Interpreting EMSG events

[sanbornhilland]

I'm looking for clarification on how to interpret the EMSG events being sent by Shaka, specifically, the timing info.

Shaka sends an event that looks roughly like this:

const mockShakaEmsgEvent = {
      type: 'emsg',
      timestamp: 168271.6,
      detail: {
        messageData: [88, 89, 90 ....],
         eventDuration: 0,
         id: 1000300,
         presentationTimeDelta: 0,
         schemeIdUri: "urn:foo:bar:id3:2016",
         timescale: 48000
         value:"0"
      }
    };

Does the mockSkakaEmsgEvent.timestamp refer to the timestamp of the event stream event? Meaning if you take ( timestamp / timescale ) - presentationTimeDelta it will give the presentation time of the start of the event?

I think my confusion is resulting from several things. The timescale property docs say:

Provides the timescale, in ticks per second, for the time and duration fields within this box.

There is an eventDuration field on the event.detail object so I assume that the timescale applies to that property. But there is no time property on the event.detail object so it's unclear to me which time the timescale applies to. Do I need to parse the messageData or does the timescale apply to the timestamp property?

Part of my confusion also is coming from the fact that I am getting different emsg timestamps for the same sample content from the Chromecast MPL library than I am from the Shaka player.

[TheModMaker]

The timestamp field is the time the event was created, so has no use here. presentationTimeDelta represents the time (in units of timescale) that the event starts. This is relative to the segment that it is embedded in. eventDuration represents the duration (in units of timescale) that the event lasts.

So the presentation times would be:

startTime = segmentStartTime + (presentationTimeDelta / timescale);
endTime = startTime + (eventDuration / timescale);

We should update the docs to be explicit about these fields. Currently there is no way to get the start time of the segment. The reason is that the parsing of the emsg box is done as a network response filter, which doesn't have access to the segment info it came from. We should consider providing this info somehow (e.g. parsing the sidx).

Do you have a public manifest that has emsg boxes?

[sanbornhilland]

Okay thanks that makes things a lot clearer.

You can use this test stream: http://demo.yospace.com/quickplay/dash/vod.mpd