Signal flow

What is synthesis?

substractive synthesis

In subtractive synthesis, an oscillator is the sound source. Subtractive synthesis starts with a rich, harmonically complex waveform, and uses filters to remove (subtract) unwanted frequencies. That’s why it’s called subtractive — because we take something away of the sound to shape the final tone.

To make the sound musical, we modulate aspects of the sound over time. E.g. if you think of plucking a guitar string it has a different phases of volume, also the the frequencies are different at those different stages. In synthesisers we use Envelopes to shape and change the volume (amp-envelope) as well as the brightness (filter envelope). many synthesisers also feature an evelope for pitch. e.g. if we pluck a guitar string very hard it will be slightly higher in the very first phase.

In subtractive synthesis, you start with a sound source that’s always “on”—a tone rich with harmonics—and then you carve away the parts you don’t want. Think of that constant tone like water flowing through a garden hose. By adding filters, pumps, and timed valves, you shape that steady flow into a musical “fountain” that swells, brightens, and fades exactly when you want. Just as real instruments change over time—guitar strings ring out then decay, trumpets brighten then mellow—synthesizers use envelopes and filters to mimic those natural evolutions.

Imagine a garden hose that’s always on, gently flowing water—you’re shaping that steady flow into the perfect garden-fountain display.

1. Oscillator ➔ “Always-On Water Source”

   • Analogy: A hose hooked up to a tap that never shuts off. Water (sound) is constantly available.
   • Real-World Parallel: Think of a bowed violin string—once you draw the bow, the string keeps vibrating until you stop.

2. Filter ➔ “Adjustable Nozzle”

   • Analogy: A nozzle with interchangeable screens that block certain droplet sizes. One screen removes fine mist (high frequencies) for a heavy stream; another blocks larger drops (low frequencies) for a light mist.
   • Real-World Parallel: A trumpet player opens and closes the lips and changes mouth-shape to emphasize or soften certain harmonics—bright at the top of the phrase, mellow as it tails off.

3. Amplifier (VCA) ➔ “Pump / Pressure Regulator”

   • Analogy: A pump that turns the flow’s pressure up or down—controlling how forceful (loud) the final jet of water (sound) will be.
   • Real-World Parallel: A piano hammer strikes with varying force: hard strikes give a loud, percussive attack; gentle strikes produce a softer tone.

4. Envelopes ➔ “Timed Valves”

   • Analogy: Small valves along the hose that open and close with precise timing to shape bursts of pressure and nozzle width.
   • Why We Use Them: Real acoustic sounds aren’t static. A plucked guitar string has a sharp attack, a quick swell, then a slow decay. A flute note can start quietly, swell to fullness, and then fade. Envelopes recreate those dynamics in synthesis.
     • Volume Valve (Amp-Envelope): Opens fast for a hard “pluck,” holds as the note sustains, then closes for a smooth fade—just like a guitar string.
     • Brightness Valve (Filter-Envelope): Starts closed (darker), then opens to let through more high frequencies (brighter) before closing again—like a “wah” or the way a clarinet’s tone blossoms then softens.
     • Pitch Valve (Pitch-Envelope, optional): Gives a tiny extra burst of pressure at the very start—similar to the slight pitch rise when you strike a piano key especially hard or snap a bass string.

Putting it all together:

1. Continuous Source: Water (sound) flows from your always-on hose (oscillator)—just as a bowed or wind instrument sustains a tone.
2. Shape Timbre: The nozzle (filter) subtracts unwanted droplet sizes (frequencies)—like a trumpet player shaping their embouchure.
3. Control Loudness: The pump (VCA) sets overall pressure (volume)—mirroring how hard you strike a piano key.
4. Animate Over Time: Timed valves (envelopes) open and close to sculpt the attack, sustain, and release—emulating guitar plucks, flute swells, and piano decays.

By combining these elements—oscillator, filter, amplifier, and envelopes—you can recreate the dynamic character of real instruments or invent entirely new sounds, all from a steady stream of “always-on” audio.

Here is also a great tutorial how you can explain substractive synthesis. A screaming person in a box as the sound source:

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