Hammond Model A, Manual, Page: 285 / 298

5-14. VIBRATO EQUIPMENT.
5-15. The vibrato effect is created by a periodic raising and lowering of pitch, and
thus is fundamentally different from a tremolo or loudness variation. It is
comparable to the effect produced when a violinist moves his finger back and forth
on a string while playing, varying the frequency while maintaining constant volume.
5-16. The Hammond Organ vibrato equipment, as shown in simplified block diagram,
figure 1 in the Vibrato and Percussion section, varies the frequency of all tones by
continuously shifting their phase. It includes a phase shift network or electrical
time delay line, composed of a number of low pass filter sections, and a capacity
type pickup or scanner, which is motor driven so that it scans back and forth along
the line.
5-17. Electrical waves fed into the line are shifted in phase by each line section
(the amount per section being proportional to frequency), so that at any tap on the
line, the phase is retarded relative to the previous tap.
5-18. The scanning pick-up traveling along the line will thus encounter waves
increasingly retarded in phase at each successive tap, and the signal it picks up
will continuously change in phase. The rate at which this phase shift occurs
will depend on how many line sections are scanned each second.
5-19. Since a cycle is equivalent to 360 electrical degrees, a frequency shift of 1
cycle occurs for each 360 electrical degrees scanned per second. For example, if the
scanner passes over the line at such a rate that 3,600 electrical degrees are
scanned each second, there will be a frequency change of 10 cycles.
5-20. For the widest vibrato, the whole line is scanned from beginning to end in
about 1/14 second, and this rate of change of phase causes about 1-1/2 percent
decrease in frequency. Note that the frequency remains constantly 1-1/2 percent low
as long as the moving pick-up retards the phase at a constant rate.
5-21. Since the pick-up sweeps from start to end of the line and then back, it
increases the frequency by an equal percentage on its return trip, the average
output frequency remaining equal to the input frequency. The exact amount of
frequency shift depends not only on the amount of phase shift in the line but also
on the scanning rate. This rate, however, is constant because the scanner is driven
by the synchronous running motor of the organ.
5-22. The degree of vibrato (or amount of frequency shift) may be varied by a switch
which causes the whole line to be scanned for No.3 (wide) vibrato, about half of it
for No.2, and about one-third for No.1.
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