The synthesizer card drew the chain: oscillator, filter, amplifier. That chain has a name, subtractive synthesis, and a method: start with a wave that is full of harmonics and take some of them away. It was the way nearly every synthesizer worked until digital ones arrived, which is why it is sometimes just called analogue synthesis.
Where the brightness comes from
A plucked string does not vibrate at one frequency. It vibrates at its fundamental and at every whole-number multiple of it: twice the frequency, three times, four times. Those are the harmonics, and the mix of them is what makes one tone sound different from another. A sawtooth wave, the one you will find on almost every synth ever built, contains every harmonic, each one quieter than the last: the nth harmonic has 1/n the strength of the fundamental. That is why it sounds so bright and buzzy, and why it is the usual starting point. Square and triangle waves and white or pink noise work too: anything with overtones to remove.
The filter takes them away
Remove all but the first five harmonics of a sawtooth and you get a different wave and a different sound. The device that does it is a filter. A low-pass filter lets the low frequencies through and attenuates those above its cutoff frequency; a high-pass filter does the opposite; a band-pass filter is a low-pass and a high-pass in series, leaving a band in the middle.
| Filter | Lets through | Attenuates |
|---|---|---|
| Low-pass | Below the cutoff | Above it |
| High-pass | Above the cutoff | Below it |
| Band-pass | A band around the centre | Both ends |
| Band-reject (notch) | Both ends | A band in the middle |
How hard a filter cuts is its slope. The simplest circuit, one resistor and one capacitor, cuts by 6 dB per octave above the cutoff; synth filters stack stages to cut harder, and 24 dB per octave is the figure you will see quoted, for the PPG Wave's filters among others.
Resonance is the knob next to cutoff. A resonant filter still attenuates the frequencies far above the cutoff, but it boosts a band around the cutoff itself, making the harmonics there louder than they were going in. Sweep the cutoff up and down with the resonance turned up and the boosted band sweeps with it: the analogue filter sweep, probably the most recognisable synthesizer sound there is. Turn the resonance all the way up and many filters start to sing on their own: self-oscillation.
What moves the knobs
The trick that made this a playable instrument is that none of the knobs has to be turned by hand. Each setting can be controlled by a voltage, which is what the V in VCO, VCF and VCA stands for. A control voltage, or CV, can come from a keyboard, from an envelope or from an LFO.
An envelope is a voltage that rises and falls once each time a key is pressed. The most common has four stages, ADSR: attack, decay and release are times; sustain is a level. An organ has a fast attack and holds full volume until the key is released, so its envelope is almost a rectangle; a trombone speaks more slowly, peaks, then settles to a lower level while the player keeps blowing. Send the same envelope to the filter cutoff and the sound opens up as it starts and darkens as it dies, which is most of what makes a synth bass sound like a bass.
An LFO is an oscillator too slow to hear as a note. On the amplifier it gives tremolo; on the pitch, vibrato; on the filter, a slow sweep.
The instruments that fixed the layout
| Synth | What it settled | Year |
|---|---|---|
| Moog voltage-controlled modules | First shown at the AES convention in New York; the oscillator, filter and amplifier as separate boxes | 1964 |
| Moog Minimoog | The chain in one portable, non-modular keyboard: the first synth sold in retail stores | 1970 |
| Sequential Prophet-5 | The first fully programmable polyphonic synth: five voices, and sounds that could be saved and recalled | 1978 |
| Roland Juno-106 | Six voices, digitally controlled oscillators that do not drift, one ADSR shared by filter and amplifier | Feb 1984 |
The Minimoog's three oscillators were never completely in tune with each other, because the engineers could not fully stabilise the power supply. That accident is the "warm" sound people still chase. The Juno-106 went the other way: its oscillators are digitally controlled, so the pitch holds, but their sawtooth and pulse waves are still analogue.
Sources
Every date, name, figure and definition on this page comes from these pages, read on 2026-11-05.
- Sound On Sound, Synth Secrets 1: What's in a sound?
- Sound On Sound, Synth Secrets 3: Modifiers and controllers
- Sound On Sound, Synth Secrets 4: Of responses and resonance
- Bob Moog Foundation: Bob Moog timeline
- Sequential: Prophet-5
- Roland: Juno-106
- Wikipedia: Subtractive synthesis
- Wikipedia: Minimoog