Back to Basics (Part 4)
Steve Howell continues our beginner's guide to synthesisers by describing how sounds are shaped by amplifiers and envelope generators.
We've covered how synths generate sound and how that sound can be filtered - now our synth beginner's series looks at how things are shaped using amplifiers and envelope generators.
The third stage in the creation of a synthesiser's output comes under the loose title of 'shaping'. In order that this process can take place, most analogue synths come equipped with two distinct bits of circuitry, a Voltage Controlled Amplifier (VCA) and an Envelope Generator (EG).
The first of these is extraordinarily simple, and the best way to think of it is as a voltage-related volume control. All VCAs work on the principle that the higher the voltage you feed into them, the louder their audio output will be.
Figure 1 shows the block diagram of a typical VCA, and the first thing to notice is the similarity between it and the diagram of a VCF we published last month. The major difference is that, in the normal run of things, the VCA has only one CV input, normally connected to the voltage output of the Envelope Generator.
There are a few exceptions to this rule. Some synths such as the Roland Jupiter 6 have a second input that enables you to modulate the VCA with the output of another controller such as a Low Frequency Oscillator; old ARP synths have a DC voltage available on a slider which allows you to keep the VCA permanently 'open' for indefinite drones; and the new breed of touch-sensitive models such as the Prophet T8, Akai AX80 and Siel DK80 have their keyboards' dynamic voltage routed to another CV input on the VCA. But hang the exceptions - let's concentrate on the average design that gives the VCA one CV input from the EG.
Because of its inherent simplicity and lack of user-variable parameters, the VCA doesn't actually appear on many synth control panels, unless it's at the machine's overall volume control. This stage might consist of just that - a simple potentiometer between the VCA and the synth's output jack - or it might be designed in such a way as to let you regulate the output level by the amount of voltage you send to the VCA's EG control input, in which case you'll find a level control between the two modules. It's unlikely your synth will be fitted with both design options, unless it's a programmable model on which the EG modulation amount can be stored as part of a program, so that you can match the levels of all your own sounds and still have the overall level control thrown in for convenience.
I hope all that has made sense, because I'm going to go straight on to the Envelope Generator without any further ado.
Figure 2 shows the block diagram of a typical one.
Any sound, whether it's the result of a load of synth modules doing their stuff, the gentle breath of a flute, or an embarrassing burst of unexpected flatulence, has an envelope shape of some kind. This shape is best described in layman's terms as the way the sound's amplitude varies during the course of its existence.
Some sounds reach their full volume very rapidly but decay in level with similar speed; others take rather longer to get to their loudest point but will sustain at that level for as long as a note is played. The possibilities are endless, and on a synthesiser, there are (at least) four controls that allow you to regulate the comings and goings of a sound's amplitude changes.
The controls in question are represented by that mysterious set of initials ADSR, and I'll explain each of these in turn.
The time it takes for a sound to reach its full volume is known as the Attack (hence 'A') time, and the appropriate control allows you to vary this time over a range of between, say, a few milliseconds and 10-15 seconds. The Decay ('D') portion enables you to set the time it takes for a sound to fade away to the Sustain ('S') part. Finally, the Release ('R') segment of the envelope lets you predetermine the length of time it takes for a sound to fade away to nothing after your finger has left the key.
If all this has succeeded only in leaving you all the more confused, have a look at Figure 3. This graph actually shows the voltage the EG generates whenever a note on the keyboard is pressed. Note how the voltage rises during the attack phase, falls during the decay portion, remains constant for the sustain time and falls to zero during the release phase. And bearing in mind that the VCA is sensitive to incoming voltages, you should be able to work out that the audio input will get louder as that voltage increases. It follows, therefore, that as the voltage output from the EG rises and falls, so the sound will get louder and softer in accordance with those voltage changes.
And if that's confusing, a quick glance at Figure 4 should rectify matters. You'll see that the sound at the VCA's audio input is at a constant level, but that it becomes the shaped signal appearing at the output once it's been tampered with by the EG's voltage changes. It's that same shaped signal that appears at the output jack of your synth, and Figure 5 shows a few examples of the sorts of shapes that are made available by an ADSR-type Envelope Generator.
Many of you will no doubt be in possession of synths with more elaborate envelope-shaping systems, as the ADSR set-up is by no means the world's most comprehensive. However, all the more advanced systems do is split the envelope into smaller sections so that you can draw a more detailed picture of your sound's amplitude-thru-time pattern.
Only snag is, the EG can't actually generate this variable voltage until it gets the necessary inspiration, and more often than not, this takes the form of a trigger or gate pulse derived from the keyboard. Because at the same time as it delivers a control voltage to the VCOs and VCF, the keyboard also sends a pulse to the EGs.
In fact, a synth's VCOs produce sound continuously, so it's left up to the VCA to shut that sound off completely until it receives the EG's voltage shape, which it won't unless the EG has been triggered by the depression of a note on the keyboard.
But why do you need a trigger and a gate? Well, some sounds require a style of playing in which the whole envelope shape is heard for each new note played, whilst others do not. A pianist effectively re-triggers the entire sound envelope every time he or she plays a new note, but a flautist only has to blow once to play a whole series of notes. To imitate the sound characteristics of the former, you need to create a patch that lets you hear the attack and decay phases for each new note played, but a realistic recreation of a flute sound needs only the provision for a change of pitch during the sustain portion: the attack and decay phases don't have to be repeated. And this is where gate and trigger pulses come in, because whereas the former is responsible for the entire envelope, the latter concerns itself only with making sure that the attack and decay portions are heard each time a new note is played on the keyboard.
The true value of having both these means of triggering simultaneously available probably won't become apparent until you start programming on a regular basis, because it's only then that you'll realise the number of different playing styles it allows you to use.
For instance, legato styles can be easily accommodated by removing the trigger pulse and relying only on the gate for firing the EGs, while more percussive playing styles can be used if both gate and trigger systems are used to initiate the envelope cycle.
True, older synths usually had either one or the other. The Minimoog and Moog Prodigy, for instance, incorporated only a gate system, which meant that percussive phrases were difficult to play unless you employed impeccable fingering: if you didn't, you got some notes coming out without any attack or decay portions. That 'gate only' system was (still is) referred to as Single Triggering, while any synth that uses both gates and triggers to initiate an envelope cycle is said to use Multiple Triggering. Figure 6 shows the difference between the two systems in visually-digestible form.
Most recent monophonic synths incorporate a switch that allows you to select between the two methods, while polysynths employ Multiple Triggering in all modes except mono ones.
Well, if all that hasn't left you utterly dazed and perplexed, collect your souvenir badge on the way out. And if it has, don't despair: it's probably the single most confusing element in the theory of sound synthesis, save the decoding of Japanese instruction manuals. At least we've got it out of the way.
Feature by Steve Howell