AC3 unwanted stereo downmix

corug7 wrote on 8/14/2008, 2:54 PM
I have a very basic AC3 question that I cannot seem to find a satisfactory answer to anywhere, on these forums or elsewhere. A client has provided 6 discreet mono channels to be combined with a video we fixed for him. I have synced everything up on the timeline and brought audio levels within an acceptable level. There is to be no "mastering," so to speak, just line 'er up and render. However, each of my renders to 5.1 have resulted in 5.1 stereo downmixed files, despite each track being panned to the correct spot on the surround panner.

I am only monitoring on a 2.0 system currently, as we are not required to tweak the audio. Could this have something to do with it? IE, is the transcode being routed through my soundcard for some reason? This is the only thing I can think of. All other settings appear to be correct (to the best of my knowledge).

In addition, should the center channel generally be set to "constant power?"

Thanks in advance.
Corey

Comments

blink3times wrote on 8/14/2008, 3:23 PM
In order to deal with 5.1 audio, you NEED the proper system. You say you have a 2 channel system so you can't use it for testing. For all you know your sound card is down mixing the 5.1 that you have properly encoded.

If you have a multi track audio editor you could always import the ac3 and check it visually..... but you need to verify it faulted before you can call it that.

One other thing.... if you have 6 mono channels then you can use 6 different tracks and when you do this you should MUTE the speakers that are not to be used on a given channel and not just set the panner. If you have a track for the left front, then all other speakers except the left front should be muted (left click on the speaker to mute and it will turn a dark gray)
corug7 wrote on 8/14/2008, 6:42 PM
Exported to AIFF and used a competing product. I'm feeling a little stung about it, as there is very little documentation in the Vegas Manual regarding AC3 settings.

Still, I'd like to know why my files were downmixed to stereo when I used a 5.1 timeline with 5.1 tracks and output to 5.1 Dolby Digital.
TGS wrote on 8/15/2008, 12:13 AM
Make sure your Audio Project Properties are set for 5.1 (not Stereo)
Make sure your Audio Device, in Preferences, is set to 'Direct Sound Surround Mapper' in 'Audio Device Type'
Be sure to check this again, if you open any other vegs.
In DVD-A, the 5.1 waves will still look like Stereo waves, but should have a different Icon if it's the 5.1 mix.
If listening with only 2 speakers, your sound card or software playback may be downmixing it to 2 speakers, but it may be fine. Check in DVD-A, which will also downmix it into stereo on playback, but the 5.1 Icon will tell you if it's 5.1 or not.

corug7 wrote on 8/15/2008, 8:10 AM
Thanks all for your replies.

I'm getting the 5.1 symbol in DVDA, but the ac3 file metadata says "Dolby Digital 5.1 (stereo downmix)", and after burning a disc, I have no center channel and the surround speakers are the same as the front L/R channels. It is somewhat baffling.

I have done all that has been suggested and I'm still getting a downmix. I'm sure it is somewhere in my settings but tracking it down is the hard part. Any other suggestions are welcome.

Corey
riredale wrote on 8/15/2008, 8:59 AM
Corug7:

This would be a great time to install a surround-sound audio card and buy/borrow some additional speakers for the surrounds.

I can also speak from experience that you need the "Direct sound" setting under Preferences/Audio Device. Other drivers will mix the rear into the front.

It is assumed that each audio track on your timeline has a surround mapper, and that the panner for each mapper goes to its own unique corner, right? In my haste I once forgot to do this.
corug7 wrote on 8/15/2008, 10:06 AM
Plugged in some basic speakers to the surround card and all channels seem to work.

Direct sound is definately on. The three "default" selections below it in preferences are all greyed out. I do not know if this is normal or not.

Each audio track has a surround Mapper, and each track is mapped to its own channel.

In other words, it seems the main problem I have appears at render. Downmix output is set to 5.1, but is there any other setting outside of what appears in the render settings that could have an impact on this?
TGS wrote on 8/15/2008, 10:57 AM
I just noticed an Icon, just to the right of the "Project Audio Properties" Icon, to the right of the Surround Mixer that says "DownMix Output" (Hover mouse to see it). I don't know if that applies to just listening or encoding. I've never tried it.
TGS wrote on 8/15/2008, 11:09 AM
I guess you're talking about the same "DownMix Output"
The only other thing I've seen, is in the Custom settings of ac3, on the "Audio Service" tab, the 'Audio coding mode' has some settings that might downmix. My default for 5.1 says: 3/2 (L, C, R, Ls, Rs,)
I don't know what else to suggest, unless you didn't choose "5.1 Surround DVD" setting in ac3 templates.