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RE: MediaStream and time-shifted TV playback

From: HU, BIN <bh526r@att.com>
Date: Fri, 5 Jun 2015 13:56:20 +0000
To: Chris Needham <chris.needham@bbc.co.uk>, TV Control API Community Group <public-tvapi@w3.org>
Message-ID: <179FD336116F754C876A9347238FE29A03FB011C@CAFRFD1MSGUSRIA.ITServices.sbc.com>

Great analysis. Let's talk about it on Tuesday.

Thank you and have a good weekend

-----Original Message-----
From: Chris Needham [mailto:chris.needham@bbc.co.uk] 
Sent: Friday, June 05, 2015 4:32 AM
To: TV Control API Community Group
Subject: MediaStream and time-shifted TV playback

Hi everyone,

Following from my previous email [1], I've done some further investigation into the use of MediaStream as video source for TV Control API, and identified some specific details in the MediaStream specification [2] that would need to be addressed to enable pause/resume for live TV and timeshifted playback, and in particular allowing buffering of the media by the user agent.

The Media Capture and Streams spec is now being finalised [3], so we're unlikely to get any changes made in the short term, but I post this information here for feedback and discussion of next steps.

On to the detail...

# 4.1 Introduction

> All tracks in a MediaStream are intended to be synchronized when rendered..
> This is not a hard requirement, since it might not be possible to synchronize
> tracks from sources that have different clocks. Different MediaStream objects
> do not need to be synchronized.

For a TV (or radio) tuner source, the tracks within the MediaStream must be synchronized.

# 4.3.4 Track Source Types

Add a "tuner" (or similar) value to SourceTypeEnum.

# 6. MediaStreams as Media Elements

> A MediaStream is not preloadable or seekable and represents a simple,
> potentially infinite, linear media timeline.

We want to allow the stream to be seekable.

> The timeline starts at 0 and increments linearly in real time as long as the
> MediaStream is playing. The timeline does not increment when the MediaStream
> is paused.

What is time 0 in the context of a MediaStream obtained from a tuner? Is there a timestamp we can use, obtained from the broadcast stream? How would we specify this for the various broadcast protocols (DVB, ATSC, ISDB, etc)?

I know that HbbTV 2.0 includes a synchronisation protocol for companion screens, so there's work we can look at there.

> The User Agent must not buffer data from a MediaStream. When playing, the User
> Agent must always play the current data from the stream.

Again, we want to allow the stream to be seekable.

# 6.3 Media Element Attributes when Playing a MediaStream

This section contains a table of MediaElement attributes and describes their behaviour when the element is used with a MediaStream.

> readyState: A MediaStream may be created before there is any data
> available, for example when a stream is received from a remote peer.

When the source is a tuner there is no remote peer, so maybe this constraint does not apply in our case.

> currentTime: The value is the current stream position, in seconds. On any
> attempt to set this attribute, the User Agent must throw an InvalidStateError
> exception.

It should be possible to set the currentTime to any time within the seekable time range.

> seeking: A MediaStream is not seekable. Therefore, this attribute must
> always have the value false.

> defaultPlaybackRate (also playbackRate). A MediaStream is not seekable.
> Therefore, this attribute must always have the value 1.0 and any attempt
> to alter it must fail.

> played: A MediaStream's timeline always consists of a single range,
> starting at 0 and extending up to the currentTime.

> seekable: A MediaStream is not seekable.

As we want to allow the stream to be seekable, these attributes should have the usual MediaElement behaviour defined in HTML [4].

> duration: A MediaStream does not have a pre-defined duration.

> loop: Setting the loop attribute has no effect since a MediaStream has no
> defined end and therefore cannot be looped.

These constraints also apply when playing streams from a tuner.

Also, HTMLMediaElement does not have an equivalent to the OIPF stopTimeshift() method, although the same effect could presumably be achieved using:

    element.currentTime = Infinity;

because the current "live" playback position should be the latest time in the seekable range. See [4, section].


[1] https://lists.w3.org/Archives/Public/public-tvapi/2015Mar/0013.html

[2] http://w3c.github.io/mediacapture-main/, draft 23 May 2015
[3] http://www.w3.org/blog/news/archives/4600

[4] http://www.w3.org/html/wg/drafts/html/master/semantics.html

Chris Needham
Senior Software Engineer
BBC Research & Development


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Received on Friday, 5 June 2015 13:57:12 UTC

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