A Few Questions About Dolby Pro Logic

[seven_deuce offsuit]

First question, is Dolby Pro Logic Left, Right, Center, Rear Left, and Rear Right? Second, I'm using Vegas for editing along with the V.I. plugin to create my separate wav files for my surround project, when I render out to ac3 what audio coding mode should I use for Dolby Pro Logic? I figure it might be one of the last three: 3/1 (L, C, R, Ls), 2/2 (L, R, Ls, Rs), or 3/2 (L, C, R, Ls, Rs) while disabling LFE. Lastly, I've separated my stereo track (the one I'm trying to make a surround project for) into two channels, Left and Right, would I need to pan 100% left for the Left channel and 100% right for the Right channel? Or would I just leave them at the center?

[aedipuss]

if you only have 2 channels of wav input then use dolby 2.0 - just use the defaults and it should be fine. you didn't need to separate them, vegas will convert a stereo wav to ac-3 2.0 without any problem.

[seven_deuce offsuit]

I'm actually trying to make a Dolby Pro Logic surround track though. 4 channels mixed into 2 channel stereo.

[aedipuss]

call me slow but i don't get it. what do you have for input for the audio? 2 separate wavs or 4?

dolby pro logic was 4 channel audio: left, right, left rear, right rear.
dolby pro logic II was 5 ch. added center
dolby digital is 5.1 added subwoofer

[Video Head]

Dolby Prologic is a 4 channel phase shift matrix technology. It is 4.0 - left, right, center and mono rear channel. You should matrix encode the 4 channels into a standard l/r stereo track. LFE will not be available.

[seven_deuce offsuit]

When you say matrix encoding do you mean combining or in Vegas, render the 4 tracks into a dolby stereo track?

If including the rear channels, does that mean 5 tracks are actually being used: Left, Center, Right, Rear Left, Rear Right? Or do I simply combine the 2 rear channels as mono?

[aedipuss]

vegas only renders dolby digital(ac-3).

[Midzuki]

http://en.wikipedia.org/wiki/Dolby_Pro_Logic

http://en.wikipedia.org/wiki/Ac3

HTH.

[Video Head]

When you say matrix encoding do you mean combining or in Vegas, render the 4 tracks into a dolby stereo track?

If including the rear channels, does that mean 5 tracks are actually being used: Left, Center, Right, Rear Left, Rear Right? Or do I simply combine the 2 rear channels as mono?

The rear channel is mono. There are no discreet left or right rear channels available as you would find in a Dolby Digital 5.1 set-up. You can have as many rear speakers as you want, they all produce the identical output. To produce a Dolby Prologic encoded soundtrack you need a program to matrix encode the 4 channels (left, right, center and rear) to a matrix encoded stereo track. Anyone playing the file with a Dolby Prologic decoder will recieve 4.0 surround, all others will receive stereo audio.

[seven_deuce offsuit]

So Dolby Pro Logic is a decoder while Dolby Surround is decoder/encoder? If that's the case, then I actually meant Dolby Surround.

[Video Head]

Dolby Prologic is encoded and decoded.

[seven_deuce offsuit]

I doubt this is the way to achieve Dolby Surround, but if there's software out there I can try out even as a trial then I'll do it, otherwise I won't bother.

1. First I import my WAV stereo file into Soundforge and use the V.I. plug-in (CLFE) to create a center channel. Save.
2. Re-import the file and use the same plug-in to create my rear channels (sLsR) as a stereo track. Save as a mono track instead.
3. In Vegas, I split the original WAV into two channels, Left and Right
4. Import the files I made in Soundforge, Center and Mono Rear tracks.
5. Render all 4 tracks as a Dolby 2/0 (Left, Right) ac3 for Dolby Surround.

[Video Head]

Why do you need to produce a Dolby Prologic encoded track? It is old matrix technology dating back to the '70's. Used in movie theatres in the 80's. It's primary use these days is in the TV broadcast realm...that's because stereo is the upper limit of most broadcast TV...until digital achieves 5.1 across the board. Is this for broadcast or school?

