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Re: MSE: Ad-insertion and seeking

From: David LaPalomento <dlapalomento@brightcove.com>
Date: Wed, 26 Aug 2015 10:34:42 -0400
Message-ID: <CACh87oeTEXfkBQibLEGdbjN9QYW=hXHXRWr4df7F6y5iukfVTQ@mail.gmail.com>
To: Matt Wolenetz <wolenetz@google.com>
Cc: public-html-media@w3.org
Hi Matt,
Thanks for the response! "sequence" mode does sound like it could make
discontinuity handling less of a pain. Just to make sure I follow your
suggestion, tell me if this sounds right to you:

1) video.currentTime is set to a value that causes playback to cross over a
known timestamp discontinuity.
2) The application selects a value for sourceBuffer.timestampOffset to
adjust the target media's timestamps to account for the discontinuity and
allow playback to begin.

This works today in my testing so far. My problem happens a little bit
further on from this, though. Since I'm dealing with somewhat unreliable
third-party content, I can't be confident of their duration without
downloading and inspecting them. That leaves me with the tough choice of
taking a guess on the appropriate timestampOffset in step 2) and risk
overlapping content if the user seeks back and plays through the content
again; or downloading all intervening segments before the seek can complete
and forcing the user to sit through a painful amount of buffering.

Basically, ad insertion requires dealing with third-party content and (in
my experience, at least) you can't rely on those parties for accurate
duration information to set timestampOffset. Does that make sense? Did I
miss something from your advice?

On Tue, Aug 25, 2015 at 5:43 PM, Matt Wolenetz <wolenetz@google.com> wrote:

> Hi David,
>
> I am one of the current co-editors of the MSE spec. Thanks for your
> question.
>
> This appears to be a problem that "sequence" appendMode may alleviate: it
> collapses all discontinuities into one continuous buffered region, so long
> as there are no other intervening operations like explicitly changing
> timestampOffset or appendMode. This "sequence" appendMode is in
> experimental support currently in Chrome M46 (it's hidden behind an
> experimental flag). The caveat for using sequence appendMode (beyond, of
> course, having implementations in user agents) is that the appends must be
> done in the order they are desired on the timeline, not in a scattered
> fashion.
>
> While implementations are pending "sequence" appendMode support,
> explicitly updating timestampOffset to collapse potentential
> discontinuities would be feasible if you:
> 1) know the current SourceBuffer.buffered() range(s) end time(s): this is
> available in MSE.
> 2) know the start timestamp of media about to be appended (by inspection
> offline, or even in a js parser)
>
> Combined, these are like timestamp rewriting, except the rewriting is done
> implicitly by timestampOffset, rather than updating the timecodes in the
> appended byte stream.
>
> Is the latter method (explicitly updating timestampOffset using data from
> the API and from the byte stream (or offline inspection/metadata/some other
> assumption)) sufficient for your use case?
>
> Matt
>
> On Tue, Aug 25, 2015 at 7:53 AM David LaPalomento <
> dlapalomento@brightcove.com> wrote:
>
>> Hi all,
>> I'm a contributor to video.js and am working to convert an existing
>> Flash-based HLS playback plugin to using Media Source Extensions. We
>> support a number of server-side ad insertion services, all of which seem to
>> compose existing media files without doing timestamp rewriting and signal
>> this to the player through metadata in the HLS manifest.
>>
>> One of the technical challenges we've faced in the existing
>> implementation is handling seeking across multiple timestamp
>> discontinuities before the entire video has been downloaded. HLS v3 rounds
>> segment durations to the nearest whole number which can introduce a
>> significant amount of timeline error in long-form content. Ignoring the
>> shortcomings of HLS though, the duration values provided by ad-insertion
>> services may lack precision and the wild-west of ad creatives doesn't help
>> the situation.
>>
>> We handle this issue today by recalculating the media timeline whenever a
>> new segment is downloaded and processed. Since the buffer always grows
>> forward, media timeline adjustments occur ahead of the current playback
>> position and the player's media timeline converges on reality as more
>> content is buffered.
>>
>> Preamble out of the way, here's my question for this group: how would one
>> seek across discontinuities without frame-accurate durations using Media
>> Source Extensions? If we had perfectly accurate duration information, I
>> believe we could use timestamp offsets on the source buffer to place the
>> new content at the appropriate position. With inaccurate or low-precision
>> duration information, it seems like we run the risk of mis-placing the
>> media data and creating overlaps at discontinuities and misreporting the
>> total content duration. Is there a solution in the spec I'm missing?
>>
>
Received on Wednesday, 26 August 2015 14:35:11 UTC

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