5.1 Surround and Dolby ProLogic and me

[Mark Spano]

[Matt Gorecki] “how do I work with this audio?”

Pro Logic encoded audio is LCRS (left, center, right, surround, aka 4.0) encoded to two channel Lt/Rt (Left total, right total) stereo-compatible audio. What that means is that you can use it just like regular stereo audio in your edit. If this audio is played through a Pro Logic decoder, it will decode back into LCRS.

[Matt Gorecki] “Does the Blu-ray player and/or receiver automatically figure out that the audio stream is Dolby encoded?”

Most likely it would be the receiver with a Dolby Pro Logic decoder built-in. As long as the end-user’s receiver is set up to decode Pro Logic, the material will play out LCRS. If not, it’ll play in stereo.

[Matt Gorecki] “Would I theoretically be able to drop other audio samples onto these two tracks without affecting the Dolby encoding?”

You could add audio into the mix, but it would make it less likely that a decoder would play back LCRS. It would still sound like you want it to in stereo, though, and many music tracks and other source materials are used this way.

[Matt Gorecki] “I realize that they would only play on the primary left and right channels, but would the other channels be affected?”

Yes. The way Pro Logic encoding works has to do with summing and phase relationships between the channels. Once you add something into this mix that isn’t encoded, the decoder might not work correctly. The only real way to find out is to try it out – play your mixed audio back through a Pro Logic decoding receiver in a surround monitoring setup. Other than that, you might have to give up hoping it’ll play in LCRS and concentrate on making it still sound good in stereo.

[Matt Gorecki]

Thanks, Mark.

That’s about how I thought it worked. I must be doing something wrong then during recording because the rear channels never show up.

Can audio compression affect the Dolby encoding? Say a bad mp3 encode?

[Mark Spano]

[Matt Gorecki] “That’s about how I thought it worked. I must be doing something wrong then during recording because the rear channels never show up.”

When you record with this microphone, you get an already encoded signal, which is two-channel stereo Lt/Rt. You’ll only hear the rear channels when that signal is played through a Pro Logic decoder.

[Matt Gorecki] “Can audio compression affect the Dolby encoding? Say a bad mp3 encode?”

Two types of audio compression you might be referring to: one is dynamics control, the other is lossy perceptual coding compression. Dynamics control is what you’d use when you want to control the overall level of a track – bringing peak levels closer to average levels. That can sometimes adversely affect an encoded track, but usually not too bad. Lossy perceptual coding compression is what you’d use to shrink the bandwidth/file size of an audio file (like MP3). That will probably completely destroy Pro Logic encoding. MP3 encoding basically throws information away that it thinks your brain won’t miss – this is precisely the information that the Dolby people have harvested to use to encode more ‘channels’ into a stereo pair. MP3 encoding often messes up phase relationships on a sample level and Pro Logic encoding requires precise phase relationships in order to direct the subsequent decoder to put certain information in the rear and center channels.