The Swedish dictionary of music 'Sohlmans Musiklexikon' is also in the
camp of Tremolo = AM (amplitude modulation), Vibrato = FM (frequency
modulation), though often coupled with AM.  In its 1st edition of
1948-1952 you find about the rotating valve mechanism of the Deagan
Vibraphone (my translation), "... whereby the tone from the beaten bars
becomes strongly tremulating (sic!) ..."

Same publication, 2nd edition of 1979, has a rewritten text: "...
a rapid variation in the strength of its sound, which is perceived (!)
as a vibrato."

A colleague at my lab, Eric Prame, looked at commercial CD recordings
of 'Ave Maria' by Schubert.  (Eric Prame: "Measurements of the vibrato
rate of ten singers", J. Acoust. Soc. Am. 96 (1994), pp 1979-1984.)
He found their vibrato rates to be narrowly confined in the range from
5.5 Hz (Pavarotti) to 6.7 Hz (Marian Anderson) in the long stationary
notes.  The extent was typically +/- 60 cents.  Three violinists used
the same rate but a significantly smaller extent [modulation depth],
say +/- 25 cents.  (100 cents = 1 semitone).  Eric's pet finding is
that the vibrato rate goes up a couple of Hz during the last few
vibrato cycles of a long note.

The vibrato rate is critical for its artistic value and tends to
decrease with age in a singer, more than once the cause to terminate
a career (by chevrotage = goat's bleating)!

Johan Liljencrants

 [ The frequency ratio between adjacent notes of the chromatic scale
 [ (one semitone) is 1.05946.  For convenience this interval is divided
 [ geometrically into 100 tiny intervals of ratio 1.000578.  So what
 [ does this sound like?  At 'A' = 440 Hz the piano tuner will count
 [ one beat in 4 seconds!  -- Robbie