Aren't
they called 'quadrature' mice or something like that?
Exactly. each has
two outputs, spaced slightly out of phase with
each other.
More than slightly. Most versions ideally place them 90 degrees out of
phase with one another. This has two advantages: (1) it maximizes the
minimum time between seeing one edge and seeing the next, and (2) if
you count all four edges per cycle instead of two or one, using 90
degrees of phase shift gets you more regular counting.
(1) is admittedly not very relevant if you're using a real mouse and
even vaguely modern electronics, since on even a high-res mouse edges
are very far apart on digital logic timescales.
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