Subtractive Synthesis Part 3

Robbikal Adlim Friday, February 8, 2013

For this part of the tutorial, it is definitely best if you follow along with a subtractive synth of your choice. I have provided some examples for each part, but nothing beats learning the principles by actually manipulating the sound and tweaking the controls yourself.

What is modulation?

"Modulation" is a fancy word for a very simple concept. To say that "A is modulating B" just means that the behavior of A over time is controlling the behavior of B over time. Envelopes are used to modulate settings of subtractive synths from the time when a note is struck to when it is released.

What is an envelope?

An envelope is a part of a synth which controls the behavior of some characteristic of the sound that your synth produces from when a note is first struck to when it fades out completely. Most subtractive synths have at least three envelopes built in: an amp envelope, a filter envelope, and a modulation envelope. Each of these envelopes typically has at least four controls, often referred to as "stages" or "phases." These four controls are attack (A), decay (D), sustain (S), and release (R); because of the abbreviations used for these four elements, envelopes on subtractive synths are often called "ADSR envelopes." Now to explain the function of each type of envelope:

Amp Envelope

When you play a note on your synth, the amp envelope controls the volume of the resulting sound over time. The attack controls how long the sound takes to reach its highest volume once a note is struck. A shorter attack time will give you a "punchier" sound, while a long one will give you a sound that "fades in." The decay controls how long the sound's volume takes to drop down to the sustain volume. The sustain volume is the volume maintained when you hold a note after the sound has gone through the attack and decay "phases." How long the sustain phase lasts depends on how you long you hold down the note(s) on your synth. If you have the sustain set all the way at the top, the "held note" volume will be the same as the "attack" volume.

If you are following along with your own synth, try to imitate the volume characteristics of each of the above files by adjusting the ADSR controls on your amp envelope.

Sounds that typically have short or short-medium attack times are Synth "stabs," keyboard sounds, basslines, any synth used in a "percussive" manner, and some pad sounds. Sounds that typically have medium or long attack times are synth "washes," noise sweeps, bass that gradually fades in, and pads.

Filter Envelope

The filter envelope controls what the cutoff frequency of your synth's filter does as a note is struck and then held. Here the attack setting controls the amount of time that passes before the filter cutoff reaches its highest frequency and the decay setting controls how quickly the filter cutoff decreases after hitting its highest frequency. The sustain setting controls the frequency of the filter as you continue to hold the note down after the attack and decay phases have ended. The release setting controls how quickly the cutoff frequency of the filter goes from the sustain level back to nothing again. Here is an audio example to give you some idea of what a filter envelope does:

ADSR filter envelope applied to a saw wave

In the above file, the filter envelope has a long attack time and a short decay time, so it takes a while before all the frequencies are let in, but then they get cut out quickly during the quick decay phase. The sustain is at a medium level, so the filter is cutting out and leaving in pretty even amounts of frequencies. The release time is short, so once I let go of the note, the cutoff frequency drops right away.

Modulation (Mod) Envelope

Unlike the amp envelope and the filter envelope, the modulation envelope can be "assigned" by you to control different settings on your synth, and on some synths it can even control more than one setting at a time. Typical parameters for the modulation envelope to control can include the following:

Oscillator pitch: If you assign the mod envelope to the pitch of an oscillator, the oscillator changes the pitch played from the time a note is pressed to when it is released according to the ADSR model explained above.

Oscillator mix: If you have more than one oscillator playing at once, the "Mix" setting on the mod envelope can be used to vary the volume levels of two or more oscillators relative to one another.

FM: Many subtractive synths allow you to use the frequency of one oscillator to modulate another waveform; on the mod envelope, the FM setting can be used to vary the level of frequency modulation over time. FM will be discussed in the next part of this series.

A second frequency filter: Many subtractive synths have a second frequency filter that acts upon the sound which results after the first filter has been applied to the raw sound. With the mod envelope, you can control the behavior of the cutoff frequency of this second filter.
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