It’s not necessarily just dead center. Phantom imaging is any image generated some place between the speakers when they both play the same thing in phase. Or if they play the same thing with a time delay on one of the speakers. It’s only dead center if both speakers are at the exact same level and in phase, which means the comb filtering is at it’s worst. (There are potential combinations of level difference and phase/timing difference that can bring the phantom image back to dead center.) A lot of content has a lead singer or instrumentalist panned dead center, so it makes sense in a lot of ways to move that to a center speaker. The devil is in the details of how to do that, so it’s no surprise that audiophiles have often not been satisfied with the results of a center channel. It depends a lot on what characteristics of the sound they are most sensitive to.
If two or more omnidirectional microphones that are not coincident in location are used to record an orchestra, there will be delay characteristics introduced into the recording that may be considered technically less than optimal. I personally think a lot of the Mercury Living Presence recordings sound great despite this issue. Blumlein demonstrated that two coincident microphones could produce a coherent stereo image that was correct in both amplitude and phase at each of the listener’s ears for imaged sound objects placed anywhere between the speakers, with no delay echoes introduced as the microphones are right on top of each other, just pointed in different directions. Unfortunately it only works perfectly when sounds are hard panned to one speaker or the other. Anything in between will sound like it’s coming from the correct direction, but the tone will be increasingly corrupted by interference patterns, with it being at its worst when the imaging is dead center. The interference patterns are not caused by the microphones, but by the 2 speakers when they play the recording back, and this happens with 2 speakers regardless of how the mics were set up. Fortunately this doesn’t sound as bad as it looks like it should. But still, it doesn’t sound as clear and pure of timbre as does a hard panned sound. So, center channels, and perhaps a couple more speakers between the center and side speakers have desirable potential. The problem is how to make the recording, or how to upmix a good 2 channel recording to more channels without doing more harm than gain.
I learned recently that there are some French multi-channel recordings that were made with 5 microphones spaced apart in a row in front of the orchestra. The intent is to play the recording back with 5 speakers arranged in your room with the same spacing as the microphones. This allows for a proximation of wave front reproduction that a 2 speaker system cannot reproduce at all. Unfortunately it also generates lots of comb filtering/interference from delays when played back, which could be alleviated to various degrees with more microphones and more speakers. 100 channels would probably do a marvelous job of pushing comb filtering/interference patterns above the critical range, but would also be ridiculous to pull off. 100 speakers and 100 amp channels. Yikes! I’ve thought about ordering some of these 5 channel recordings and having a listen, but honestly I’m enjoying 2 speakers well enough and it’s considerably more convenient.
