Change bitrate of input ac3 to output ac3

[Chumbo]

I've been wanting to do what seems like a simple thing; convert a 448Kbps ac3 to 384Kbps to an ac3 output file.

This seemed pretty straightforward with BeLight or BeSweet GUIs. The output file's channel mapping becomes skewed. The channels are shifted over by one is what it sounds like.

So I tried saving it to a 5.1 WAV file. The wav file's mapping is fine. So I tried converting the 5.1 WAV file to an ac3, but the result is the same as when I went ac3 to ac3. I'm guessing that BeSweet just does the ac3-to-wav and then wav-to-ac3 internally any way.

So much for this. So I tried ffmpeg. The output again was skewed. The channel mapping is even worse. I tried the older ffmpeg gui and the more recent SUPER gui to help make sure I was making the right selections and no go.

I'm at a loss why the output when going from ac3 to ac3 is messed up when all I'm doing is changing the bitrate.

Has anyone done this in one step using a tool that doesn't mess up the channel mapping in the output file? Thanks for any help or pointing me in the right direction.

For now I have to export the ac3 to seperate wav files or a 5.1 wav file and then use an app like Vegas to create an ac3 file. Needless to say, this is a very time consuming process to just change the bitrate of the file.

[Krismen]

You can use Aften (http://win32builds.sourceforge.net/aften/index.html) & AviSynth + BePipe (http://www.gotdotnet.com/workspaces/workspace.aspx?id=1bb59ddf-901b-43a5-bd54-b0999e8e223e) for recompression:

bepipe --script "directshowsource(^ac3_file.ac3^)" | aften -b 384 - out.ac3

[tebasuna51]

Instead directshowsource I recommend use a dedicated decoder plugin for AviSynth NicAudio.dll (http://nic.dnsalias.com/NicAudio_alpha3.zip) (at ...\Avisynth 2.5\Plugins folder)

And preserve the full dynamic range using original dnorm/dynrng, for instance:
bepipe --script "NicAc3Source(^ac3_file.ac3^)" | aften -b 384 -dnorm 27 -dynrng 1 - out.ac3"
Or to be played with equipment with volume problems:
bepipe --script "NicAc3Source(^ac3_file.ac3^, DRC=1).Normalize()" | aften -b 384 - out.ac3"

[Chumbo]

Thanks so much for the info. I'll check it out in the next few days and let you know how it worked. Much appreciated.

[Chumbo]

Okay, I was able to use this method, however, the command line stuff just wouldn't work which made no sense. It's pretty straight forward.

I wound up grabbing BeHappy and the GUI worked after I updated the aften.exe to the latest version. Not sure why the command line piping didn't work.

I did find another thread here that shows bepipe was a problem which was later fixed and I'm using the latest of everything. When I have time I'll look into the command line stuff.

Thanks again for pointing me in the right direction. I really appreciate it. :)