AC3 Normalisation

farss wrote on 10/17/2003, 7:32 PM
I know this has been done to death here and in other areas. Yes there is heaps of info on this from Dolby and some very good info in the Doom9 forum.

I mostly understand it, and it's certainly a lot to get the brain around but I've never been one to shy away from doing the hard yards.

However most of the information I feel doesn't really relate to what most of us here are on about. I suspect not that many VV users are mixing audio for cinema release. Basically were producing soundtracks in VV that are already normalised for listening in the average home. All the complexities of handling a huge dynamic range just aren't on our horizon.

We've got our audio where we want it and basically are encoding to ac3 simply to go onto a DVD in the most efficient way possible i.e. leave more room for the video. Any audio tracks that have too much dynamic range we've probably already compressed.

So what we want to do is really quite simple, encode our PCM soundtrack to ac3 withour anything happening to it, no more compression, we just want it to come out the same as if we used a PCM track on the DVD.

Now I'd be prepared to invest some time in this to work it out BUT unlike playing around with the audio in VV running tests on ac3 is very tedious. There is no way to do this in real time and no objective way to monitor the ouput. I could setup a DVD player with meters but that's a lot of work, it means I've got to encode, burn a DVD and then try it in the player, tweak a few things and repeat.

I could live with waiting for the encode but not being able to bring the encoded file back into VV at least to see / hear how it is is a pain. I understand that ac3 is only an output format but it would be mighty handy to know what its got in it.

This issue is further clouded by some suggestion that the preview in DVDA doesn't do a proper ac3 decode so perhaps even there we don't really get to preview our soundtrack.

I know this really shold have been posted under VV audio or AC3 encoder but:

a) Not to many of us read those boards.
b) I suspect this is of interest to most of the VV users.

Comments

Spot|DSE wrote on 10/17/2003, 8:34 PM
Actually, this is quite easy to test, if you render test tones to AC3 vs trying to single out the differences with your ear on music or voice media.
We have a room that is 27 x 18 that the audio is tested/played back in, and monitored in both AC3 and PCM formats. AC3 on encode, then fed to the DVD player via the burned DVD. We're familiar enough with the room, and tweak it/check it before any major mix, so it helps tremendously.
To put an AC3 decoder in Vegas would be fairly expensive, not to mention unstable given the file source, therefore while it's one of those 'nice to have things' it is also in the 'but what does it do to performance and price' sorts of things.
ANY time you compress a file, something happens to it. Regardless of what type of file it is. So I guess I'm not certain what you are asking for here, excepting that I can say it takes a LOT of experience to do a proper encode, spending time on the file authoring and knowing what tools do what to the encode when prepping the media for compression.
farss wrote on 10/17/2003, 10:46 PM
SPOT,
I take all of your points, hadn't really thought through the issue of cost adding a ac3 decoder to VV.

I'd also agree most wholeheartedly that knowing your tools is the hallmark of a pro in any field, one of the things I took a lot of time to fathom is why most pros don't use the latest and greatest. The answer is of course they've spent many years acquiring knowledge on the toolset that they have. To change to something else can mean throwing away a large part of that knowledge and starting again.

If I was in the process of producing anything for commercial release or even for my own pleasure I'd certainly be doing what you suggest, trying, testing, tweaking. Not just the audio but also the impact of the mpeg encoders etc. I'd be checking it not just on studio monitors but also how it looks and sounds on the average tele.

Unfortunately what I am doing for part of my living is transferring old home movies to DVD. Needless to say there isn't much money in this, the competition just records straight from a VCR to DVD using a Philips burner! I'm trying to do a somewhat better job, cleaning up the video and audio a little and then going to DVD. Using ac3 audio means I can use a higher bitrate on the DVD which is a good thing given the amount of noise or grain in the source. Yes I'd love to have the budget to clean that up too.

I'll take your advise on board and do some more exhaustive tests. I know I'm never going to come up with a 'one size fits all' solution and maybe it sort of sounded like that was what I was after but if I can maybe get it norrowed down to Small, Medium and Large.