Modularity v1.00 by Grant Richter
Background: The use of envelopes in electronic music is one of the most important items in electronic music production. Essentially, anytime you grab a knob and twist it, you are producing an envelope for that parameter. Let's call this a "Manual Envelope". In the case of controlling the volume of a tone, you can do this manually by varying the setting of the knob which controls the volume. But there are limitations on speed and repeatability to this approach. We would like to have a device which will automatically generate an envelope and has wider range and is faster than what we can do manually.
A number of problems come up right away, first off, when do we start the envelope? And how do we tell how long the envelope should last? The problem is addressed by what is called a "gate" signal.
The gate refers to the original image of a "sound gate" like a swinging gate which when it is open allows sound to pass, and when it is closed, we have silence. The gate signal itself is a kind of envelope with only two features: it is either "on" or "off" with no in between states, and the amount of time it is on, determines the length of the sound. An example of this is the gate pushbutton on the Wiard manual controller. When it is not pressed, the jack outputs zero volts, when pressed it outputs +10 volts.
This can be used directly to gate on and off sounds using the ZMOD in put on a Wiard Mixolator or the envelope input on any VCA. This is also the volume envelope that corresponds to pipe organ tones which could only switch the flow of air on and off. Electronic music would not have progressed much if this was the only envelope available. One of the simplest things to do is to slow the raising and falling edges of the gate signal with a lag processor (also called a slew limiter). Let's call the time period after the gate has gone on, but the envelope has not reached the gate level yet, the "attack" period.
Let us call the time period where the envelope has reached the gate level and the gate is still on, the "sustain" period. Let us call the time period where the gate signal has gone off, but the envelope has not reached zero yet, the "release" period. Example of this are the simple envelope generators included in the Arp 2600 and the Wiard Classic VCO.
This three part envelope allows us to imitate a much greater variety of acoustic instruments. Virtually all acoustic instruments have a characteristic volume envelope which can be used to describe them.
Some examples:
Marimba - Very short attack, no sustain, short release
Gong - some attack, short sustain, long release
String - moderate attack, variable sustain, some release
The typical range of envelope times in modular systems is from 0.001 second (1 millisecond) to approximately 10 seconds. The range allows synthesis of most acoustic approximations.
The three part envelope "model" covers many acoustic approximations for volume envelopes. There are other parameters that can be varied with an envelope which are important to acoustic simulation. Examples are pitch and timbre. Let us expand on the pitch example. In the synthesis of a skin drum, the pitch of the drum is raised briefly by the pressure of the striker stretching the head. This is a very brief period (in the range of milliseconds) and is an envelope which has a sustain portion too brief to be manually controllable.
It would be useful to have a gate type signal with an extremely short duration to control envelopes of this type. This signal is called a "trigger" signal and has the same voltage levels as a gate signal but the sustain portion is (usually) less than a millisecond. The trigger signal may be derived from the gate signal by using a capacitor to block the sustain portion of the gate signal and pass only the raising edge. Or more complex circuitry may be used as in the example of a keyboard.
The type of envelope which is generated by a trigger signal is called a "Attack - Decay" envelope. During the decay period the gate level may still be on, but the envelope is "decaying" anyway. Putting all of these together, gets us an Attack-Decay-Sustain-Release envelope (or ADSR) which was originally specified by Vladimir Ussachevsky of Columbia University and has become a standard model.
The initial engineering realization of this design used a capacitor to store electric charges, and a small sequencer to switch different resistors in series with the capacitor and different voltage sources to generate this envelope. Due to the mathematics of electric circuits, the capacitor does not charge in a straight line, but rather follows a curve called "exponential". Now this is fortuitous in that the ear responds to a number of musical parameters in an exponential way. One of them is volume, so in combination with a linear acting VCA, the volume envelope produced by this circuit sounds correct.
A difficulty arises when you try to use these envelopes for completely general control. For example, panning a signal is a linear process, and a exponential envelope would spend too much time in one channel. Don Buchla and Serge Tcherepnin put a great deal of thought into the problem and realized that a truly general purpose envelope should use straight line segments (linear envelopes). The idea is to make the INPUTS of the modules respond to a linear control voltage in a way that makes psychoacoustic sense. For pitch and volume, exponential converters are used at the inputs so that the linear envelopes "map" properly.
As it happens, the circuitry used to produce these straight line segments resembles that of an oscillator. And by feeding an "end" pulse back to the trigger input the same circuitry can be used as a low frequency oscillator.
Additionally, the end pulse can be used as a trigger delay without recycling. These designs use only a two section model, Attack and Decay/Release. The more complex ADSR envelope is built up from appropriate routing of the gate and trigger signals, and combining the outputs of two such sections with a control voltage mixer.
