This was a very interesting discussion four years ago and I'm glad it got revived. A lot of things could have changed under the hood since then so I'm curious if my thoughts and guesses then are still valid now.
First, since the RT's are a main part of PGM's "Secret Sauce", I don't think they're going to release all the goodies that go into producing a Real Track, these user templates are dumbed down versions.
Second, that list of chords that Biab recognizes is for midi, not necessarily the RT's. We all know RT's are recorded by a real person playing that instrument in a studio. If s/he didn't play a proper 6/9 chord then that chord gets ignored if you're using that RT. Or maybe it gets subbed with a standard major 6th or 9th.
Third, the file sizes. This is part of the secret sauce I think. To me it's logical to assume there is a max file size for an RT because if there wasn't and somebody produces a massive piano RT that has all of those chords in the list played in all four keys so they can be properly transposed, the generation times would be huge, like measured in minutes not seconds.
And even if that massive RT was produced, what I described is only one chord inversion in one position on the piano and it's the same for guitar. You should have at least two different inversions but three or four are really needed to cover jazz properly so multiply that list of chords by one of those factors.
For any noobies, understand this is no problem with midi. We have printed out and discussed midi piano tracks generated by Biab and they are really good. Lots of different chord positions, left hand voicings etc and all based on the style. Not so with the Real Tracks and I'm pretty sure it's because of file size limitations.
Fourth, I don't think it would do any good to take an RT soloist and run it through the ACW because soloists are plying single notes and the ACW doesn't work well with single note lines, it needs the bass and a chording instrument.
Just some thoughts.
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