[vlc-devel] DASH specific MP4 boxes

[Robert Forsman]

On Wed, 14 Mar 2012 11:43:37 -0400
Frederic YHUEL <fyhuel at viotech.net> wrote:

> 2012/3/14 Frederic YHUEL <fyhuel at viotech.net>:
> > Hello,
> >
> > Does anyone know where I can find the definition of DASH specific
> > "sidx" and "tfdt" boxes?
> >
> 
> Ok, they are described in a amendment of ISO/IEC 14496-12:2008 that
> can be found there:
> 
> www.3gpp.org/ftp/Inbox/LSs_from_external_bodies/ISO_IEC_JTC1_SG29_WG11/29n12310.zip


The sidx box is mildly redundant.  I think all of the info can be
computed from other boxes within moof and moov.  I welcome
clarification.

The tfdt tells you the temporal offset of the samples in the trun
relative to the beginning of the overall media, so if the fragment was
43 minutes into the movie, the tfdt would tell you that.

The tfdt can be used as an alternative to the tfma to compute A/V sync
when you're jumping into a fragment (they contain mathematically
redundant information).  One of the two should be used regardless,
because you can not rely on the video and audio lining up, especially
if they come from different <Representation>s.