A virtual center (just the right and left speakers) works well if you just have a single seat (where one can get a decent center image from the pair). Once you have several people or sit off center … the center action/dialog will not be centered. To correct this, to anchor centered sounds to the center of the screen, the center channel was created. Since most screens (TVs) are not acoustically transparent (AT) the center is put above or below the screen (usually below). The problem here is as screens get bigger you once again notice that the sound is not coming from the correct place … but from below the screen. An AT screen allows you to put the sound exactly where intended.
Note that this can be taken even further…. If your screen is wide enough you might have 5 speakers behind the screen (L, LC, C, RC, R). Of course you need a fairly sophisticated prepro to drive it.
Note that a center channel is really manufacture speak for a main channel optimized to be placed under your tv.
