RE: TTML and aspect ratio

There is no authoring intent signaling for 608.  It was and is always 4:3.

 

708 has metadata for signaling the authoring intent to one of 4:3 or 16:9.
This is OOB of the caption data and it's signaling is transport dependent.
For example in ATSC, it is in a Caption Service Descriptor of the MPEG-2 TS.

 

SMPTE defined an encoding of this metadata in its forthcoming publication of
ST 2052-11, which maps 708 into TTML.  A metadata item was defined to
provide this signal, m708:aspectRatio.

 

SMPTE has recently asked W3C to consider adopting its extensions of TTML
from 2052-1, 2052-10 and 2052-11.  See:

http://lists.w3.org/Archives/Public/public-tt/2013Jan/0006.html 

 

Today, when dealing with 608 and 708 sourced content, one should rely on the
SMPTE work.  Except for 2052-11, which is not yet published, the work is
available for free at www.smpte.org.

 

Regards,

 

                Mike

 

From: David Ronca [mailto:dronca@netflix.com] 
Sent: Tuesday, January 29, 2013 6:45 PM
To: public-tt@w3.org
Subject: Re: TTML and aspect ratio

 

CEA-608 mandates a 32x15 grid in a 4:3 display area.  CEA-708 allows either
32x15 (4:3) or 42:15 (16:9).  For 4:3, the position 10%,10% is a different
location on the screen than it is for 16:9 content.. Further, displaying
CEA-608 caption in a 16x9 display area will result in very poor presentation
(as will presenting 16x9 CEA-708 caption in a 4:3 display area).  In
reviewing the W3C-TTML, we can find no well-defined means to inform the
client of the proper aspect ratio of the caption display area.  My question
is how should a client determine the correct aspect ratio for the caption
viewing area?  

David Ronca
Manager, Encoding Tools
Netflix, Inc.