7.1 is a home theater conceit. Theaters don't need it because they use arrays of speakers on the side and rear walls. The rears are typically fed using Dolby EX. So why do we have 7.1 when the actual audio data is typically 5.1 with a rear channel matrixed into the surrounds? Because single point sources directly behind the listener have a tendency to create a perceived reversal in sound, making them sound like the audio is coming from in front of you instead.
There are several ways around this reversal issue:
1) Stick to 5.1. With proper speaker placement, the matrixed audio in the surround channels will phantom image behind you anyway.
2) Dolby Pro-Logic IIx Music mode on a 6.1 channel setup. DPL-IIx Music creates the rear channel from the matrixed audio in the surrounds in much the same way as Dolby EX does, but during the steering, it leaves part of that rear-channel audio in the left and right surrounds in varying increments. Because of this, the perceived reversal is defeated because the audio is being produced by multiple point sources.
3) Dolby Pro-Logic IIx Movie on 7.1 systems. With two rear speakers, audio placed directly behind the listener doesn't suffer from perceived reversal because the sound is being produced by multiple point sources in equal amounts, making it image directly behind you. For this same reason, DPL-IIx Movie is able to steer audio more precisely across the rear soundstage based on the level and phase of the audio data encoded in the 5.1 bitstream, effectively recreating the sound of an array system.
4) Discrete 7.1 soundtracks. Movies are mixed theatrically for 5.1, with matrixed rears kept in mind since all the mixer has to do is place those sounds equally and in-phase in the surround channels (or out of phase to make them diffuse, in which case EX leaves them in the side surrounds - or in the case of DPL-IIz, can steer that out-of-phase data to the height channels for diffuse ambient playback). Most so-called 7.1 discrete soundtracks are basically created from the 5.1 mix using steering processing (such as DPL-IIx, etc.) to create the rear channel data, then encode fully discrete channels to the bitstream. The end result is essentially the same, which is why 7.1 is so effective if you have the room to properly place the speakers, even when listening to 5.1 tracks and using matrix schemes to create the rear surrounds.
So why aren't movies done in 7.1 in the theater? There's no need to, because the large space and use of speaker arrays makes it unnecessary. Meanwhile, home 7.1 technologies such as DPL-IIx can help to recreate the sound created by a theatrical array system with far fewer speakers, making it easier to get cinema sound in the home.
