Synthesis Fundamentals: Oscillators, Filters, Envelopes and LFOs.

A new synthesizer’s screen looks like a control panel with dozens of knobs, colours and arrows. The good news is that almost all instruments, software or hardware, are built from the same blocks: an oscillator generating material, a filter cutting it, envelopes shaping it over time, an LFO moving it in a loop and a modulation section connecting things together.

This guide explores those blocks in the order you will use them, always describing what changes to your ears, rather than only what the parameter does. By the end, you will know why a preset sounds dull, bright, moving or flat, and can diagnose it in any instrument: Serum, Vital, your DAW’s synthesizer or a hardware box. If you come from the booth, you will also understand why two tracks with similar bass fight each other and the spectrum’s role in 4/4 structure.

The Blocks of Any Synthesizer

The classic chain is short: oscillators generate a continuous signal, that signal passes through a filter deciding which frequencies keep coming through, and an amplifier with an envelope decides how long it lasts and how the volume develops. Around that backbone is modulation: LFOs, extra envelopes and expression controls moving any parameter.

What matters is the order of decisions, rather than the physical order on the screen. First choose the material (which waves sound and at what volume), then sculpt it (filter and resonance), then move it over time (envelopes and LFO), and finally decide how many copies of the note sound at once (unison and voices). Subtractive synthesizers do exactly this; other families change the engine but retain the same controls, as you will see in the synthesis types guide.

Oscillators: The Material Before You Touch Anything

An oscillator repeats a waveform at a particular frequency. The shape is everything: it defines which harmonics exist before the filter touches anything.

• Sine: one frequency, with no harmonics. It is the basis of any sub because it occupies one clear line in the spectrum without muddying what is above.
• Triangle: a sine wave with a few very weak odd harmonics. It sounds rounded, with a little air, useful for bass that should not sound flat.
• Saw: all harmonics, ordered and bright. It is the favourite wave for filtering, because closing the low-pass makes the sweep audible.
• Square: odd harmonics only; a hollow, characterful sound, typical of classic bass and full-bodied lines.
• Pulse width: the width of the square pulse. Narrowing or widening it changes the harmonic balance without touching the filter, and modulating that width over time produces the familiar pad chorus.
• Noise: white or pink noise, energy across the whole spectrum at once. It is not for pitched tones, but supplies air, synthetic percussion, a kick’s transient or the basis of a riser.

Phase matters more than it seems: two identical oscillators sounding together in opposite phase cancel. That is why one tuned wave is preferable for the sub, rather than four copies.

Filters: Cutoff, Resonance and Slope

The filter decides which part of the spectrum passes. The most common is low-pass (LP), which passes below the cutoff and attenuates above; high-pass (HP) does the opposite, and band-pass (BP) passes a narrow band. Slope indicates attenuation per octave: 12 dB/octave is gentle and 24 dB/octave is steep and more forceful.

Resonance (also called Q or emphasis) boosts a narrow band right at the cutoff. Turned up, it makes the sweep audible and adds presence in a busy mix; at maximum, the filter starts howling and behaves like an almost sinusoidal tone, a technique used for percussion and extreme textures.

The typical mistake is thinking that closing the cutoff always darkens the sound: with lots of resonance, closing it sounds more nasal and high-pitched. Listen to the boost, not just the roll-off.

Envelopes: ADSR for Amplitude and Filter

Envelopes draw a shape triggered with each note. The most common is ADSR: Attack is the time to reach maximum, Decay the time to fall to Sustain level, Sustain a level maintained while holding the note (not a time), and Release the time to fall to silence after releasing.

Almost all instruments apply one envelope to volume, always beginning and ending in silence, and another to the filter. The volume envelope decides whether sound enters abruptly or gradually; the filter envelope decides whether the attack sounds bright then fades, or the reverse.

The detail that changes everything: a filter envelope needs an amount. If the cutoff is fully open and amount is zero, the envelope works in vain and you hear nothing. Close the cutoff slightly and raise amount: the classic attack sweep appears.

LFO and Modulation: Movement Independent of the Note

An envelope triggers once per note; an LFO repeats in a loop while there is a signal. Its classic uses are vibrato (pitch), tremolo (volume) and filter wobble periodically opening and closing the cutoff.

Two parameters govern almost everything: rate (how fast) and depth (how much). Synchronising rate to tempo turns the LFO into a rhythmic element; leaving it free makes it more organic. Not everything is a smooth loop: random sources and sample & hold create irregular movements that bring pads and textures to life.

Modulation is the system connecting a source and destination with a specific amount. In modern synthesizers it reads as “how much → destination”: how much of that source goes to that parameter. Learning to read that arrow means learning to read any interface: the same logic in a minimal synth and a synthesizer with a twenty-entry modulation matrix.

Unison and Voices: How Many Copies, How Much Detune

Voices is the number of notes the instrument can play at once: polyphony. Unison is different: stacking several copies of the same note with small tuning differences and, often, stereo differences.

Live’s manual describes unison as a switch stacking voices per note and a detune control adjusting how far each copy deviates; more detune sounds wider and more analogue, less sounds more focused. In a lead or pad, unison is the difference between a line-like sound and a mass of sound. In a sub or kick, it is usually unnecessary: detune blurs the fundamental and muddies its relationship with the kick.

CPU cost is proportional to voice count, so the usual order is to build the sound with one copy and add unison at the end, only where it contributes.

Glide and Legato: How Notes Connect

Glide (or portamento) makes frequency travel from the previous note to the new one. It can be measured in time or ratio; in time mode, long jumps take as long as short ones; in ratio mode, speed is proportional to the interval.

In legato mode, the instrument only restarts envelopes when you release all notes and press again. Combined with a one-voice mono mode, it produces bass and lead lines where notes drag into one another without each note restarting the attack. With polyphony, on the other hand, glide loses its meaning because each voice has its own trajectory.

A One-Hour Practice Path

Before collecting presets, spend an hour on one instrument with this sequence:

1. Start an empty preset and listen to a Sine with a simple volume envelope: change only Attack and Release.
2. Add a Saw and close the low-pass until you notice the difference; raise and lower resonance while sweeping cutoff.
3. Apply the filter envelope with amount: short attack for a pluck, long attack for a pad.
4. Send an LFO to cutoff and synchronise it to quarter notes; then unsynchronise it and compare.
5. Enable unison with two copies and raise detune; remove it and hear the sound’s focus again.
6. Add glide and play legato lines to hear how articulation changes.

Record the experiment and compare it the following day. You are not seeking a beautiful sound, but recognising by ear which block produced each change.

Next Step

With the blocks clear, it is time to understand the families combining them in different ways: subtractive, wavetable, FM, additive, granular and modular. That map is in Synthesis Types.