Render AC3 as Pro-Logic

mp3superfreak wrote on 5/26/2003, 11:45 AM
How do you set the AC3 decoder to produce a Pro-Logic (or PL II) output? I tried 2 channel AC3, then checked the box for "Pro Logic", but I'm not sure if this is the proper way to produce the file. If anyone can shed some light on the subject, I would greatly appreciate it.

I mainly use 5.1 AC3 for myself and family members that have the equipment for playback, but most people use their TV's, so this is overkill.

Comments

mikkie wrote on 5/26/2003, 1:09 PM
If it helps...
As far as I understand, pro logic is a modified 2 channel setup encoded more or less so that the center and surround channels are modified versions of the ft 2. I can't remember all the details, but it's something like rear channels are inverted and subtracted from the front and so on, the results being added to the audio signal in a way that they appear normal stereo (if somewhat lacking in separation) unless a decoder is present.

AC3 5.1 is actually 6 separate channels that are sent their way via the decoder + related software or circuitry - the player (and or software) downmixes the 6 channels to stereo in the absence of a 6 channel setup. For example, on many set top DVD players, there might be one or two optical outputs that send the audio un-decoded to a 6.1 capable stereo system, also providing a mixed down stereo signal for the TV via a couple of RCA jacks. Any older style surround processing similar to pro logic is done by electronics on some players, kind of like a synthetic surround up mix.
mp3superfreak wrote on 5/26/2003, 1:51 PM
Thanks for the explanation, but I am well versed on the differences b/t 5.1 AC3 and Pro-Logic.

That said, it is possible to have 2 channel AC3 Dolby Digital tracks that are not encoded as Pro-Logic. My question is how to accomplish Dolby Pro-Logic encoding within Vegas's AC3 encoder. I'm pretty sure Pro-Logic is 4 discrete channels (FR, FL, C, Surround is mono). Anyone know how this is done in the AC3 encoder via Vegas 4?



For those interested, here is the real difference between Pro-Logic, Pro-Logic II and Dolby Digital 5.1 (taken from Dolby.com):

"Dolby Pro Logic is a matrix decoder that decodes the four channels of surround sound that have been encoded onto the stereo soundtracks of Dolby Surround program material such as VHS movies and TV shows. Dolby Surround is a matrix encoding process that in essence “folds” Left, Center, Right, and Surround channels onto stereo soundtracks. A Pro Logic decoder “unfolds” the four channels on playback (without a Pro Logic decoder, the encoded program plays in regular stereo).

Dolby Pro Logic II is an advanced matrix decoder that derives five-channel surround (Left, Center, Right, Left Surround, and Right Surround) from any stereo program material, whether or not it has been specifically Dolby Surround encoded. On encoded material such as movie soundtracks, the sound is more like Dolby Digital 5.1 (see below), while on unencoded stereo material such as music CDs the effect is a wider, more involving soundfield. Among other improvements over Pro Logic, Pro Logic II provides two full-range surround channels, as opposed to Pro Logic’s single, limited-bandwidth surround channel.

Dolby Digital 5.1 is a method of transmitting and storing 5.1-channel soundtracks via newer digital media such as DVD, digital cable, digital broadcast TV (DTV), and satellite transmissions. Unlike the Dolby Surround encode/Pro Logic decode process, which sacrifices channel separation to get surround onto any stereo soundtrack, Dolby Digital 5.1 is a discrete system that keeps the multiple channels fully separated throughout the encoding and decoding processes. In addition to having full-range Left, Center, Right, Left Surround, and Right Surround channels, Dolby Digital 5.1 soundtracks carry a sixth (“.1”) channel recorded with low-frequency effects (those bass rumbles and booms you feel as well as hear in a well-equipped cinema)."
Baylo wrote on 5/26/2003, 4:55 PM
I don't think it can be done. As you point out, although ProLogic is 4 discreet channels, they are mixed into 2 via some clever maths. The algorithm that would perform this maths is different (I think) from the ac3 process which is basically a data reduction/compression algorithm.

To my mind, to get ac3 prologic, you would first have to create a prologic compatible stereo mix, then encode as ac3. Having said that, I don't know of an ac3 decoder that would then push the stereo signal through a prologic decoder. I guess you would just use the analog outs from the DVD player...

Mark
mp3superfreak wrote on 5/26/2003, 5:55 PM
Thanks for the response, Mark. I will probably just email SoFo's tech support and see if it's possible....unless they view and respond to this thread first.
SonyDennis wrote on 6/3/2003, 5:19 PM
Vegas does not have a ProLogic encoder, but your DVD player probably does!

ProLogic is not discrete, it is Left, Center, Right, (Mono) Surround (LRCS) matrixed into stereo. The ProLogic checkbox in the AC-3 encoder is a way to mark an AC-3 stereo stream as 'containing' a ProLogic encoded stereo signal.

If you have a 5.1 mix, and render to 5.1 AC-3, and put it on DVD, nearly all DVD players will produce a ProLogic matrix mix out of the stereo RCA jack outputs, so that folks with older surround amps will still get a surround mix (granted, ProLogic LRCS and not AC-3 5.1). Folks with a Dolby Digtal (AC-3) decoder/amp will hook the bitstream output of the DVD player up, and get the 5.1 discrete AC-3 playback from the same disc.

///d@
mp3superfreak wrote on 6/3/2003, 8:16 PM
Cool. My Sony ES HT Receiver has an "auto decode" function and sometimes chooses 2 channel stereo for DD2.0, and other times chooses DPL surround sound (on Hollwood DVD's such as National Lampoon's Vacation). I thought there may be a way to tell the audio file to include the DPL signal...(in the AC3 encoder)

I use 5.1 AC3 most of the time anyway, but also enjoy playing with ALL the features. I don't know exactly what settings I used, but I recently encoded a 2.0 DD soundtrack on one of my home videos, and when I put it in my DVD player, the Sony Receiver recognized it as DPL instead of 2.0 DD. I'll continue to test and post my findings.

I knew that 5.1 AC3 would "downsample" to 2 channel when used with older equipment, but I also wonder why the Hollywood studios include a 5.1 AND 2.0 DD tracks on most of the commercially released DVDs?

Thanks for the response, SonicDennis.

RBartlett wrote on 6/4/2003, 2:32 AM
I believe 5.1 is an option in the DVD "spec". So to not give 5.1 and 2.0 would be a non-spec-following format that could result in silence on some players. The percentage of population stuck with stereo AC3 (or also stereo MPEG-1-L2) audio is of course reducing as new players are bought and old ones bomb out for good.

AFAIK !
RBartlett wrote on 6/4/2003, 2:33 AM
I believe 5.1 is an option in the DVD "spec". So to not give 5.1 and 2.0 would be a non-spec-following format that could result in silence on some players. The percentage of population stuck with stereo AC3 (or also stereo MPEG-1-L2) audio is of course reducing as new players are bought and old ones bomb out for good.

AFAIK !