In this series of tutorials, we will explore the parameters and history of the legendary ARP Odyssey in its current, reissued form. Here, we will discuss the envelope generator.
Text: Ryota Hayashida (Iroha Studio)



An envelope generator produces a control signal that operates according to triggers from the keyboard. The most common type of envelope generator (ADSR) consists of four parameters: A (attack), D (decay), S (sustain), and R (release). The ARP Odyssey provides not only an ADSR but also an AR type of envelope generator.

The AR type produces an envelope curve in which the ADSR’s “S” (sustain level) is fixed at the maximum.

Here I’m going to provide a simple beginner-level explanation of envelopes. An envelope is a signal that applies modulation to the VCO (oscillator), VCF (filter), and VCA (amp) according to when you press and release a note on the keyboard. The most common use is to apply this signal to the VCA to produce volume change. If there was no envelope, the synthesizer will instantly start producing sound the moment you pressed a key, and instantly stop producing sound the instant you release the key. This would be rather boring; we would rather have the sound response more like a real instrument, such as the way in which the sound of a piano has a sharp attack the instant you play the key, a slow decay while you hold down the key, and then a subtle and short decay after you release the key. These time-varying changes in volume and tone are created by the envelope. It’s important to understand that the envelope generator module does not handle the audio signal itself; it’s a “modulator” that applies modulation to the modules that process the audio signal. In other words, a signal such as ADSR is itself a control voltage (CV), and is essentially the same as the direct current produced from a battery.


Let’s get back to the ADSR. “A” (attack) is the time from when the key is pressed until the peak of the attack is reached. When you press a key, the envelope voltage always rises to the specified maximum value. The attack setting specifies the time taken for this rise. When the attack ends, the voltage then decreases over time. This time is specified by “D” (decay). Sustain indicates the level that is maintained as long as you hold down the key. Decay is the time over which the voltage decreases from the maximum value to the specified sustain level, and helps determine how the attack is perceived. “R” (Release) specifies the time from when you release the key until the voltage returns to zero. You’ll notice that while A, D, and R are “time” parameters, only S is a “level” parameter. This means that “sustain time” is the time that you hold down the note, minus the time taken by A and D.


Now we’ll look at some standard envelope assignment methods often used on the ARP Odyssey. With the default settings, ADSR is assigned to the VCA. With more parameters than AR, it provides more flexibility. However in most cases, the envelope is also used to open and close the filter. Thus, let’s assign the AR to the VCA, and use ADSR to modulate LPF FREQ. Using ADSR to close the LPF FREQ will also decrease the volume. The envelope used for the filter typically uses a low sustain to make the filter close. If the ADSR with the same curve is also used on the VCA, the volume would decrease even more. Thus, by controlling the VCA by the AR whose sustain level is fixed at the maximum, we prevent the volume from being decreased even when the filter is being closed. This method gives us a fatter impression.


The switches below the envelope generator control the operation of the triggers produced by the envelope curve.

The typical use is to set both ADSR and AR to “KYBD GATE” so that the envelope operates when you play the keyboard. Next, switching this to “LFO REPEAT” will make the envelope trigger repeatedly at cycles of the LFO. If the ADSR repeat switch in the center is set to “KYBD REPEAT,” the envelope is triggered by the LFO only while you hold down a key, but if it is set to AUTO REPEAT, it is triggered even if a key is not being held down; depending on the settings, this can produce continuous sound as if it were driven by a step sequencer.