I had never seen the article in American Cinematographer. As to my contention regarding the 4-track master, I must defer to you - considering the source. Ioan is a class act and still something of a guru at Dolby, and has never been prone to issuing unsubstantiated information. I still submit that the 70mm and 35mm mixes are distinctly different in many ways including content. There ARE elements in the 6-track that do not exist in the Dolby Stereo version so I assume were not in the original 4-track either. I can only suppose that additional SFX were added during the 70mm dubbing session.
At the risk of contradicting my original post, I also find myself skeptical of one of Mr. Schnuelle’s statements – that the boom channel was simply derived from summing everything and rolling off the top end. Listening to even an “in theater” recording of the 6-track suggests there’s more going on than just added bass across the entire mix. An excerpt from an interview with Mike Minkler, one of the original mixers for Star Wars supports this as well:
(http://mixonline.com/recording/interviews/audio_mike_minkler/index.html),
“…It was a defining moment because of Dolby Stereo. At the same time, it was the birth of baby boom. The 6-track was devised by Steve Katz, who was the Dolby consultant on the show. When we were predubbing reel 1 spaceships, we couldn't get this big thunderous low end that we wanted on the pass-by. We were going to do what we called a “Todd spread” back then, which was to record a left, center and right, and a surround — then fill in channels 2 and 4, the left extra and right extra, with information from these adjacent channels. But Steve said, “What if we used 2 and 4 for boom only, the low-frequency information, and we'll use full-range speakers.” Well, we didn't have them; we had the Altec A4 speakers, and we put low-frequency material in there as much as we could to enhance the spaceships. And every time there was an explosion, there was a sweetener that was cut for those two channels.”
…. the last statement clearly suggesting that they were treating the boom channels selectively, like an LFE. (It should be noted that the 2 boom channels contained the same mono information ((a waste of real estate in some people’s view)). The reason for this decision was that the theaters capable of playing 6 track were all hard wired for Todd-AO – 5 full range screen channels with mono surround. Doubling up the boom channel on channels 2 and 4 allowed for the desired distribution of signal without the need for special wiring, as well as maintaining “backward compatibility”.)
The Baby Boom format (with stereo surround) essentially became the template for 5.1 – five full range channels with one LFE. The 6-track print-masters almost always contained additional LFE information that was not available on the Dolby Stereo mix or the 4-track master, simply because of the technical limitations of the optical release format. Which is why, in the case of pre-5.1 films, the THX Laserdisc Program always extracted their Pro Logic mixes (and later, the 4.1 mixes) from a 6-track source whenever possible. The only reason for sourcing the main channels from a 4-track would be if it were an earlier generation copy - as was apparently the case in Star Wars and Empire. So aside from the generation loss (which is negligible with Dolby A or SR encoded material), I’m still not clear as to why you believe the 4-track by itself represents the optimal source.