Main and Echo refers to having two speakers one the main and the other a secodary. You can chose to activate one or both speakers at will.
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 delay line, really a low-pass filter. The signal is applied to the delay line and a rotating scanner, attached to one end of the tone generator assembly, picks the signal off of the delay line 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 delay line and once back down, for each scanner rotation. As the delay line 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 chorus give the impression of "fattening up the sound".
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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