In the previous parts of this mini-series, we have explored the way a synthesiser produces raw waveforms and how the timbre of those waveforms can be manipulated using filters. However, if this were all a synthesiser was capable off it would still be quite an underwhelming device. Enter modulation. Via the use of envelopes and LFOs a synth is able to bring sounds to life, providing them with movement and variation.
An envelope generator is used to automate some aspect of the instrument’s sound over time; most commonly loudness, pitch or filter cutoff. When a musical instrument produces sound, the loudness of the sound produced changes over time in a way that varies from instrument to instrument. The “attack” and “decay” of a sound have a great effect on the instrument’s sonic character. For example, when a guitar string is plucked the sound begins abruptly and quickly fades. By contrast, sounds generated by slowly bowing a violin or cello fade in and out slowly. A synthesiser’s EG can be used to closely mimic the response of acoustic instruments or alternatively, may be tweaked to create interesting and obscure results.
An envelope generally has four stages: attack, decay, sustain and release. (Note: Some envelopes will omit sustain or utilize only attack and release parameters). Attack, Decay and Release are all time-based parameters while Sustain is a level parameter.
Attack is how long the signal will take to travel from its minimum to its maximum. It is the first stage of the envelope and begins as soon as a note is played. Attack can be set to anything from an instant on (i.e the signal reaches its peak immediately) to very long times which fade in slowly.
Decay is how long the signal will take to travel/decay from its maximum to its sustain level. The Decay, therefore, kicks in as soon as the attack stage is complete. A short decay will result in a fast drop to the sustain level while a long decay will slowly fade to the sustain level.
Sustain sets the level the signal will remain at as long as a key is pressed (or some signal keeps the envelope’s gate open). It kicks in as soon as the decay stage is complete. Remember, Sustain is a level control and it’s not defined by time. For this reason its corresponding parameter on the synthesiser sets the amount/level the signal will remain at while the key is held, rather than the length of time it will sustain for. Once the key is released (or the gate closes) the envelope will enter the release stage. There are however some exceptions to this general rule. For example, the Korg MS-20 implements an ADSHR (attack, decay, sustain, hold, release) envelope architecture. The “hold” parameter, could hold notes at the sustain level for a fixed length of time before decaying.
Release is how long the signal takes to travel from the sustain stage to its minimum (return to zero). As the name suggests it kicks in as soon as the key is released (or the gate closes).
These 4 controls work together to modulate various parameters on a synthesiser; the routing is dictated by the synth’s architecture. The Arturia Minibrute for example has fixed envelopes assigned to the filter cutoff and amplifier. By contrast, the Arturia Matrixbrute envelopes can be assigned to nearly any parameter on the synth by way of an input matrix.
Beginner Tip: Envelope Clicks
If the speed of the envelope is set faster than the original sound (i.e. if the envelope cycles faster than the sounds wave cycle) then you may hear a small clicking sound when a key is pressed. To fix this just tune your envelope to have a slightly longer attack and release time. Keep increasing the parameters until the click is gone.
The second type of modulator commonly found in synthesisers is known as the low-frequency oscillator or LFO. Like the oscillators described in Part 1 of this series, an LFO generates a periodically repeating waveform below 20Hz. Because 20Hz is below the threshold of human hearing the waveform produced cannot be heard; instead, it is used to modulate certain parameters on the synthesiser. LFO signals are often used in vibrato, tremolo and other effects. In certain genres of electronic music, the LFO signal can control the cutoff frequency of a VCF to make a rhythmic wah-wah sound or the signature dubstep wobble bass.
Controls often found on an LFO include Rate, Waveform type, Target and Intensity (although this may vary between synthesisers).
Rate is the speed of the LFO in Hz (cycles per second). This is usually a variable control covering frequencies 20Hz and below. LFO rates are typically set around 10Hz or lower. A common setting for vibrato, (essentially an LFO that modulates pitch) is around 6Hz. In some synths the LFO rate can be set to either “free”, meaning it can be swept through the frequency range independently of tempo, or “synced” to a division of the beat based on a set tempo.
Waveform Type, as the name suggests, is a parameter that allows for different waveforms to be used as the source of LFO modulation. Common types include Sine, Square, Sawtooth and Triangle. Most synths also include some form of random generator that is not periodic in nature; some examples include noise, sample & hold and side chain input that allows audio to be used as a modulation source.
Target allows the user to set which parameter on the synth will be modulated by the LFO. Pitch and cutoff are the most common targets. Some synths allow unique destinations to be set based on the architecture of the synth. One example is the Korg Minilogue, which allows LFO control over the shape of the oscillator. In essence, the LFO can be used to periodically morph between various wave shapes produced by the oscillators.
Intensity, sometimes called depth or amount, refers to the impact that the LFO has on its target. If the LFO’s amplitude is zero no modulation will be heard. As the amplitude is turned up, the depth of the LFO’s effect will increase.
To summarise so far…
An Oscillator produces a waveform. This waveform is fed into a filter, usually a low pass filter, which changes the timbre of the sound by attenuating the higher frequency harmonics while passing the lower harmonics. Envelopes are used to modulate a host of parameters in the signal path including the parameters of other modulators like LFOs. The extent to which this is possible depends on the particular architecture of the synth in question.