Synthesizer Basics: Oscillators, Filters, Envelopes and LFOs

A synthesizer can look intimidating: rows of knobs with names like "cutoff", "resonance" and "LFO rate". The secret is that almost every classic synth follows the same simple signal path. Once you understand four building blocks, you can walk up to most synthesizers, hardware or software, and know roughly what each knob does.

The best way to learn this is with your ears, so keep the online synthesizer open in another tab and try each idea as you read.

A very short history

Electronic instruments existed earlier, but the synthesizer as we know it took shape in the 1960s. Robert Moog built modular synthesizers: cabinets of separate modules connected with patch cables, each one generating or shaping sound. Wendy Carlos's album Switched-On Bach (1968), played on a Moog modular, showed a wide audience what these machines could do.

Modular systems were large and expensive, so around 1970 Moog released the Minimoog, a compact, portable synth with a fixed signal path and a built-in keyboard. Musicians could carry it on stage, and its layout of oscillators, filter and envelopes became a template that countless later synths followed. That template is called subtractive synthesis.

Oscillators: where the sound starts

An oscillator produces a repeating electrical wave. How fast it repeats sets the pitch; the shape of the wave sets the basic tone colour. Four classic waveforms cover most needs:

To hear the difference clearly, play each wave on its own at the same pitch. The online tone generator is good for this, because it lets you hold one steady tone and switch waveforms. Many synths, including the one on HarmonyHub, have two oscillators. Detuning one slightly against the other makes them drift in and out of phase, creating a thicker, chorus-like sound.

Sine: a smooth, pure tone with no extra harmonics. Soft and round, like a whistle or a tuning fork.
Triangle: a little brighter than a sine, with a few gentle odd harmonics. Flute-like and mellow.
Square: hollow and woody, rich in odd harmonics. Think clarinet or retro video-game music.
Sawtooth: bright and buzzy, containing all the harmonics. The raw material for brass, strings and aggressive leads.

Filters: sculpting by taking away

In subtractive synthesis you start with a harmonically rich wave, such as a sawtooth, and remove parts of it with a filter, rather like a sculptor carving stone.

The cutoff knob sets where the filter starts working. Resonance boosts frequencies right at the cutoff point, adding a whistling, vocal edge. Slowly sweep the cutoff of a low-pass filter on a sawtooth while holding a note, and you will hear the classic "wah" sweep of countless electronic tracks.

A low-pass filter lets low frequencies through and cuts high ones. It is the most common type: lower the cutoff and the sound becomes darker and more muffled.
A high-pass filter does the opposite, thinning out the bass.
A band-pass filter keeps only a band in the middle, giving a nasal, telephone-like tone.

The ADSR envelope: shaping each note in time

Real instruments do not just switch on and off. A piano note starts with a strike and fades; a violin can swell slowly. The envelope gives a synth note a shape over time with four controls:

A quick experiment: set sustain to zero and decay short. Every note now becomes a short pluck, no matter how long you hold the key. Then set attack and release long, and the same sound turns into a soft cloud.

Attack: how long the note takes to reach full volume after you press the key. Short for plucks and drums, long for slow swells.
Decay: how long it takes to fall from that peak to the sustain level.
Sustain: the level the note holds while you keep the key down. This is a level, not a time.
Release: how long the note takes to fade after you let go.

The LFO: built-in movement

An LFO (low-frequency oscillator) is an oscillator that runs too slowly to hear as a pitch, typically from a fraction of a cycle per second up to around 20. Instead of making sound, it moves another control up and down automatically:

Keep the depth small at first. A little movement brings a sound to life; a lot makes it seasick.

Sent to pitch, it creates vibrato.
Sent to the filter cutoff, it creates a rhythmic "wah" or the wobble heard in dubstep bass.
Sent to volume, it creates tremolo.

Three classic sounds to build

Here are starting recipes. Settings vary between synths, so treat the numbers as directions rather than exact values. On the HarmonyHub synthesizer you can start from a preset such as Warm Bass, Warm Pad or Bright Lead and compare it with what you build.

1. A punchy bass

2. A soft pad

3. A singing lead

Oscillators: sawtooth plus square, the second tuned an octave lower if possible.
Filter: low-pass, cutoff fairly low, a touch of resonance.
Envelope: attack zero, short decay, medium sustain, short release.
Play in the lower octave with short, separated notes.
Oscillators: two sawtooths, slightly detuned against each other.
Filter: low-pass, cutoff in the middle so it is warm but not dull.
Envelope: slow attack, high sustain, long release.
LFO: very slow and gentle on the filter for a breathing effect. Hold chords.
Oscillator: a sawtooth or square, alone or lightly detuned.
Filter: low-pass, cutoff fairly high, moderate resonance.
Envelope: quick attack, full sustain, short release.
LFO: gentle vibrato on pitch. Add a little glide so notes slide into each other. Play single-note melodies in the upper octave.

Keep turning knobs

Every synth sound you hear, from a film score to a pop hook, combines these same parts: an oscillator for the raw tone, a filter to shape it, an envelope to give it a life in time, and an LFO for movement. Change one thing at a time and listen closely. When you are ready to explore more electronic sound, browse the electronic instruments category and see how these building blocks appear in drum machines and other tools too.