Figure 4. A typical reed with three pickups showing their relative
positions. Notice that the time interval for the reed to sweep by the front pickup is very small.
resulting in the generation of two sharp spikes in the waveform for each complete
cycle of vibration.
Here is a Flash animation that illustrates a typical
three-pickup reed in the Wurlitzer electrostatic organ. The reed vibrates
con-tinuously as long as the instrument is powered on. Applying a DC voltage
to any of the three electrodes causes a suitable AC voltage audio waveform
to develop. The electrodes and the reed together constitute three variable
capacitors that share a common plate (the reed). Electrically, however,
they represent three separate capacitors.
Audio signals occur only when voltage is present on one or more pickups.
The amp-litude or strength of these audio voltages also depends on how high
the DC charging voltage is that is applied. If the voltage is suddenly applied,
the resulting audio tone begins virtually instantaneously. If the voltage
is initially high, and then gradually decreases over time, the tone will
likewise begin loudly and then gradually fade out.
If the voltage is high initially and then
fairly quickly drops to a slightly lower level, the resulting signal will
begin percussively. If the voltage is allowed to rise gradually to some value,
the tones begin softly and gradually increase their level.
These properties are all very useful because
they allow the musician, via suitable controls in the instrument's keying
circuits, to control the so-called "envelopes" of the instrument's
tones to a considerable degree. By allowing the voltage to rise almost, but
not quite instantly, to its steady-state value, this feature adds a very smooth
and mellow onset to the tones of the instrument. One of the faults of some
electronic instruments of this era is their so-called "telegraph-key"
tone, where when you push a key, the sound is instantly on, and when you let
go, it's instantly off. This instant-on, instant-off effect is completely
unnatural in that real (non-electronic) musical instruments don't do that.
Even percussive tones such as piano tones or xylophone notes take a slight
amount of time to develop, even if less than a millisecond. Likewise, the
tones of all instruments do not decay instantly but take at least a few milliseconds
to die out completely.
The electrostatic system lends itself to very
easy control of the voltages applied to the reed pickups by means of various
simple resistor-capacitor networks which control the application of the charging
voltage to the various pickups on the reeds. The reeds, by way of their continuous
mechanical vibration, are always "on," in the sense that they are
ready to create AC waveforms as soon as the DC charging voltage is applied.