[FFmpeg-devel] [PATCH] LGPL AC3 decoding

Jason Garrett-Glaser darkshikari
Tue May 19 02:04:16 CEST 2009

This patch contains a clean-room-written
ac3_decode_transform_coeffs_ch, written solely from the spec and
Justin's copious advice and descriptions (no knowledge of the original
function).  Oddly enough, it seems to actually be closer to the
original liba52 code, despite the fact that I never saw that either.
According to Diego, this should be sufficient to finally LGPL ffac3;
I'll let someone else handle the header/configure changes for that.

Bonus: mine's faster, too.  With GCC 3.4 on 32-bit Core 2 Conroe
(First 1234901 bytes of Broadway-5.1-48khz-448kbit.ac3):

Before: 27908.6 dezicycles (+/- 9.7)
After: 24820.6 dezicycles (+/- 12.4)
Total: 12.4% faster ac3_decode_transform_coeffs_ch

PSNR-wise it matches the original exactly; this was achieved through
Justin supplying me with the single line of code for the bap=0 noise
from the original code.  Note that I have not tested this on anything
other than that sample of Broadway, so I cannot guarantee it works for
all files.

Dark Shikari
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ac3lgpl.diff
Type: application/octet-stream
Size: 5161 bytes
Desc: not available
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20090518/2e4019bc/attachment.obj>

More information about the ffmpeg-devel mailing list