With the proliferation of inexpensive home theater sound systems and the easy access to surround sound authoring tools, theater-style mixing has become a viable option for the home studio musician. Although the 5.1 format is designed for the traditional cinema approach, with a center channel for dialogue and a subwoofer for effects, the creative musician or sound artist can also subvert the medium for other purposes. For example, in the past, electronic musicians wanting to compose in four-chanel sound have had to struggle with expensive or hard-to-come-by technology. Now it's a simple matter to author a DVD that can play four channels into a quad concert sound system, and even do a reasonable job in many home systems. With a little ingenuity, the 5.1 format can be used for true six-channel sound or for things like stereo sound with multiple click tracks.
For those working more traditionally, several conventions should be noted. First of all, one should set up one's monitoring system in accordance to AES standards. Left and Right speakers are each 30° from center, as in stereo; and the surround speakers are about 110° from center, rather than being true rear speakers. Generally, the center speaker is meant for sounds like dialogue, which benefit from the clarity of a single source. For this reason, one can adopt two different approaches to panning in front. One can pan from left to center to right, for more detailed positioning of virtual sources; or one can pan from left to right, reserving the center for special sounds.
The LFE channel (the .1) often causes confusion. Traditional practice is to use this for low frequency effects only, like earthquakes and explosions. For that reason, compression formats often use a 120Hz low-pass filter on this channel. The playback hardware (subwoofer speaker) serves two purposes, however. The subwoofer does receive the LFE signal, but it usually also serves as a common subwoofer for the low frequencies of all the other speakers. This is because, although the other five positions are theoretically full-range, for practical reasons most sound systems have only one subwoofer, and a system called bass management mixes the low-frequency content of the five speakers together with the LFE signal. Note that, although some software environments and plug-ins offer bass management, it is best to do it in the sound hardware (audio pre-amp or subwoofer electronics), to avoid the risk of doing it twice and since it is, after all, hardware-dependent.
Fold-down compatibility is another consideration. Just as a stereo mix should sound acceptable in mono, it is generally accepted that a 5.1 mix should work when folded down to stereo. Recently there has been somewhat of a trend to provide separate surround and stereo mixes, making fold-down compatibility less important and opening up the possibility of more interesting spatialization effects. However, with the advent of HD Broadcast and the quickly approaching HD DVD, bandwidth restrictions seem to be swinging requirements back to a single downmixable stream only. These broadcast compatibility concerns should make one carefully consider whether to abandon fold-down compatibility completely in choosing an UpMix algorithm. We strongly suggest getting a clear decision on fold-down compatibility (from your client) before you begin a mix.
Cycling '74's UpMix plug-in suite can be used not only to repurpose stereo recordings for traditional theater-type mixing, but also as a rich set of tools for all types of 5.1 mixing.