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-   -   Audio Codecs: Ac3 decoding/encoding with ffmpeg (http://www.digitalfaq.com/archives/audio/8973-audio-codecs-ac3.html)

tickey 04-05-2004 11:32 AM

ac3 decoding/encoding with ffmpeg
 
Like most I was very impressed with the ffmpeggui for ac3 2 channel encoding. Fast, great quality and works on my DVD player without problems.
Unfortunately ffmpeggui does not support ac3 decoding, which ffmpeg can handle.
Avalon was kind enough to implement this feature in his builds.
http://forum.doom9.org/showthread.php?s=&threadid=73629
Now that DspGuru has re-implemented ac3 encoding in BeSweet, this testing is less important.
However people might want to do it from the ffmpeg.exe.

Some test results
AC3 2.0 channel@X_bitrate to AC3 2.0 channel@X_bitrate-pass
AC3 5.1 channel@X_bitrate to AC3 5.1 channel@X_bitrate-pass
AC3 2.0 channel@X_bitrate to mp2 2 channel@X_bitrate-pass
AC3 5.1 channel@X_bitrate to mp2 2 channel@X_bitrate-fail: appends 3x2 channels end to end
mp3/avi 2 channel@X_bitrate to AC3 2.0 channel@X_bitrate-pass
mp3/avi 2 channel@X_bitrate to mp2 2 channel@X_bitrate-pass

I couldn't sort out the ac3 5.1 channel to mp2 (for svcd/vcd people).
If someone has the command line for this that works, please post.
Myself I use the AC3 5.1 channel@X_bitrate to AC3 5.1 channel@X_bitrate for reducing the size of my ac3 for my KDVD(3 short/2 long movies/dvd)

Just as a short comparison I tested AC3 5.1 channel@X_bitrate to AC3 5.1 channel@X_bitrate on the new BeSweet on a short/fixed 10 min ac3 file.
BeSweet:82 secs
FFMpeg:approx. 60 secs


Special thanks to Avalon for enabling this great feature in ffmpeg.
Sample cli
Code:

path\ffmpeg.exe -ab 256 -ar 48000 -ac 6 -acodec ac3 -i path\input.ac3 path\output.ac3
Transodes an ac3 5.1 @xbitrate to ac3 5.1 @ 256 kbps
It is also being discussed here.
http://forum.doom9.org/showthread.ph...0&pagenumber=2

incredible 04-05-2004 11:51 AM

A nice to hear that DSPGuru implementatet a new version of AC3enc in his besweet .... but whats about the mentioned Volume bug?? Still avaiable even when doing a Auto-Max-Gain ??

That FFmpeg GUI I also used one time but there wasn't a Auto-Max-Option ... maybe someone should list the parameters of ffmpeg.exe build so we can at least use it via command line like mencoder.
Does there exist a page where the parameters are listed .... I would apriciate it! ;-)

tickey 04-05-2004 12:00 PM

http://ffmpeg.sourceforge.net/ffmpeg-doc.html
gives some info.
The problem with ffmeggui is it doesn't decode ac3
Avalons build enables that.
On the BeSweet in the Azid 1 box I select LFE to LFE @ 0db, and gain 0db.
I set this as I basically want no processing other than lower bitrate.
If I don't set these the BeSweet GUI will add LFE to L/R @ -3db and Dynamic Compression -normal when I leave the Azid 1 box all by itself 8O


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