058 SECTION SEVEN: ADSR AND AR GENERATORS

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To summarize what happens when a key is played on the keyboard: The keyboard generates a trigger pulse which is sent to the EG’s. (As soon as the key is pressed, the keyboard begins generating a gate signal as well.) The trigger pulse causes the EG’s to begin the attack stage. Following the attack stage, they go into the decay stage. Then, if the key is held down, the gate voltage will keep the EG in the sustain stage as long as the gate voltage is present. When the key is released, the gate voltage is in- stantly gone, and the EG begins the release stage, during which it decreases gradually to zero volts. This brings up a rather interesting question: What happens if a key is played, but released before the EG reaches the end of the attack stage? It is possible to set the attack time so long that a key can be released before the attack is complete, but when the key is released and the gate voltage disappears, the EG will immediately jump to the release stage. This is true of releasing the key at any time during the first three stages.

It is also interesting to note that it is possible to program envelopes that have no sustain level at all, and if a key is held down, the EG will stop producing voltage after the decay. However, it is also possible to program a sound which has no sustain, but has a release time. If the key is released before the EG gets to the sustain stage, a release will occur. If the key is released after the sustain stage is reached, no release will occur. This is because of the actual functioning of the release stage.

The release stage is activated exactly when the key is released, and it will cause the voltage to ramp down from wherever it was when the key was released. Remember: release is a setting of time, not a level setting. Thus, the release stage will always cause the voltage to decrease from where it was last being produced rather than decreasing voltage from a set level every time.

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Thus far, the AR generator has received little attention. This is because it is very similar to the ADSR generator. One might ask, “but what about the decay and sustain stages?” In the AR generator, the decay stage is not present. This is acceptable, as it is the least noticeable of all of the stages. The sustain stage is still present; it is just not programmable by the user. It is permanently set to full open.

It is interesting to note that when the sustain stage is set to full open on the ADSR generator that the decay parameter has no effect on the envelope that the EG produces. This is because the decay stage sets the amount of time the EG will take to decrease from the highest point of the attack to the sustain level. When the sustain is set full open, the decay

becomes an early extension of the sustain stage, as illustrated in Figure 7-8. Since the sustain stage in the AR generator is permanently set full open, there is no need to even consider including a decay stage in this module. Although the AR generator has fewer features than the ADSR EG, it is still very important

to synthesists. Figure 7-8: The decay stage is negated by the sustain stage

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The envelope generators on the ARP 2600 are used exclusively as a source of control voltages. They can be used to control the frequency or pulse width of an oscillator or the cutoff frequency of the filter. The ADSR EG is normalled to FM inputs on each oscillator, as well as one of the control inputs on the VCF. When used to FM a VCO, an envelope generator can produce a wild disturbance in pitch depending upon how deep the modu- lation is set. More importantly, the EG’s can be used to raise the filter’s Fc every time a key is pressed, and thus stop or gate the sound when a note isn’t being played. Essentially, the envelope generators are a useful tool whenever one wants to have a voltage contour created whenever a key is played. Of course, there are other ways to cause the EG’s to fire, but their use is generally tied to a key press.

The level or time of each stage of the EG’s is set using sliders which can be seen in Figure 7-9. One will note that the EG’s only have outputs, but no inputs, which means that they cannot be modulated. It is possible to design EG’s which allow voltage control of each stage, but such features are a rare item in a commercially-produced synthesizer.

Observant persons may note that there is a jack and a switch just below the AR generator which has gone unexplained. However, this jack is inti- mately connected to the module which will be discussed in Section 10, so it is best left unexplained until that time.

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Modern developers have changed the EG in many different ways, but the most simple change has been the addition of more stages. Some compa- nies offer DADSR generators, which have a programmable delay time before they begin the attack stage. This is particularly useful if many EG’s are available, as they can all fire at slightly different times after a

key is pressed. Other synthesizer companies such as E-mu Systems have developed a DADHSR EG which not only had the delay stage, but an extra ‘hold’ stage as well. Many modern synthesizers have abandoned the ADSR concept entirely and have begun to just allow users to set four different times with four different levels. Some EGs even have up to eight stages! These super flexible EG’s are explored in depth in the second volume of this series.

Figure 7-9: The ADSR and AR generators