creating an AC3 file

Question

So long story short, I need to make an 5.1 AC3 for a local showing of a recent feature project. I don't have enough time to completely study up on the subject, but I'm picking up bits and pieces. I'm using Compressor 4 to create the AC3, and I got everything where I want as far as dialnorm, mixdown settings and DRC. But the only thing I can't change is the bite rate. It's stuck at 16 bit (AC3 Dolby Digital). I have a 5.1 book that I'm referring to, and it mentions bit reduction, LPCM vs MLP packaging, run time, DVD capabilities, etc. But it doesn't seem to directly address what's going on. Can someone explain this to me?

Also: - if I use MLP packaging can I create my AC3 at 24 bit? Does compressor have this feature? - I suspect I'm completely ignorant and it has to be burned at 16 bit for 5.1. In which case should I re-bounce the film using the proper dither vs letting Compressor make the change to the 24 bit 5.1 mix I have?

Thanks for any and all help guys. This is definitely a subject I'm going to learn extensively for the future.

Answer 1

It's been a while since this was posted and as it has had no replies I thought I would tell you I know. Hopefully others will fill in additional details.

AC-3 file has 48KHz sample rate, 16-bit resolution and the data is encoded using a lossy compression format. As far as I am aware 24-bit data is not supported and the data cannot be left unencoded (PCM), certainly using Compressor 3.5.3. (I know DVD-Audio does allow higher bit depths and uncompressed data, but the encoder that comes with compressor does not seem to support.)

So to create your AC-3 file, first bounce out six WAV files (preferably 48K, 16-bit) from your DAW project for each of the output channel. I append my file names with L, C, R, Ls, Rs and LFE so that they are easily identifiable. Then in compressor create a surround sound group and you can drag and drop each of your WAV file to the appropriate monitor location. Select the format setting required "Digital Dolby Professional 5.1" and drag it to the surround sound group in the project window. (You can find Digital Dolby Professional 5.1 on the settings tab: Apple -> Formats -> Audio)

Should you need to you can make any changes required in the inspector window, but the default settings should be okay for what you want. Hit the submit button and you are done.