Since a cycle is equivalent to 360 electrical degrees, a frequency shift of one
cycle occurs for each 360 electrical degrees scanned per second. For example, if the
scanner passes over the line at such a rate that 3600 electrical degrees are scanned
each second, there will be a frequency change of 10 cycles.
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 l ½ decrease in
frequency. Note that the frequency remains constantly l ½ low as long as the moving
pick-up retards the phase at a constant rate.
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.
The degree of vibrato (or amount of frequency shift) may be varied by a
switch (not shown in figure 1) which causes the whole line to be scanned for
#3 (wide) vibrato. about half of it for #2. and about one third for #1.
A vibrato chorus effect, similar to the effect of two or three slightly out-of-tune
frequencies mixed together, is obtained when the vibrato output signal is mixed with
a portion of signal without vibrato. For vibrato chorus, part of the incoming signal
appears across the vibrato line and the rest across a resistor in series with the
line. As the vibrato effect is applied to the part of the signal appearing across
the line but not to the part appearing across the resistor, the combination produces
a chorus effect. For normal vibrato, this resistor is short-circuited.
In selective vibrato consoles the vibrato effect can be applied to either
manual separately or to both at once.
CONSTRUCTION OF COMPONENTS
Figures 2 and 3 show different models of the vibrato line box. Each of the air core
inductance coils is connected with one or more condensers to form one filter
section.
Figure 4 shows the construction of a typical vibrato switch. Some models differ in
wiring and number of contacts, but all are similar in mechanical arrangement.
The scanner (figure 5) is mounted on the main generator synchronous motor and driven
at 412 revolutions per minute. It is a multi-pole variable condenser with 16 sets of
stationary plates and a rotor whose plates mesh with the stationary ones. In figure
5B two sets of plates have been removed to show the rotor.
Signals coming from the line through the vibrato switch appear on the stationary
plates and are picked up, one at a time, by the rotor. Connection to the rotor is
made by carbon brushes as shown in figure 5A. Two brushes touch the sides of the
contact pin and a third presses on the end, in order to eliminate the possibility
of contact failure.
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