W3C home > Mailing lists > Public > public-html-a11y@w3.org > May 2011

RE: [media] issue-152: documents for further discussion

From: Bob Lund <B.Lund@CableLabs.com>
Date: Wed, 18 May 2011 09:44:21 -0600
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
CC: HTML Accessibility Task Force <public-html-a11y@w3.org>, "Mark Vickers @ Comcast" <mark_vickers@cable.comcast.com>, Eric Winkelman <E.Winkelman@CableLabs.com>, David Agranoff <d.agranoff@CableLabs.com>
Message-ID: <114DAD31379DFA438C0A2E39B3B8AF5D0185B200B1@srvxchg>


> -----Original Message-----
> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
> Sent: Tuesday, May 17, 2011 7:36 PM
> To: Bob Lund
> Cc: HTML Accessibility Task Force; Mark Vickers @ Comcast; Eric
> Winkelman; David Agranoff
> Subject: Re: [media] issue-152: documents for further discussion
> 
> On Wed, May 18, 2011 at 12:44 AM, Bob Lund <B.Lund@cablelabs.com> wrote:
> > Hi Silvia,
> >
> > I had considered @data- attributes but was unsure of the implications
> of this statement in section 3.2.3.8 of the current HTML5 spec
> (http://dev.w3.org/html5/spec/Overview.html#embedding-custom-non-
> visible-data-with-the-data-attributes):
> >
> > "User agents must not derive any implementation behavior from these
> attributes or values. Specifications intended for user agents must not
> define these attributes to have any meaningful values."
> >
> > In the case of in-band tracks, the user agent will have to create the
> DOM equivalent of the @data- attribute for metadata tracks. This
> appeared to me as being in conflict with the second sentence of the
> above quote. Is this not the case?
> 
> 
> Where would a UA get the information about the special track type from
> from in-band metadata tracks?

MPEG-2 transport streams contain program map tables that identify each program id with a type, e.g. video, audio, EISS (http://www.cablelabs.com/specifications/OC-SP-ETV-AM1.0-I06-110128.pdf)m etc. MPEG-2 TS may be used directly over HTTP or as the fragment format in HTTP live streaming and DASH.

> Do you know fields in MP4, Ogg and WebM
> that provide such information?

Fragmented MP4 will be carried in some adaptive bit-rate containers, e.g. certain DASH profiles. In this case, the metadata tracks will be identified in the manifest file. However, with respect to the HTML5 "timed text track" API these are still in-band, i.e. not sourced from an external file. In this case, there is still the need to identify the type of metadata. Discussions are taking place now in MPEG and other places regarding requirements for identifying metadata tracks in DASH.

> If there is such a field that you need
> exposed on top of what is already there, then it would indeed make sense
> to include that.

As described above, there is or will be such a field.

> But I honestly doubt that you will find in-band tracks
> that will tell you that they contain adinsertion information or
> syncwebcontent data.

See above.

> This is all very application-specific 

You are right these are application specific but in the broadcast industry these applications are common: ETV (http://www.cablelabs.com/advancedadvertising/etv/), ad insertion (http://www.scte.org/documents/pdf/standards/ANSI_SCTE%2035%202007%20Digital%20Program%20Insertion%20Cueing%20Message%20for%20Cable.pdf) and parental control content advisories (http://www.ce.org/Standards/browseByCommittee_2524.asp)

> and therefore can only be solved with external text tracks IMHO.

Out-of-band timed text tracks work well for file based content but I don't think will work for linear streams with no start or end.

> Always open to hear a real-world use case though!

> 
> Cheers,
> Silvia.

Thanks,
Bob
Received on Wednesday, 18 May 2011 15:44:51 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Friday, 27 April 2012 04:42:38 GMT