AC3 bit & 16-bit Dithering

orca wrote on 1/6/2005, 2:43 PM

Just curious about the AC3 settings here. I know I've been doing this for a while but never really bothered with it until I saw that quite a few Hollywood DVDs use 16-bit/48 5.1 Dolby Digital or 16-bit/48 DTS. Is AC3 by default 16-bit? Or can we set it to 24-bit? Now I'm talking when rendering from Vegas. Or is the bit setting on the PROJECT audio properties is irrelevant once we render to AC3? Pardon my ignorance here.

The next question, if it renders to 16-bit/48 by default and our source is 24/48, don't we need to use DITHER by default then?

Thanks!

Comments

MJhig wrote on 1/6/2005, 4:12 PM

The project properties are relevant in many ways but how the AC-3 encoder deals with it exactly, I haven't researched enough to know.

If you are concerned about quality audio and have the DVD space you should use PCM (wave). AC-3 is a compressed format so you lose quality.

Vegas dithers and re-samples on the fly when rendering so if DVD space requires 16 Bit, a better way to do it would be (if your source is 24 bit) to have the project properties set to 24 bit, after setting all your audio FX, render to a new track, now you'll have a 24 bit rendered wave file and you took advantage of the 24 bits during FX processing.

Then open that track in Sound Forge and Process > Bit Converter or use Ozone to dither (see Ozone's directions on dithering), then save the file as a wave from Sound Forge to the same folder you will be saving the DVD-A MPEG 2 video stream only file with the same name as the MPEG 2.

Now when opening the MPEG 2 in DVD-A, since the wave is the same name it should also be imported.

If DVD space is not a problem just render the audio as wave from Vegas in the 24 Bit project with the same name to the same folder as above and stay in 24 Bit.

MJ

orca wrote on 1/6/2005, 6:30 PM

Thanks MJ. Could you point out where I could research this further. I'm already browsing the dolby site (still looking), but haven't found the info I'm looking for here. Thanks again.

MJhig wrote on 1/6/2005, 8:01 PM

Sorry, I do have the Dolby .pdfs and skimmed them but the fact remains AC-3 is a compressed format. I got the links to the .pdfs here, you could search the forum.

If I have to use AC-3 I simply set Diaglog Normalize to -31 and turn off all the FX on the last page (Processing tab) in Customize.

MJ