Advanced Music Synthesis
Envelope Generators - Part 2
This month, the topic of the workshop is envelope generators, also known by various manufacturers as contour generators (Moog) and envelope shapers or trapezoid generators (EMS). The title 'envelope generators' is the name given to these devices by most manufacturers as they are usually used to generate a voltage which, if applied to the control input of a voltage controlled amplifier (VCA) or voltage controlled filter (VCF), shapes the envelope of any given sound. But before we look at envelope generators let's first see what an envelope is.
The envelope is a major constituent of sound. Every acoustic instrument has its own unique envelope which, even on what might appear to be a simple sound, can be very complex indeed if analysed. On most synthesisers we have the ability to define the attack, initial decay, sustain level and final decay (or release) elements of a sound in the form of an ADSR envelope generator. These devices allow us to get a fairly close approximation of the envelopes found on acoustic sounds, however, they do fall short of the inherent complexities and irregularities of naturally occurring sounds and instruments. There are ways around these shortcomings which I will show later, but first let's have a look at the envelope generator's function and how it works.
Basically, the envelope generator is a device that puts out a varying voltage every time it receives an incoming trigger or gate pulse. This pulse is usually derived from the keyboard so that every time a key is depressed, as well as sending a control voltage to the voltage controlled oscillators (VCOs), the keyboard also sends a pulse to the envelope generator(s) so that the note can be shaped (or some other task can be performed). The basic patch for a simple envelope shaping system is shown in Figure 1a. The four controls associated with the envelope generator can now be used to define the shape of the note or sound. These controls are:
Attack — This control allows us to vary the time it takes for the note to reach full volume.
Decay — This control determines the length of time it takes for the note to die away to the sustain level.
Sustain — This control sets the volume or level at which the decay is interrupted while a note is held.
Release — This control determines the time it takes for the sound to finally die away after the key is released.
Whenever a pulse is received from the keyboard (or wherever) the voltage output of the envelope generator rises and falls in accordance with the settings of these four controls as in Figure 2a. If one considers that a VCA will not allow any sound to pass through it until the voltage applied to its control input increases, you should be able to see how it is possible to shape the amplitude envelopes of your sound. Figure 2b gives examples of possible envelope shapes and, as I'm sure you can see, there are many permutations.
Apart from simply increasing and decreasing in volume, a sound will also vary its harmonic content throughout its duration. This is where the VCF comes in. By applying the output of the envelope generator to the control input of the VCF, as in Figure 1b, it is possible to create many interesting and dynamic sounds which have a continuous change in tone and volume throughout the course of the note. Many synthesisers have two envelope generators one assigned to the VCF and the other to the VCA (as in Figure 1c) which allows a greater range of sound variation than on synths having only one envelope generator. I'm sure most of you are familiar with the various effects that are possible by applying the envelope generator to control the VCF as this was discussed in the previous article on filters.
If you have a modular synthesiser such as the ARP 2600, Roland System 100M or 700 then it is possible to route the outputs of two envelope generators to the control inputs of the VCF as shown in Figure 3a. With careful setting of the respective controls many varied and interesting sounds can be produced which resemble more closely the complex envelopes of acoustic instruments. One such envelope is given as an example in Figure 3b. If applied to the control input of the VCF which is being fed with a sawtooth wave, it could be used to successfully imitate a French Horn, where the initial attack/decay cycle is followed by an increase in tone. By experimenting with the various envelope generator controls many more 'animated' sounds can be produced than if you had used only one envelope generator.
Another option available to those of you with modular systems is to route the various VCOs through independently shaped VCF/VCA combinations as in Figure 4a. This patch can be used to create a variety of sounds. For example, tune VCO1 an octave and a fifth above VCO3 and give it a very short, 'plucky' envelope. Now tune VCO2 an octave above VCO1 and give it a slightly longer envelope than on VCO1. Finally, VCO3 should have a longer envelope than VCO2. By using sine or triangle waves or by using filtered square waves you will be able to produce some very realistic marimba, xylophone and celeste sounds amongst others. What is actually taking place here is that VCO1 and VCO2 (the high frequency content of the sound) die away leaving only VCO3 to sustain a little. The end result being more natural-sounding.
However, if you don't have a modular synthesiser but have access to a few monophonic synths with control voltage and gate/trigger inputs and outputs, you could try experimenting with this technique by hooking them up as in Figure 4b. Those of you with a poly-synth capable of layering two sounds together might also like to try this. If you want to hear good examples of how multiple envelope generators can be used to create a more 'animated' sound then I recommend you take a listen to Wendy Carlos' version of the Brandenberg Concerti. Carlos said, in a lecture in America some time ago, that she finds multiple envelope shaping far more musically creative than the usual approach, which is to use multiple oscillators through a single envelope generator. She can use up to eight envelope generators on one sound alone which probably accounts for the fact that the music she creates is much more 'alive' and dynamic than a lot of other synthesised music.
Envelope generators need not only be used for tonal or level shaping, however. They can be used for a variety of other control purposes such as pitch bend if routed to the VCO(s). The output of an envelope generator can also be applied to the CV input of a low frequency oscillator (LFO) to enable vibrato or other modulation effects to be produced that speed up or slow down as the note either sustains or dies away. Many interesting synth sounds and realistic string sounds can be created using this technique. An extension of this is to route the output of the LFO through a VCA which is being controlled by an envelope generator (as in Figure 5) so that every time a note is played, the modulation effect fades in (or out if the inverter is omitted) thereby creating delayed vibrato and modulation effects that are dependent on the settings of the ADSR controls.
So far, we've only looked at fairly standard ADSR types of envelope generators but some manufacturers provide envelope generators which are more flexible. RSF, a French firm, for instance, provide the facility to voltage control the attack, decay and release times. Many things are possible with this feature by routing the keyboard voltage to control the envelope so that the various times of these parameters can vary across the range of the keyboard in much the same way as on an acoustic instrument. Many of the latest polysynths have this feature 'onboard' in the form of a Key-follow switch associated with the envelope generators; so those of you with a Jupiter 8, Memory-Moog, Jupiter 6 or DX7 may like to experiment with this.
By using a velocity sensitive keyboard and assigning the velocity voltage output to the control voltage input of, say, the attack time, one could vary the attack of the sound simply by hitting the key a bit harder or softer. This is a very expressive technique that until recently has only been available on the large modular systems but now the Rhodes Chroma, Prophet T8 and RSF Polykobol have this feature.
Other envelope generators worth looking at are made by an American firm, Serge. They make a device known as the 'dual universal slope generator'. As well as functioning as an envelope generator the device can be patched up to act as a VCLFO, a portamento device that can be voltage controlled, a VC trigger delay, an oscillator, a sub-harmonic series generator and a fairly primitive lowpass filter. As if this weren't enough, there's a separate trigger output that fires when the unit has reached the end of it's cycle (when being used as an envelope generator). This can be used to trigger another slope generator. In this way, it is possible to produce multiple stage envelopes that allow far more sound variations than with the ordinary four stage ADSR type unit.
That just about concludes this workshop, although we will be looking at envelope shaping using other modules in future articles. Next time, as a logical follow on to this month's article, we will be looking at trigger and gate pulses and their application.
Feature by Steve Howell
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