Just a quick post on this, since this is a rare find.
Natural Tuning vs Un-natural Tuning
Demonstrated with a choir.
17.35 - That which was foisted upon us.
21.25 - Natural tuning
Much is explained in the 26 minute video, but I'll mention you may find the un-natural tuning more titillating. I'll leave it to you to find an adjective for the Verdi (natural) tuning.
Not proposing a slide discussion here, just wanted to share.