[Bug 22137] changes in number of audio tracks during advert insertion

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22137

--- Comment #12 from Aaron Colwell <acolwell@google.com> ---
(In reply to comment #11)
> (In reply to comment #10)
> > [...] 
> > Just disable/unselect the tracks that are provided by that SourceBuffer.
> > That should cause the SourceBuffer to be removed from activeSourceBuffers
> > which in turn should prevent it from causing playback to stall.
> 
> it's my understanding that enabling/disabling tracks takes effect
> immediately (at playback time), not at append/buffering time.

Yes given the current HTML5 track selection mechanism. I believe there is an
assumption here that append time is the only time that you'd want to decide
when to adjust the track selection. I believe decoupling track selection from
appending would be more flexible of a solution.

> 
> if you scheduled a track disable for a particular point in playback, you
> might not be able to reach that point because playback could already be
> stalled for lack of data in a SourceBuffer.  if you let buffers run dry,
> then enable/disable the appropriate tracks, then begin appending again,
> you'd avoid the SourceBuffer stall but you'd have a playback stall/glitch.

It is up to the application to make sure that there has been enough data
appended and the proper tracks are selected to reach the next switch point. I
agree that with the current HTML5 spec there would be a slight stall/glitch if
switches were keyed off of the existing events. This is why I'm advocating for
a separate time-based track switching spec, so that we have a generic way to
signal to the media engine when these track changes should occur in a way that
is independent of how the media is being sourced. All the application has to do
is make sure the switches are described to the UA some reasonable time before
the current position reaches the switch point. The web application could choose
to do this alongside its appending activity or defer the decisions until a
later time.

> 
> if you enable/disable tracks while appending, tracks would enter or leave
> playback at the wrong times, potentially not presenting the intended tracks
> to the user or showing inappropriate tracks to the user.

Yes. If the web application is not careful about how it constructs
presentations, I agree this could happen.

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Received on Monday, 22 July 2013 22:46:31 UTC