Your dealer gets to sell you an extra pair of speakers.
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A 7.1 channel system incorporates all the elements of a 5.1 channel system, but instead of combining both surround and rear channel effects into two channels, a 7.1 system splits the surround and rear channel information into four channels.
In other words, side sound effects and ambience are directed to left and right surround channels, and the rear sound effects and ambience are directed to two rear or back channels. In this set-up the surround speakers are set to the side of the listening position and the rear or back channels are placed behind the listener.
One simple benefit is that it can provide a more immersive experience. Like when a live audience applauds at the end of a performance, some clapping comes from the rear, helping to create the impression that you are in the performance hall.
During the performance, some echo might be audible from the rear, suggesting that you are in a large hall and some sound is reflecting off the back wall.
And of course for movies, the back channels offer the opportunity for sounds from behind, like explosions or the snap of a twig as someone approaches you from behind.
*** 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.
It's not rocket science; when you go to Forums, you see several discussion groups, including Home Theater, Digital, Speakers, Analog, Music, etc. Since surround sound has been in the digital doman for about two decades and analog generally refers to vinyl records and open reel tape, the logical forum for 7.1 surround sound would be home theater or digital, It's not like you needed an official memo to sort that one out.