[seven_deuce offsuit]

It's just that I see on some commercial DVDs an audio option for Dolby 2.0 Surround. So I figured if commercial DVDs used them as an alternative to normal Dolby Stereo then I should try it. I'm trying to make a few DVDs for a few friends and since they don't have 5.1 surround systems I thought I'd do Dolby Surround instead but I guess it's outdated. I was just trying to find out a way to carry surround sound through stereo speakers which I should have asked in the very beginning.

[Cornucopia]

Commercial DVDs that offer AC3 tracks designated "2.0 surround" do so because:
1. The original movie soundtrack was Dolby Surround encoded (4-matrixed-into-2), and
2. They don't have the budget/inclination/material to properly upmix/remix to 5.1. and/or
3. They don't want to use the higher bitrate needed for 5.1

If you use the VI plugin, you'll end up with 5 channels. If you had a PROPER Dolby Surround encoder (and merged the LsRs from stereo to mono), you could create a matrixed 2.0 (called in the industry "Lt/Rt") output. But you DON'T NEED TO. If you just used the 5 (or 6) channels as input for the Vegas-provided AC3 5.1 encoder. And a 5.1 would be able to automatically downmixed to Dolby Surround in all software and hardware DVD players (if encoded correctly). Besides, you haven't mentioned that you even have a Dolby Surround Encoder.

BTW, just so we're all on the same page:

Dolby Surround Encoding, which may start with 4 channels in DS or PL, or with 5 channels in PL2, either way results in a 2 channel file.

There are 3 types of Decoders:
Dolby Surround (3 or 4 channels output, rarer oldest version)
Dolby ProLogic (4 channels output-includes the "Pro" Logic of automatic level adjustments which increase inter-channel separation)
Dolby ProLogicII (5 channels output, newest version-include different coefficients for optimizing different types of material)

(Synopsized from the Dolby website)

Scott

[aedipuss]

what they call dolby 2.0 on a dvd is not dolby surround. it is ac-3 dolby digital 2 channel. you can make it from a stereo wav directly in vegas.

[seven_deuce offsuit]

Thanks guys. I think I'm just going to stick with 5.1 surround for myself. On the other hand, if 5.1 was downmixed to stereo (as I'm making a few DVDs for a few friends who don't have 5.1 surround systems), wouldn't it be logical just to stick with stereo? If Dolby 2.0 sounds better, as I've heard, on stereo speakers than downmixed 5.1, I'll just forget about Dolby and stick with PCM all the way.

[Cornucopia]

what they call dolby 2.0 on a dvd is not dolby surround. it is ac-3 dolby digital 2 channel. you can make it from a stereo wav directly in vegas.

AC3 2.0 can be made up of stereo files, dual mono/language files, binaural files, or Dolby Surround-encoded files.

seven_deuce_offsuit: A "specifically-encoded" Dolby surround-matrixed AC3 2.0 file could sound better than the automatic downmix of a 5.1 surround AC3 file; this is why some studios put BOTH 2.0 and 5.1 on some DVDs. However, they are few and far between, as the automatic downmixing is actually quite good.

Here's what I don't understand--You've started with 2.0 plain stereo. You could just as easily LEAVE it like that and encode it as 2.0 (tagging it either as "stereo", "unsure", or lying and tagging it as "surround") and most HomeTheatre decoders have good options for synthesizing/extracting the surround channels anyway (which, BTW, is what the VI plugin is doing). They may be "expecting" Dolby-Surround-encoded 2.0, but if you give them plain stereo 2.0 they'll still try to work their magic on them anyway, and it can be quite an appropriate sounding output. Which leads me to the next thing to consider...BITRATE.

You can do a regular 2.0 file at the usual bitrates--256, 224, 192 (maybe even 128!). Dolby Surround encoded files, even though they may be 2.0, REQUIRE somewhat higher bitrates in order to correctly convey the low level but important matrixed difference surround signal information. So if plain stereo 2.0 at 192 is good enough sounding, you ought to use say 224 or even 256 to get an equivalent-sounding surround file. This is just like the need for a 5.1 file to have 384 or 448 as acceptable bitrates. Now, this has to be balanced out with the need to budget for the video, so your idea of using PCM for a 2.0 Surround-matrixed file is a little extreme unless you've got something that is Music-centric like a concert video. Otherwise, your best bet is to do what I suggested already, and save the bits for the video file's needs.

Scott