My own random thoughts I just dump all over these forums. No one has objected so far. Well, at least no one I care about.
As for your comparisons of DTS:X top middle speaker vs front height speakers, I am guessing that angle of altitude of the sound object will be the deciding factor in how much use that gets, and also azimuth. You will have a lot more content nearer to the front stage than you would directly overhead. I bet if you contacted DTS and asked them about this, that is what they would say.
That's true, but with atmos content height information is always mixed into the height channels and the respective bed channel in order to portray the correct imaging regardless of speaker configuration. Overall it just works and sounds much better with top middle speakers. Think of it like a 6.1 system playing back 7.1 content, the best way to handle a sound coming from the left rear would be to mix it into both the left surround and the single surround rear, not just the left surround, this is exactly what a down mixer does.
Either way, you're not likely to get much overhead sound at all with dts x using top middle speakers in a 5.1.2 setup, which is why I suggest utilizing a front height configuration. I also think front height speakers do a better job of portraying height information when only one set of height speakers are used anyways, even with atmos. Humans aren't that good at localizing sound directly overhead at 90 degrees, but that's a topic for another article.
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