      video.play();
    }
    readChunk(++i);
  }
};

Setting playback order

Chrome 50 adds additional support to the SourceBuffer mode attribute, allowing you to specify that media segments are played back continuously, in the order that they were appended, no matter whether the media segments initially had discontinuous timestamps.

Use the mode attribute to specify playback order for media segments. It has one of two values:

  • segments: The timestamp of each segment (which may have been modified by timestampOffset) determines playback order, no matter the order in which segments are appended.
  • sequence: The order of segments buffered in the media timeline is determined by the order in which segments are appended to the SourceBuffer.

If the media segments have timestamps parsed from byte stream data when they are appended to the SourceBuffer, the SourceBuffer's mode property will be set to segments. Otherwise mode will be set to sequence. Note that timestamps are not optional: they must be there for most stream types, and cannot be there for others: inband timestamps are innate to stream types that contain them.

Setting the mode attribute is optional. For streams that don't contain timestamps (audio/mpeg and audio/aac) mode can only be changed from segments to sequence: an error will be thrown if you try to change mode from sequence to segments. For streams that have timestamps, it is possible to switch between segments and sequence, though in practice that would probably produce behaviour that was undesirable, hard to understand or difficult to predict.

For all stream types, you can change the value from segments to sequence. This means segments will be played back in the order they were appended, and new timestamps generated accordingly:

sourceBuffer.mode = 'sequence';

Being able to set the mode value to sequence ensures continuous media playback, no matter if the media segment timestamps were discontinuous — for example, if there were problems with video muxing, or if (for whatever reason) discontinuous segments are appended. It is possible for an app to polyfill with timestampOffset to ensure continuous playback, if correct stream metadata is available, but sequence mode makes the process simpler and less error prone.

MSE apps and demos

These show MSE in action, though without SourceBuffer.mode manipulation:

Browser support

  • Chrome 50 and above by default
  • For Firefox, see MDN for details

Specification

API information