Centre channel derivation using correlation
Audio up-mixers are fairly complicated and the &quot;better&quot; ones are either protected IP or trade secrets.
To extract a center channel you would look at the correlation between the left and right input channels and dynamically steer the correlated content towards the center and the uncorrelated towards left and right. The devil is in the details though: it's tricky to do this without creating undesirable steering artifacts. There are lot of implementation issues tow work out: time constants, energy detectors details, correlation metrics, steering curves, frequency band splicing, etc.
Here is on old patent you can plow through if you are interested: https://www.freepatentsonline.com/7016501.pdf