Roland System 700 Analog Synthesizer Modules
4. The Envelope Generator (ADSR) and the Keyboard
The Envelope Generator (or ADSR) is classified as a "controller." As can be seen by the diagram below, the ADSR is NOT a module that the audio signal passes through. Rather it controls some other module (most typically a VCA, VCF, or VCO) on the synthesizer. As such it does have an impact on the audio signal, but only via the parameters associated with that module.
Similarly, audio signals do not pass through the Keyboard. But control voltages which come from the keyboard are used to control the VCO (causing that module to produce different pitches) or the VCF (causing the cut-off-frequency of that module to "track" the VCO).
The following diagram represents the type of ADSR found on the Roland. It is actually called a "Dual Envelope Generator" because it is made up of 2 identical ADSR modules which function totally separate from each other. For that reason each item is labeled only once on the diagram.
The name ADSR is derived from the fact that, once activated, this module usually goes through 4 successive stages: Attack, Decay, Sustain, and Release. Notice that there are 4 sliders on the ADSR, one dedicated to each one of these stages. Those which refer to Attack, Decay, and Release are marked in terms of time. The slider for Sustain is calibrated for level.
When a key is depressed on the Casio keyboard, two different voltages are sent out of the Roland control box: the Keyboard Control Voltage and the Keyboard Gate Voltage. The Keyboard Control Voltage has already been discussed. It varies from key to key, and at unity (full strength) produces a voltage/octave relationship where playing a pitch an octave higher results in double the output voltage and playing a pitch an octave lower results in half the voltage. The VCO is designed to respond to these changes and produce the proper pitch.
At the same time as the control voltage is sent, the keyboard/control box simultaneously sends a Gate Voltage. This voltage is the same no matter what key is depressed . Its sole function is to control an ADSR. When the ADSR receives a gate voltage it begins to "step through" the sucessive stages and send out control voltages in the following manner:
Attack. In this portion of the cycle the control voltage rises from 0 volts to 10 volts. The speed of this ascent is determined by the attack time slider setting. It can be very fast (almost immediate) or very slow (c. 10 seconds).
Decay. Once reaching 10 volts the ADSR immediately enters the Decay portion of its cycle. In this portion the control voltage descends from 10 volts down to a voltage level determined by the setting of the Sustain slider. The Decay slider sets the speed at which this descent occurs.
Sustain. This portion of the ADSR cycle does not vary in voltage. It remains at whatever level the slider is set as so long as the key remains depressed. This is because the keyboard will continue to send out the Gate voltage until the key is released. At that time the Gate voltage will drop to zero and the ADSR will immediately enter the Release portion of its cycle.
Release. In this part of the cycle the control voltage descends from the level set in the Sustain portion to 0 volts. The Release slider sets the speed at which this descent occurs.
The following diagram illustrates the various stages of the ADSR, their relationships to the control voltages produced, and the relationship of the keyboard gate voltage to the cycle as a whole.
Bear in mind that whenever the key is released the ADSR automatically jumps to the Release stage of its cycle. This means, for example, if the attack stage is long (say 5 seconds) and the key is depressed, held for 1 second, and released that the ADSR attack stage will never get all of the way up to 10 volts and go directly to the release stage where the voltage will drop back to zero.
Similar "irregularities" can occur if the sustain level is set at maximum or zero. If at maximum (10 volts) there really isn't any decay portion of the cycle. Actually there is, but it takes the ADSR from 10volts (at the end of the attack portion) to 10volts at the sustain portion and the result is no different from having an ASR (devoid of the decay portion).
Similarly, if the sustain level is at zero and the key is left depressed long enough for that stage to be reached, when it is released the voltage is already at zero and the release stage has no impact. In this case the decay stage has really become the release portion of the cycle even though it doesn't correlate to the performers key action.
Noting these irregularities becomes useful in creating envelopes which have dual functions.
So far only the slider controls have been discussed on the actual ADSR module. The remaining features will be discussed below.
1. Output jack. The only ADSR output is available as a jack on each module. On the main unit ADSR outputs are hardwired into the control inputs of both VCAs and both VCFs as well as VCO-2. On the auxilliary units they must be connected via patchcords.
2. ADSR trigger select switch. A three position toggle switch which allows the ADSR to be activated from the (a) gate and trigger, (b) gate only or (c) an external source (jack provided for that source). If in the (a) or (b) position the ADSR is activated from the main gate bus. All ADSRs regardless of what unit they are housed in can be activated from this bus. If in the (c) position, some form of gate voltage must be patched into the corresponding jack.
3. Manual gate button. Manually triggers both ADSRs in the module simultaneously. Useful when setting up the ADSR; not used for "performance."
4. Time attenuator switch. Changes the times of the Attack, Decay and Release settings from unity (as indicated on the slider) to 1/10th. Does not affect Sustain level.
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