Dolby Surround

sean@oregonsound.com wrote on 8/8/2007, 4:27 PM
In the bitstream options for creating an AC-3 file, there is an option for "Dolby Surround Encoded" available when creating a stereo file. Does that create a Pro Logic mix, or the equivalent of Lt/Rt? I wouldn't think so, since I thought the AC-3 and Lt/Rt formats were completely different. But there it is, so I wonder what it's for? Could it take my 5.1 layout and downmix it to Pro Logic stereo? Any clues?

I'm doing a theatrical 5.1 mix but may also be asked to provide a Lt/Rt and would love to be able to do that without buying any pricey encoders.

Comments

reidc wrote on 8/9/2007, 12:47 AM
It just sets a flag to let receivers know definitively whether to decode for ProLogic. However, if there's a 90 degree phase shift selection available to you, selecting it will get you close to better LtRt compatibility off the 5.1 stream during decoding. This gets complicated though. The consumer would have to select the 5.1 stream on their DVD player, while setting their receiver to ProLogic instead of 5.1 Who's going to tell them to do that, and why would they anyway? They about most likely have 5.1 setup in their homes anyway. No harm in checking the 90 degree box though, I suppose. I have a high end DVD authoring client who insists on this. As for real LtRt without a dedicated encoder, you're out of luck. Encoder is only about $500, and someone like me will charge you almost that to do the work: realtime once through for compatibility, which may well be more than realtime if the mix doesn't translate well, and an encoding charge for the stream. Still, I can do it if for some reason doing it yourself is an issue.

Reid C
newhope wrote on 8/9/2007, 6:23 AM
The flags, when set in the AC3 encode, do tell the decoder how to handle the 5.1 mix when replayed on a system not set up for 5.1

For instance, someone with a Prologic (Dolby surround) capable amplifier who is feeding a 'stereo' line level analogue signal from their DVD Player into the amp. The DVD player does a realtime downmix to an LtRt compatible stereo feed using the parameters you set in the AC3 encoder. The amp then decodes that into Dolby Surround.

It's better to do a separate LtRt mix and encode that as a 'stereo' AC3 or MPEG audio on the disk as an alternate track because, if you have the correct encoder, you'll end up with a better mix but checking the correct boxes in the AC3 results in the next best thing.

By the way if you want to create an LtRt 'compatible' mix, try rendering a stereo downmix (WAV) from the 5.1 output. Although this isn't a real LtRt ,and you don't have control over the way your surround channels are encoded into the mix, it does produce a mix that will decode reasonably well in Dolby Surround.

I have done this from a 5.1 short feature mix to supply a guide stereo mix for 2 channel video and when I replayed the mix through a Proligic capable amp it decoded the surround quite well. It's a cheat but actually produces a surpisingly 'acceptable' result.

New Hope Media
sean@oregonsound.com wrote on 1/24/2008, 4:10 PM
Sorry to resurrect this old thread, but I've been experimenting to try to find the best method for downmixing my 5.1 tracks to a Dolby Pro Logic compatible mix. My first surprise was that Vegas wouldn't include all six tracks in an AC-3 2.0 file unless the Audio Properties of the EDL were switched from 5.1 to stereo. Fair enough. But what would you recommend as the closest approximation of a proper downmix? I've been creating an AC-3 2.0 from my six discrete .wav files, with the "Dolby Surround" encoding option turned on. Is there another option, or a better option? I wouldn't think you'd want to render a 2.0 file from an already compressed 5.1 AC-3 file, but what do I know?