Possibly the Dolby Digital print has the 5.1 remix from the early 2000’s and an optical stereo track. I would think a dual mono track insured compatibility with projectors with a stereo sound head?
That newer print has all audio-options: Optical Stereo (most likely Dolby SR), Dolby Digital, a DTS-Timecode and SDDS-sound.
Older Mono-prints just had one channel, I think from sometime in the 60s it became more common to use Dual Mono. Don’t know the exact reason, but SilverWook could be right that it had something to do with the upcoming popularity of Stereo-tracks and the soundheads used for them.
http://www.film-tech.com/ubb/f1/t008255.html
Brian Dooda draw a very specific timeline when Mono and Stereo-tracks has been used and he backs it up with the noise-reduction types, which were used back in the days. In Conclusion Dr. Cooper nailed it with Dual-Mono and silver-wook with the compatibility of the sound system used in different theatres. 😃
You all are absolutely right. I found this:
-There had been experiments with stereo optical tracks, but there was too much noise to make that sound system worthwhile. But when Dolby Laboratories introduced Dolby A in 1965, a noise reduction method originally developed for professional recording studios, the movie industry saw an opportunity to reinvent the optical track.-
Then, before 1965 there could be no dual mono 35 mm prints. So it would be more accurate if sound films shot between 1927 and 1965 were released on Blu-ray and UHD with a DTS-HD Master Audio Mono (1.0) track (or LPCM) instead of the more usual 2.0.
With “The Shining” print (1980) I wonder what year dual mono 35 mm prints start to appear. Maybe it’s hard to determine the exact year, but there would be Blu-ray releases where a DTS-HD Master Audio Mono (2.0) (or LPCM) would be accurate because the 35mm prints already included 2 channels. Indeed, “The Shining” seems to have been released on LaserDisc with dual channel sound as we can see here:
For this preservation, what would be more accurate? 2.0 or 1.0? In principle, both. But not for a film before… what year? I don’t know how you see it.