The center channel also helps by allowing the listener to localize the dialogue differently than the music/FX. When the dialogue comes from a different point in space, it doesn't need to be louder than the music/FX for sufficient intelligibility. When downmixed to stereo or mono or any other non-center-having configuration, the dialog must be mixed well above the music/FX to avoid being masked by it. Sometimes a downmix occurs without this step being given enough consideration, resulting in complaints about the mix, when the original 5.1 (or whatever) mix was perfect but an engineer wasn't involved for downmixing.
This is also what Dave Rat is on about, in addition to IM and combing concerns, when explaining why a speaker per instrument sounds better than a mix: not just that it's a single point source, not just that it's doing one single job, but that all the sounds come from discrete points nowhere near each other. Just like an acoustic band using no reinforcement.
This is also what Dave Rat is on about, in addition to IM and combing concerns, when explaining why a speaker per instrument sounds better than a mix: not just that it's a single point source, not just that it's doing one single job, but that all the sounds come from discrete points nowhere near each other. Just like an acoustic band using no reinforcement.