Chewtobacca said:

Jonno said: And it's otherwise a strong sounding track - impressive bass on the Nostromo landing sequence, particularly in comparison to the 2.0 surround track (which presumably comes from the 35mm stereo mix).

I've never understood how a presumably pure stereo track can become a 2.0 surround track. Where does the matrixed surround sound come from if the source is two channel?

The surround channel (and the centre channel) are matrix-encoded into the left and right channels. You sometimes see these referred to as Left Total (Lt) and Right Total (Rt). A Dolby Pro-logic decoder extracts the additional channel information to retrieve the 4 channels L, R, C, S.

I also don't understand why the other mix on the BD is 4.1 and not 5.1. If it comes from a six-channel mix, why is it not 5.1?

Early 70mm 6-track mixes were actually 4.2 - 3 front, mono surround and 2 LFE channels. You can't encode 2 LFE channels into AC3 hence it becomes 4.1. However, the quote in Jonno's post above suggests the mix came from the 4 channel master used to make the Dolby Surround mix (which had no LFE channel - so I don't know where the .1 came from).

Later 70mm 6-track mixes used a single LFE channel and split surrounds, the same configuration used in Dolby Digital 5.1 mixes today. According to Jambe's filmumentary, Raiders of the Lost Ark used split surrounds, but this is not shown in the list on in70mm.com.