Re: EME and timed text

From: Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>>
Date: Wednesday, April 3, 2013 9:58 AM
To: David Singer <singer@apple.com<mailto:singer@apple.com>>
Cc: Henri Sivonen <hsivonen@iki.fi<mailto:hsivonen@iki.fi>>, Pierre-Anthony Lemieux <pal@sandflow.com<mailto:pal@sandflow.com>>, "<public-html-media@w3.org<mailto:public-html-media@w3.org>>" <public-html-media@w3.org<mailto:public-html-media@w3.org>>
Subject: Re: EME and timed text
Resent-From: <public-html-media@w3.org<mailto:public-html-media@w3.org>>
Resent-Date: Wednesday, April 3, 2013 9:59 AM



On Wed, Apr 3, 2013 at 8:44 AM, David Singer <singer@apple.com<mailto:singer@apple.com>> wrote:

On Apr 3, 2013, at 3:00 , Henri Sivonen <hsivonen@iki.fi<mailto:hsivonen@iki.fi>> wrote:

>  Timed text should not be DRMable just for principled
> symmetry with video and audio.

There are some cases where people want to supply text tracks 'in multiplex' (and meta-data tracks, and so on).  It seems to me that though, as you say, we shouldn't spend our time optimizing the EME and MSE for other track types, we shouldn't try to stop them working (or ban them from these interfaces) just for principled considerations from some people, either.

There is a difference, though, between the CDM decrypting a multiplexed text track and simply passing that decrypted text data to the browser and the CDM being responsible for rendering. We need CDM rendering to be possible for audio and video, but I am not sure anyone is asking for that for text. It has a lot of technical implications, as Henri points out. Given what we've already said in the specification, it seems like scope creep for this first version.

If text tracks are always to be processed by the UA (either rendered or passed to script), and these tracks MIGHT be encrypted, then there always needs to be enough unencrypted track metadata, e.g. "kind", "language",  "label", available to the UA so that UI can be provided to allow the user to choose a particular text track. Can this be guaranteed?


...Mark



David Singer
Multimedia and Software Standards, Apple Inc.