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.