*** the Analog Forum answer***
As mentioned by others, this is an analog forum. Analog recordings (with rare exceptions) are limited to 1 or 2 channels. The question of whether it's better to play them back on a 5.1 or 7.1 system is a red herring, since the correct answer is "neither".
Most audiophiles believe that the goal of a playback system is to reproduce as accurately as possible whatever the engineer put on the recording. Therefore, the optimal number of playback channels is equal to the number of channels on the recording:
- Mono recording? Use a 1 channel system
- Stereo recording? Use a 2 channel system
- Multi-channel recording? Use an x.y channel system that matches the channels on the source
In reality, few of us have the space or means to maintain 3 separate systems. Like many, I use a 2 channel system for mono/stereo recordings and a multi-channel (7.2) system for multi-channel recordings. Less than ideal, but a reasonable compromise.
BTW, despite some suggestions above, using fewer channels than are on a recording is no more desirable than using more. In either case, you're not experiencing what the recording was intended to offer.
*** the answer you were probably seeking***
As Mofimadness and Jameswei said...
On a 7.x channel recording, the 2 additional channels vs. 5.x are typically the Back L&R channels. If you play the recording on a 5.x system, those channels will either be dropped or (depending on your surround processor) mixed to the L and R surrounds. Having a 7.x system allows those sounds to come from behind you, which is what the engineer/director intended.
A good example is in LOTR: Fellowship of the Ring, Disc 2, when the black birds swoop down on the Fellowship hiding in the rocks. The viewpoint is a helicopter shot and the birds approach from behind the movie viewer. If you have a 7.x system, the birds come from straight behind you, swoop around both sides and envelop the entire soundfield. The effect is quite startling. Without the 2 Back channels, it's less immersive, less effective.
Of course you need enough real estate. The Back speakers need to be several feet behind your listening position. If your listening seats are backed up against the wall, don't bother. You won't be able to position the Back speakers properly.
