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[Bug 20900] New: drop constraint that MPEG2-TS segments must comprise one or more complete access units

From: <bugzilla@jessica.w3.org>
Date: Thu, 07 Feb 2013 23:11:36 +0000
To: public-html-media@w3.org
Message-ID: <bug-20900-5436@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20900

            Bug ID: 20900
           Summary: drop constraint that MPEG2-TS segments must comprise
                    one or more complete access units
    Classification: Unclassified
           Product: HTML WG
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Media Source Extensions
          Assignee: adrianba@microsoft.com
          Reporter: mthornbu@adobe.com
        QA Contact: public-html-bugzilla@w3.org
                CC: mike@w3.org, public-html-media@w3.org

the current Media Source Extensions specification constrains MPEG2-TS media
segments such that "Each PES packet must be comprised of one or more complete
access units."

this most likely intent of this constraint (that is, that PES packets contain
at least one complete AU, and only complete AUs) is unnecessarily restrictive
and will exclude a lot of existing Apple HTTP Live Streaming (HLS) content, and
exclude existing content encoders (including transport stream encoders supplied
by Apple and others).

one of the tricks used by some encoders to reduce transport stream overhead is
to allow the end of an access unit to spill into the beginning of the next PES
packet, to avoid having to potentially pad the end of the last transport stream
packet in a PES packet with wasted bytes.  this method yields PES packets that
do not contain a complete access unit, wherein a PES packet begins with the
last few bytes of a first access unit, then all but the last few bytes of a
second access unit.

this placement of access units in PES packets is consistent with and allowed by
the access unit semantics defined in ISO/IED 13818-1, and should be allowed in
MSE.

recommendation: drop the constraint "Each PES packet must be comprised of one
or more complete access units" from the "MPEG-2 Transport Stream Byte Streams"
section of the spec.

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Received on Thursday, 7 February 2013 23:11:37 UTC

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