#164 Improve AC3 encoder

[Anonymous]

Three points to make ffmpeg the best free ac3 encoder:

1) The ac3 output volume is always 50% from wav input
volume.
Using -vol 512 the output is clipped at -6 dB then
there are a internal limitation inside the encoder.
Is possible to reach the 100%, Dark Avenger in
HeadAC3he can do this using a 6dB Gain.

2) New ffmpeg parameters, and valid values, to be
passed to output_frame_header (ac3enc.c)
-ac3c 0,1,2 Default: 0 (-3,-4.5,-6 dB) Center Mix
Level
-ac3s 0,1,2 Default: 0 (-3,-6,0 dB) Surround
Mix Level
-ac3d 0,1,2 Default: 0 (Ignored,Not,Yes) Dolby
Surround Mode
-ac3n 0..31 Default: 31 (-0..-31 dB) Dialogue
normalization

3) First 2 points are easy to implement and maybe
enough for a first release, but the true challenge is
generate Dynamic Range Compression info.
I suggest a new ffmpeg parameter:
-ac3r 0..5 Default: 0 (None, Film St., Film Light,
Music St., Music Light, Speech) Dynamic Range Compression