Surround sound mixing offers new creative options but also can create new problems. One area that possibly causes more debate than any other is the use (or abuse) of the center channel. Its primary function is to provide hard center anchoring for key components (such as dialog in film postproduction, or lead vocals or solo instruments in music) with greater stability than phantom centering, and without any of the comb filtering problems that occur with phantom centers.
However, relying too much on the center channel alone can be problematic due to the fact that the center speaker in many home theater systems is smaller than the main left and right speakers. As a result, signals routed to the center channel alone can be severely compromised in terms of their frequency spectrum during playback. (Some consumer surround sound systems don’t provide a center speaker at all; however, most consumer receivers provide an option to route center channel information at equal level to the left and right speakers if no center speaker is connected.)
Another problem stems from the fact that most playback systems – even the most rudimentary consumer systems – allow each channel to be heard in isolation. Placing a lead vocal “naked” in the center channel, without other instrumentation to help mask poorly intonated notes, “auto-tuning” glitches, or bad punch-ins, can therefore potentially expose weaknesses in a performance and make your artist and record label very upset.
For these reasons, most surround sound music mixers treat the center channel with caution, rarely if ever using it to carry any mix components exclusively. Instead, those instruments routed to the center channel (most often lead vocal, bass, snare drum, kick drum and instrument solos) are also generally sent to other speakers as well. Placing selected instruments in the center channel and one or both front speakers helps emphasize their sound within the front wall and also aids in localization if the listener moves around the room. As an alternative you can create an acoustic triangle by placing selected instruments in the center channel and one or both rear speakers. This produces an interesting psychoacoustic effect in which the sound appears to come out into the room, closer to the listener. However, care must be taken to decorrelate those signals in each speaker (most often, by slightly altering equalization, delay times or pitch); otherwise, masking and/or phase cancellation problems can occur.
Some surround mixers prefer to leave the center channel dry (free of reverberation), while others opt to add a small amount of decorrelated reverb in order to prevent the signal from feeling too disembodied. If a decision is made to route reverb to the center channel, early reflections and/or reverbs with short delays (i.e., “room” presets) are generally a better choice than long reverb tails.