[Bug 746109] qtdemux: Support for gapless playback for .m4a files

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=746109

--- Comment #1 from Carlos Rafael Giani <dv at pseudoterminal.org> ---
Created attachment 316610
  --> https://bugzilla.gnome.org/attachment.cgi?id=316610&action=edit
proof-of-concept iTunSMPB support

Here is one proof-of-concept patch for handling iTunSMPB data in .m4a files. It
basically overwrites segment data with the information from iTunSMPB. The
adjustment excludes the decoder delay, which is either 1024 or (very rarely
outside of DAB streams) 960 samples. In the patch, 1024 is hardcoded. Note that
this is done only if (a) the MP4 container has only one stream and (b) if that
single stream is of type AAC.

The decoder delay is excluded for two reasons:
(1) segment adjustments didn't work properly without the exclusion
(2) some decoders automatically exclude the first 960/1024 samples, some don't
(FAAD does, avdec_aac doesn't)

Also, I do think that the decoder delay always equals the first frame, but I
didn't find .m4a files with iTunSMPB data and 960 sample frames to test.

Comments welcome.

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