Oscillators and filters define a sound's tone. Envelopes and LFOs make it move. Movement - change over time - is what makes a synth sound alive instead of static, and it is the difference between a preset you put up with and a patch you designed.
The amplifier
The amplifier controls how loud the sound is. On its own it would simply turn the sound on and off with each note. An envelope shapes it.
ADSR envelopes
An envelope is a shape that unfolds every time a note is played. The ADSR envelope has four stages:
- Attack - time to rise from silence to full level after a key is pressed.
- Decay - time to fall from full level to the sustain level.
- Sustain - the level held while the key stays down. (A level, not a time.)
- Release - time to fade to silence after the key is released.
The amp envelope
Applied to the amplifier, the envelope shapes loudness:
- Short attack, short decay, no sustain, short release - a pluck or a percussive hit.
- Short attack, full sustain, short release - an organ: on and off.
- Slow attack, long release - a pad that swells in and fades out.
- Short attack, medium decay, medium sustain - a typical bass or lead.
The filter envelope
Applied to the filter cutoff, an envelope makes the tone change with each note. A fast attack and short decay on the filter, with the cutoff set low, creates the "pluck" or "wow" at the start of each note - the bright attack that fades to a darker sustain, like most real instruments. The envelope amount sets how far it moves the cutoff, and many synths let you invert it.
Pitch envelopes
A short envelope on pitch creates a quick pitch drop at the start of each note - the click and thump of a synthesised kick drum, or the zap of a laser.
LFOs
A low-frequency oscillator is an oscillator running below the range of hearing - from many seconds per cycle to around 20 Hz. You do not hear it directly; you use it to modulate other controls.
- Shapes - sine and triangle for smooth movement; square for switching between two states; sawtooth for ramps; random (sample and hold) for stepped, unpredictable changes.
- Rate - free in hertz, or synced to tempo (1/4, 1/8 and so on).
- Depth/amount - how much it moves the target.
- Retrigger - whether the LFO restarts with each note.
Classic LFO uses
- LFO to pitch - vibrato.
- LFO to amplitude - tremolo.
- LFO to filter cutoff - wah, wobble, rhythmic filtering. The wobble bass of dubstep is a tempo-synced LFO on a low-pass filter.
- LFO to pulse width - rich, chorused movement.
- LFO to pan - auto-pan.
Velocity and performance controls
- Velocity - how hard you play - can modulate loudness, filter cutoff or envelope amounts, so harder playing sounds brighter and louder.
- Mod wheel - often routed to vibrato depth or filter, for expressive control while playing.
- Pitch bend - usually set to two semitones, or an octave for dramatic dives.
- Aftertouch - pressure after the key is down, on keyboards that support it.
The modulation matrix
Many synths have a modulation matrix: a table where you route any source (envelopes, LFOs, velocity, mod wheel) to any destination (pitch, cutoff, pan, waveform, effect parameters), with an amount for each. It is where simple synths become deep ones.
Glide and voice modes
- Polyphonic - several notes at once, for chords.
- Monophonic - one note at a time, for basses and leads.
- Legato - in mono mode, envelopes do not retrigger when notes overlap.
- Glide (portamento) - pitch slides between notes. Essential for acid lines and many leads.
Four recipes
- Bass - two sawtooth oscillators detuned slightly, sub oscillator on. 24 dB low-pass, cutoff low, resonance low. Filter envelope with fast attack, short decay, moderate amount. Amp envelope fast attack, full sustain, short release. Mono mode.
- Pad - two sawtooths detuned, unison on. Low-pass with moderate cutoff. Slow attack and long release on the amp envelope. Slow LFO on the filter cutoff. Reverb.
- Pluck - one sawtooth or square. Low-pass with cutoff fairly low. Filter envelope with fast attack, short decay, zero sustain, high amount. Amp envelope fast attack, short decay, zero sustain. Delay.
- Lead - sawtooth plus a square an octave up, detuned. Low-pass with cutoff open, a little resonance. Mod wheel to vibrato. Mono mode with a little glide.
Do this now
- Build all four recipes from an initialised patch.
- On the pluck, change only the filter envelope decay and hear how it changes the instrument.
- Add a tempo-synced LFO to the bass filter cutoff at 1/8 note for a wobble.
- Route velocity to filter cutoff on the lead and play softly and hard.