transcode supported by Rygel

[Soho Soho123 <soho123 2012 gmail com>]

Dear Jens,

I can find the code in Rygel about transcode,
like
rygel-xbox-hacks.vala
rygel-xbmc-hacks.vala
rygel-wmv-transcoder.vala
rygel-wmp-hacks.vala
rygel-visual-item.vala
rygel-video-transcoder.vala
rygel-v1-hacks.vala
rygel-aac-transcoder.vala
rygel-avc-transcoder.vala
rygel-l16-transcoder.vala
rygel-mp3-transcoder.vala
rygel-panasonic-hacks.vala

I confused about aac file can not play , but rygel include aac transcode.
Could you kindly help to explain the detail function of the files above?

Thanks!
Soho