W3C home > Mailing lists > Public > public-html-media@w3.org > July 2012

Re: [MSE] Establishing the Presentation Start Timestamp

From: Aaron Colwell <acolwell@google.com>
Date: Tue, 24 Jul 2012 14:13:55 -0700
Message-ID: <CAA0c1bAR8a_pkYUxNDu8HAKuUXVa=DcPsdoHyNQXHVCN+bL8hA@mail.gmail.com>
To: Mark Watson <watsonm@netflix.com>
Cc: "<public-html-media@w3.org>" <public-html-media@w3.org>
Hi Mark,

Thanks for your comments. I too am starting to believe that we should just
have the SourceBuffer timelines start at 0 and NOT derive the presentation
start timestamp from the first segment appended. I agree that the timestamp
offset mechanism should be used to handle any content that doesn't already
start at 0. This might make things a little annoying for live streams, but
it is a one time operation at the beginning of playback to figure out what
the appropriate timestamp offset needs to be. I think the 90-99% case will
be with content that starts at 0 so we should optimize for that. If offsets
need to be applied the app needs to know about them or "discover" them by
appending a segment to a "scratch" SourceBuffer and see what
SourceBuffer.buffered reports.

I do have a question about your first append at 10 minutes example. Are you
saying that you want the HTMLMediaElement to implicitly seek to the the
start of the first media segment appended? I think it might be less
surprising if default playback start
0 and the app manually sets HTMLMedaElement.currentTime to
HTMLMediaElement.buffered.start(0) or the desired seek time if you know it.
Otherwise, I'm not sure how to make the behavior you desire fits into the
descriptions specified in the offsets into the media
of the HTML spec.


On Wed, Jul 18, 2012 at 11:55 AM, Mark Watson <watsonm@netflix.com> wrote:

>  On Jul 18, 2012, at 11:03 AM, Aaron Colwell wrote:
>  Hi Mark,
>  Comments inline...
> On Thu, Jul 12, 2012 at 2:32 PM, Mark Watson <watsonm@netflix.com> wrote:
>>   4. How close do the starting timestamps on the first media segments
>> from each SourceBuffer need to be?
>>   - In this example I've shown them to be only 30 milliseconds apart, but
>> would 0.5 seconds be acceptable? Would 2 seconds?
>>   - How much time do we allow here before we consider there to be missing
>> data and playback can't start?
>>   - What happens if the gap is too large?
>>  I think this is roughly the same question as 'what happens if I append
>> a video segment which starts X ms after the end of the last video segment' ?
>>  if X <= one frame interval, this is definitely not a 'gap' and playback
>> continues smoothly. If X > 1 second this is definitely a gap and playback
>> should stall (in the same way as it does today on a network outage).
>>  For X values in between, I am not sure: implementations have to draw a
>> line somewhere. A gap of multiple frame intervals could occur when
>> switching frame rate. You might also get a couple of frame intervals gap
>> when switching if you do wacky things with frame reordering around segment
>> boundaries.
>>  When looking at differences between audio and video, we need to be
>> tolerant of differences as much as the larger of the audio frame size and
>> the video frame interval.
>>  if the gap is too large, this element just stays in the same state.
>> Perhaps I append video from 0s and audio from 2s and this is because my
>> network requests got re-ordered and any millisecond now I am going to
>> append the 0-2s audio. Playback should start when that 0-2s is appended.
>  [acolwell] I agree. We need to come up with some spec text for this and
> then we can then debate the merits of these various magic numbers. Care to
> volunteer for this? :)
>  Ok, assign me a bug.
>>  Any insights or suggestions would be greatly appreciated.
>>  We have the same problem with push/popTimeOffset. Suppose I want your
>> media above to appear at offset 200s in both audio and video source
>> buffers. What I really want is for the audio to start at 200s and the video
>> at 200.030ms.
>>  In this case the application knows better than the media what the
>> internal media times are. I know that the video segment has all the video
>> from time 0s, even though the first frame is at 30ms. I really want to
>> provide the actual offset to be applied to the internal timestamps, rather
>> than providing the source buffer time that the next segment should start at.
>  [acolwel] One way I think we could get around this is to mandate that
> the media segments actually have a start time of 0. In WebM there is a
> Cluster timestamp and then all blocks are relative to this timestamp. If
> the Cluster timestamp is 0 and the first frame in the cluster is at 30ms
> then there is enough information for the UA to "do the right thing". I'm
> not sure if a similar mechanism exists in ISO.
>  Not really - the rather complex combination of decode times, composition
> offsets and edit lists results in a presentation timestamp for each sample
> on a global timeline (shared across all bitrates etc.). But if the
> timestamp of the first sample is X there is nothing to say, for example,
> "there are no other samples between time Y (< X) and X".
>   The application that creates the demuxed files just need to make sure
> the separate files both have the same segment start time.
>  That's not always possible because of skew caused by audio frame
> durations being different from video frame intervals.
>  I think we need to say that all source buffers share a common global
> timeline and that timestamps in the media segments must be mapped to that
> in a way that is common across source buffers. This means any offset
> applied to media internal timestamps needs to be the same across source
> buffers. It means that establishing such offsets needs to be done
> explicitly by the application or, if they are derived from timestamps in
> the media it needs to be done in a consistent way (in terms of which out of
> audio and video the time offset is taken from).
>  I think this has implications for the push/pop time offset as well. They
> should be global methods which establish a global offset based on the
> next-appended segment(s).
>  We do also need a way to handle the user starting content in the middle.
> If I have a 30 min content item and the user wants to start at minute 10
> (because of a bookmark, say) then I should be able to start appending data
> at position 10min in the source buffer timeline. The seek bar needs to show
> the playback starting at minute 10 and if the user seeks backwards this
> should be ok.
>  pushOffset isn't right for this case because the media internal
> timestamps are correct: the first segment appended really does start at
> timestamp 10min.
>  I wonder whether we should just say that the source buffer timestamp
> starts at zero and not derive a start point from the appended media. If the
> media internal timestamp corresponding to the start of the content is not
> zero you need to explicitly handle this with a pushOffset call ?
>  Applications could also just append the segments to a "scratch"
> SourceBuffer to see what the initial timestamp is and then use that
> information to compute the proper offset to apply. It's not the greatest
> solution, but it does provide a way for people to handle this if they
> aren't as careful about how they create their demuxed content.
>  Aaron
>>  Hmm - no clear answer here - I'll think about this some more.
>>  ůMark
>>  Aaron
Received on Tuesday, 24 July 2012 21:14:24 UTC

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