Page 66
LP Stage II User’s Manual - Appendices
Page 67
LP Stage II User’s Manual - Appendices
Returning to our Subtractive Synthesis model, the first of the auxiliary components is the keyboard. The
keyboard provides a familiar musical instrument ‘interface’ that produces a control voltage and trigger signal
whenever a key is pressed. The level of the control voltage signal is a function of which key is pressed - the
higher up on the keyboard you play, the higher the level of the control voltage.
The keyboard’s control voltage signal is commonly routed to the oscillators to control the pitch, and it can also
be routed to other voltage-controlled components like the filter, to vary the cutoff frequency. The keyboard
trigger signal is routed to the Envelope Generators to trigger the envelopes.
The second
auxiliary component is the Envelope Generator, or EG. The
EG makes no sound by itself. Rather, it creates a time-varying control
voltage that is typically used to control the gain of the amplifier, or the
cutoff frequency of the filter. Many synthesizers, including the Little
Phatty, provide several EG’s for independent envelope control of the
amplifier and filter circuits.
The EG is triggered from a Gate signal that is generated every time a
key is pressed on the keyboard. Once triggered, as long as the key is
held down (i.e. the Gate signal is present), the EG envelope will evolve
according to the control settings.
The LP’s Envelope Generators have four stages that can be set individually:
Attack – The time to go from zero volts to the maximum voltage (the fade in time).
Decay – The time to go from the maximum voltage to the Sustain level.
Sustain – The maximum level of the envelope after completing the attack and decay stages (if the
key is held). If the sustain time is zero, the envelope consists of just the attack and decay
stages, and the Release control has no effect.
Release – The time to go back to zero volts when the key is released (the fade out time).
The last auxiliary component is the Low Frequency Oscillator, also known
as the LFO. The LFO operates like the main oscillators in almost all
respects, but generally at a much lower frequency. LFO’s are typically used
to send modulation control signals to the main components. For example,
if you route a 6Hz LFO signal to an oscillator, it will produce vibrato by
varying the pitch of the oscillator. If you send that same LFO signal to
the amplifier, and you’ll get tremolo. LFO’s are used to create cyclical
variations in the sound, making the sound more dynamic and interesting.
