of Microprocessors

VOLTAGE-CONTROL METHODS 91
TRIGGER
GATE
.-J
AR-TYPE
GENERATOR
OUTPUT
SUSTAIN
I
I
I
RETRIGGER
ADSR-TYPE
GENERATOR
OUTPUT
Fig. 3-6. Typical envelope generator outputs
mechanical control. As long as the gate is held high, the envelope remains in
its steady state. When the gate turns off, the envelope makes a decay transition
back toward zero. Again, the decay period is adjustable with a panel
control. Note that the trigger signal is not really needed, since it has no
influence on the envelope output. Such a simplified envelope generator is
called an attack-release (AR) generator.
A more sophisticated envelope generator is the attack--decay-sustainrelease
(ADSR) type. Four parameters define its shape. The slope of the
initial attack and the final decay are adjustable as with the AR type. However,
the initial attack overshoots above the steady-state sustain level. Thus,
the additional parameters are the amount ofovershoot and the duration of the
initial decay from the overshoot level to the sustain level. Note that the
trigger pulse can reinitiate the overshoot cycle even if the gate remains h.igh.
The ADSR shape is preferred, since the initial overshoot resembles the action
ofmany musical instrument types. Also, because ofits retriggering capability,
rapidly repeated notes at the same frequency are readily distinguished. If
desired, the controls can be set to simulate the simpler AR shape. Although
the transitions are shown as linear ramps, typical circuit implementations of
envelope generators usually result in a decaying exponential shape.
Music Keyboard
The most common manual input to a synthesizer is through a standard
organ-like keyboard with a three- to five-octave range. Outputs from the
keyboard are a control voltage and trigger and gate digital signals. The
control voltage output always corresponds to the key last pressed. Depending
on the design, the output voltage with two keys pressed is equal to the lowest
key, the highest key, or some intermediate value. The trigger pulse is
generated on the initial depression ofa key, and the gate is on as long as a key
is pressed.
Most keyboards have three mechanical controls. One is for adjusting
volts per octave and another is for adjusting the voltage level of the lowest