Tsk, tsk, Robbie --  Ya got it bass-ackwards!  Tremolo is AM (amplitude
modulation) whilst vibrato is FM (frequency modulation).

Regarding the Mills Violano vibrato, I know very little about these
instruments, but as to whether the frequency varies above or below,
or both, about a centre frequency can be easily checked with a strobe
or equivalent.  It could very well be that the tuning weights may
bounce a little under the influence of the shaking action of the
vibrato mechanism causing the pitch to both rise and fall at the
vibrato rate.

Jim Heyworth,
Sechelt, B.C., Canada
mailto:james_heyworth@sunshine.net