What is the matter with people? I am reading on YouTube--probably a mistake--a nonsensical discussion that has to do with two kinds of trills. Jeeze. Then, of course, they rant and rave against whoever is performing incorrectly this thing they have invented.
The voice of any opera singer--maybe not Emma Kirby, but everybody else--fluctuates in pitch. It does this ALL THE TIME on every note. The brain consolidates this into a single pitch somewhere near the center of the fluctuation. Your brain may put these higher or lower than someone else's brain, giving you the idea that they are flat or sharp when they are in fact both flat and sharp at the same time. And they are both flat and sharp at the same time on every note they sing. This is called a vibrato, and it is the primary feature of opera singing.
The brain does this consolidating until the fluctuation exceeds about a half step in extent, at which time the listener hears a trill. Your brain consolidates the fluctuation into a single pitch or it doesn't. There is no third possibility. To be most effective as a trill the fluctuation needs to be clearly wider than the normal vibrato.
There seems to be something about the internet that encourages the arrogant ignorant.
If the singer gets old, their vibrato may slow down. At this time it becomes a wobble. The trill effect is also distorted if the fluctuation is too slow.
The rich and famous are chosen in part because their vibratos are
pretty. A vibrato can just as easily be ugly. It is also possible that
the precise same vibrato will seem pretty to some and ugly to others. Your opinion of any given singer's vibrato (and by extension trill) did not drop from heaven. It's just your opinion.