Audio surround question on Dolby Pro Logic I & II.

November 29, 2004 8:15 AM

I have an audio surround decoder that supports Dolby Pro Logic I and II. (In separate modes.) If it's in Pro Logic II mode, will a Pro Logic I encoded signal still be "correctly" decoded?
posted by Mwongozi to Technology (2 answers total)

There's no such thing as a "Dolby Pro Logic I" or "Dolby Pro Logic II" signal, really. There's only Dolby, a means of encoding multi-channel audio down to two stereo channels. Thousands of theatrical films were released with two-channel Dolby soundtracks, and the various analog Dolby decoding systems were developed to produce better and better results from this material.

As Dolby Pro Logic offered better separation than simple Dolby, so Dolby Pro Logic II offers better separation and frequency range in the surround channels than simple Dolby Pro Logic. DPLII also has a music mode that works with most stereo recordings (this is apparently optional, so some receivers might not have it).

Now it's possible that some of your older VHS tapes were mixed to stereo with the original Dolby Pro Logic (or even original Dolby) in mind -- but more likely, the 2-channel theatrical mix was used, in which case DPL II will provide an experience more like a theater than DPL. Thus, you will probably use DPL II most of the time. Your receiver has the original Pro Logic in case you prefer it with certain source material (and/or just as a marketing checkoff item).
posted by kindall at 9:24 AM on November 29, 2004

In Pro-Logic II mode the decoder will attempt to pan the rear-surround speakers to the left or right depending on the overall balance of the track, whereas in PL I mode the rear speakers are mono. It's unlikely to cause any ill-effects and may even improve the surround effect, but to be pedantic, no it won't be "correct".
posted by cillit bang at 10:41 AM on November 29, 2004