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Hammond Vibrato

(Excerpted from the OriginalHammondLeslieFaq.)

Provided on the organs so equipped are vibrato and chorus settings V1,V2,V3 and C1,C2,C3.

SpinetOrgans, at least the M series, have only two vibrato/chorus depths: small and normal --- these correspond approximately to the '1' and '2' settings on ConsoleOrgans.

Vibrato is the periodic raising and lowering of the pitch, and is thus fundamentally different from tremolo which is a variation in only the loudness of the pitch. The hammond vibrato is implemented using a tapped DelayLine: really a low-pass filter. The signal is applied to the DelayLine and a rotating Scanner, attached to one end of the ToneGenerator assembly, picks the signal off of the DelayLine at the tap points. The Scanner, a single-pole 16-throw air-dielectric capacitor switch, is wired so that the tap point will traverse the entire delay line twice, once up the DelayLine and once back down, for each Scanner rotation. As the DelayLine is traversed phase is added-to and then subtracted-from the signal.

The chorus signal is produced by adding non-pitch-shifted signal to the pitch-shifted signal.

The three settings each of vibrato and chorus correspond to different amounts of total delay thus different amounts of total pitch shift.

In addition to the pitch shifting function, the vibrato, as implemented in the Hammond organ, also acts as a sweeping low-pass filter. There is some frequency response and amplitude variation as the tap point of the filter is swept.

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