pinnahertz
Headphoneus Supremus
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Great info. Just to clarify myself: I'm using "Dolby Surround" to broadly refer to non-consumer/cinema formats, while "Pro Logic" encompasses the consumer-facing stuff of the late 1980s (1987 sounds right but may not be).
Dolby's cinema surround format was called "Dolby Stereo", which was immediately confusing because it could be used for LCR only, surround was optional and not always mixed in. Dolby Surround was in fact a label attached to the earliest of consumer surround decoders in 1982. I've forgotten the manufacturer that first licensed the Dolby Surround logo for consumer use, but it may come to me. I had one for a while. It had no steering, but did have modified Dolby B NR on the surround channel plus adjustable surround delay and volume.
The Dolby Surround logo appeared in the opening of some TV shows like The Tonight Show that were mixed for the consumer decoders without steering. That went on for five (long) years until 1987 when ProLogic was introduced. I still have the T-shirt they handed out with a block diagram on it. The Dolby Surround label was never meant for cinema use AFIK. Cinemas with 70mm magnetic capability marqueed "Dolby 6 Track Surround" or something like that. Few releases, only big ones, but 70mm mag with Dolby A, then later SR just befor Dolby Digital was pretty darn good. Discrete channels, decent dynamics.
I know that, technically speaking, Dolby has re-used "Dolby Surround" as a brand for many things, even including digital multi-channel, and frankly I find it unnecessarily confusing on their part. I do know that on VHS, for example, some are labeled as "Dolby Surround" to refer to Pro Logic compatible matrix encodes, but that audio track is not necessarily what was in theaters in the 1970s or 1980s, nor is the equipment decoding it directly comparable to what was in theaters either.
Yes, Dolby is one of the masters of brand confusion. Dolby Surround on any consumer format was actually just a dub of the LtRt master, though. They didn't remaster for the home market at all. That was on VHS, Beta HiFi, and LaserDisk (both digital and AFM). ProLogic was directly compatible with the cinema version, which eventually upgraded their old Cat.150 cards to similar performance devices. There was only one encode matrix for Dolby Stereo (meant for steered decoders) and Dolby Surround (meant for non-steered decoders). It was just a question of what decoder you monitored your mix with.
The funny thing about that was when the first stereo audio tracks appeared on LaserDisc (predated Beta HiFi and VHS HiFi), some of us "discovered" there was hidden surround. I confirmed my suspicion with a friend deep in the industry, who tipped me off to how it all worked. I purchased a Dolby CP50 manual and studied the schematics. I built my own surround decoder just about the same time as the first commercial decoder was being designed. I still have a few of the PC boards dated 1980.
Pro Logic II is, as I understand it, an entirely distinct development, designed by Fosgate Labs and licenced over to Dolby; you're 100% that it will take the Dolby Surround/Pro Logic I matrix input and unpack it "correctly" for backwards compatibility, but the actual DPL-II matrix looks a bit different, one of the biggest changes is that the surround channels are discrete (it actually provides 5 channels out). And as you pointed out, channel separation is meant to be higher.
The only difference is in the Ls and Rs extraction from LtRt tracks, which is somewhat ambiguous in fact because tracks aren't actually encoded that way. Remember, there was only ever one encode matrix to create LtRt masters. The PLII decode matrix has to be different for the L/R extraction, but it's an extraction with assumption. They had to still allow for legacy decoding.
However for actual "1980s content" I'm not sure if this makes much of an appreciable difference.
PLII is a consumer product, and part of the reason for the Ls/Rs extraction from LtRt tracks has to do with home systems have few, usually only two, surround speakers that don't provide the diffuse surround field the tracks are mixed for in large cinemas (with a dozen or more surround speakers). THX did it first in Home THX with surround decorrelation, which was not a LsRs extraction, but rather made Ls and Rs dissimilar so they couldn't be localized to the speakers as well. That and dipole surrounds did a fine job of replicating a diffuse surround field.
PLII actually does a better job decoding a 1980s LtRt track in the home. Home systems are different from large theater systems in several ways, like I just said. But nothing is/was ever specially encoded for PLII in an LtRt track. PLII made provision for other extractions as well, like 7.1 from 5.1. But again, it's an ambiguous approximation because the home 7.1 speaker plan is different from the theatrical one, and no 5.1 theatrical track was ever specifically mixed for the home format. It works because of the size and limited speakers in the home. Dolby has done other channel extractions, as did THX with their modification to the 7.1 home plan, then added a different 7.1 caming plan.
If we thought Dolby were masters of market confusion then THX would be the Jedi Masters of marketing confusion, which has spelled their demise.