captainsolo said:
SW: The 6 channels were done this way originally- Left, Center, Right, Mono surround and two baby booms on each side of the center channel. These only augmented the bass instead of being discrete. You essentially would just need a 4.0 channel mix and a sub to get the idea. If you added in two speakers you could recreate the full experience but this would necessitate a 6.0 mix or a 7.1 receiver where you could just turn on extra speakers as mirrors cut out the treble and place them accordingly.
ESB and ROTJ were done in stereo surround without the baby booms so this would be much easier.
Not so; in addition to augmenting the low end of the main channels, all three films had real LFE content in their 70mm releases. This seems to be a point of confusion, but the research I've done on the topic indicates it to be the case, most particularly the remarks of THX engineer Tomlinson Holman (who incidentally is responsible for the adoption of 5.1 as the industry standard audio format). Also, none of the movies ever had stereo surrounds originally.
In fact, the reason the second and fourth channels of 70mm magnetic sound were used for bass purposes in the first place was because that was the only way to achieve a 'thunderous low end' when the Star Destroyer first goes by overhead. It's possible that it wasn't quite as loud as you'd expect of such things these days, true, what with subwoofers not yet being standard issue and headroom not quite as high as digital formats, but it was definitely there. Even listening to a low-fi tape recording made at a 70mm screening the discrete bass content can sometimes be discerned as clearly distinct from the rest of the mix.
The augmented portions of the boom tracks would be a bit problematic for fitting into modern formats, particularly for the first movie since the low pass filter was first set at 250 hz, though by the time the other two came out that had been changed to 125 hz to avoid noticeable crosstalk with the main channels. Dolby Digital only allows for LFE going up to 120 hz: a flat limit, not just a rolloff, in order to save bandwidth by taking advantage of the Nyquist frequency and only using a sample rate of 240 for the LFE. In general mixing practice, it tends to be rolled off around 80 hz to avoid the upper limit, and is only used for discrete effects and never as a general crossover. In making the 1993 mixes for the Definitive Collection laserdiscs, the boom tracks were deemed unsuitable because of these conditions, and the low end was instead added from separate sound effects masters as is the present custom.
So—the 70mm mixes cannot be presented exactly as they are in a remastering. The LCRS main channels could simply be ported over directly, but the bass needs some care taken to achieve an optimal effect. Either it would have to take the same path as the '93 mixes and be created over again, hopefully following the aesthetic of the original; or else the actual LFE content isolated from the boom tracks and summed together into one at a proper level. Either way, the result would be a 4.1 mix with monaural surround, similar to what appears on the Bluray releases of Alien and Aliens. (The fact that those include their 70mm audio shows that it can be done easily enough as long as proper care is taken.)