I started out responding to Moth3r's post in the thread about what constitutes an acceptable restoration, but moved it here instead since this is the more appropriate place, really.
Moth3r said:
Unfortunately the 70mm Dolby 6-track mix cannot be simply ported over, as the "4.2" Baby Boom configuration is not compatible with modern 5.1 (or 6.1, 7.1) speaker arrangements. The most authentic solution would be to remaster the audio track from the original elements, by merging the two sub channels into one and using a mono surround channel.
I believe this is how the Blade Runner 70mm mix was presented.
Quite so, the 70mm version can't be duplicated exactly as it was on modern home video; it needs a bit of remastering to adapt to the format. The two LFE channels would need to be added together and merged into one, and the mono surround channel either duped into two rears at -3db each for 5.1, or simply presented as-is in a 4.1 layout and let the receiver distribute it among the appropriate number of speakers.
There is an additional difficulty in that the bass tracks were originally mastered with content going up to 250 hz, whereas Dolby Digital only allows for a 120 hz upper limit on frequency response, so something would need to be done about this additional bass. It could possibly be level-corrected and put back into the main channels, or it might actually have to be discarded altogether, since there are some reports that the initial use of separate bass tracks may have been to duplicate/augment what was already in the mains in addition to the separate baby booms, which would be problematic if it contained music or dialogue. In making the 1993 mix, THX chose to use only the main channels of the 70mm version while taking the bass from a separate sounds effects master, in order to avoid any potential bass 'bloat' that could have arisen from using the boom tracks directly.
So, any theoretical future transfer of the 70mm audio would have to be careful about how it handled the bass for best results. Depending on exactly how it sounded, taking a similar approach to the '93 mix and mastering a new LFE channel from the separate sound effects master might actually be the most practical option. If in the course of doing so some parts were to be augmented or added, it might be acceptable as long as it didn't stray too far from the aesthetic of the source; though of course the idea of allowing some changes opens the door for the slippery slope of others. The LFE upper limit was soon changed to 125 hz for 70mm, so ESB and RotJ probably wouldn't have this issue, but with SW it would need to be taken into consideration.
The Bluray releases of Alien and Aliens have 4.1 transfers of their 70mm soundtracks, I believe, which I haven't heard but would dearly like to. I've never seen Apocalypse Now, but I have read about it in some detail since it was another pioneering 70mm mix, one of the first to use stereo surround channels. The home video transfers have all used a close adaptation of the original--the 70mm solution for stereo surrounds was to place everything over 500 hz in the same tracks as the LFE, with the lower portions coming from the sixth channel which was still the traditional mono surround (for theatres that didn't have stereo rears). So that film had to be remastered somewhat for home video as well, though reportedly it is still very close to the way it sounded back then.
Without being able to hear the real thing, it's hard to know exactly how much bass Star Wars actually had. On the one hand, it is often reported to have been very strong, but on the other, most movie theatres didn't actually have subwoofers installed when it was first released (though its aural success prompted them to upgrade quickly), and the mixers may have assumed that the bass would have been reproduced from standard loudspeakers instead. Add in the fact that impressions of this kind of thing can be fairly subjective anyway, and that the '93 mix and the two SE's all employ their bass response differently, and the relative obscurity of the topic is made plain. I think I made a pretty good guess of what it ought to be (certainly it sounds wonderful to my ear), but sometimes I almost dread that hearing the real thing would prove me wrong. ;)