Hopefully, the above provides a good overview of the function and history of envelope generators. The envelope generator is truly one of the "core" modules in a modular system, and "you can never have too many"!
A number of problems come up right away, first off, when do we start the envelope? And how do we tell how long the envelope should last? The problem is addressed by what is called a "gate" signal.
The gate refers to the original image of a "sound gate" like a swinging gate which when it is open allows sound to pass, and when it is closed, we have silence. The gate signal itself is a kind of envelope with only two features: it is either "on" or "off" with no in between states, and the amount of time it is on, determines the length of the sound. An example of this is the gate pushbutton on the Wiard manual controller. When it is not pressed, the jack outputs zero volts, when pressed it outputs +10 volts.
This can be used directly to gate on and off sounds using the ZMOD in put on a Wiard Mixolator or the envelope input on any VCA. This is also the volume envelope that corresponds to pipe organ tones which could only switch the flow of air on and off. Electronic music would not have progressed much if this was the only envelope available. One of the simplest things to do is to slow the raising and falling edges of the gate signal with a lag processor (also called a slew limiter). Let's call the time period after the gate has gone on, but the envelope has not reached the gate level yet, the "attack" period.
Let us call the time period where the envelope has reached the gate level and the gate is still on, the "sustain" period. Let us call the time period where the gate signal has gone off, but the envelope has not reached zero yet, the "release" period. Example of this are the simple envelope generators included in the Arp 2600 and the Wiard Classic VCO.
This three part envelope allows us to imitate a much greater variety of acoustic instruments. Virtually all acoustic instruments have a characteristic volume envelope which can be used to describe them.
Some examples:
Marimba - Very short attack, no sustain, short release
Gong - some attack, short sustain, long release
String - moderate attack, variable sustain, some release
The typical range of envelope times in modular systems is from 0.001 second (1 millisecond) to approximately 10 seconds. The range allows synthesis of most acoustic approximations.
The three part envelope "model" covers many acoustic approximations for volume envelopes. There are other parameters that can be varied with an envelope which are important to acoustic simulation. Examples are pitch and timbre. Let us expand on the pitch example. In the synthesis of a skin drum, the pitch of the drum is raised briefly by the pressure of the striker stretching the head. This is a very brief period (in the range of milliseconds) and is an envelope which has a sustain portion too brief to be manually controllable.
It would be useful to have a gate type signal with an extremely short duration to control envelopes of this type. This signal is called a "trigger" signal and has the same voltage levels as a gate signal but the sustain portion is (usually) less than a millisecond. The trigger signal may be derived from the gate signal by using a capacitor to block the sustain portion of the gate signal and pass only the raising edge. Or more complex circuitry may be used as in the example of a keyboard.
The type of envelope which is generated by a trigger signal is called a "Attack - Decay" envelope. During the decay period the gate level may still be on, but the envelope is "decaying" anyway. Putting all of these together, gets us an Attack-Decay-Sustain-Release envelope (or ADSR) which was originally specified by Vladimir Ussachevsky of Columbia University and has become a standard model.
The initial engineering realization of this design used a capacitor to store electric charges, and a small sequencer to switch different resistors in series with the capacitor and different voltage sources to generate this envelope. Due to the mathematics of electric circuits, the capacitor does not charge in a straight line, but rather follows a curve called "exponential". Now this is fortuitous in that the ear responds to a number of musical parameters in an exponential way. One of them is volume, so in combination with a linear acting VCA, the volume envelope produced by this circuit sounds correct.
A difficulty arises when you try to use these envelopes for completely general control. For example, panning a signal is a linear process, and a exponential envelope would spend too much time in one channel. Don Buchla and Serge Tcherepnin put a great deal of thought into the problem and realized that a truly general purpose envelope should use straight line segments (linear envelopes). The idea is to make the INPUTS of the modules respond to a linear control voltage in a way that makes psychoacoustic sense. For pitch and volume, exponential converters are used at the inputs so that the linear envelopes "map" properly.
As it happens, the circuitry used to produce these straight line segments resembles that of an oscillator. And by feeding an "end" pulse back to the trigger input the same circuitry can be used as a low frequency oscillator.
Additionally, the end pulse can be used as a trigger delay without recycling. These designs use only a two section model, Attack and Decay/Release. The more complex ADSR envelope is built up from appropriate routing of the gate and trigger signals, and combining the outputs of two such sections with a control voltage mixer.
Hopefully, the above provides a good overview of the function and history of envelope generators. The envelope generator is truly one of the "core" modules in a modular system, and "you can never have too many"!