Introduction to Sound Synthesis The classic voice
The classic voice
The classic subtractive voice is an oscillator, a filter and an amplifier in a line, with a keyboard to play it, two envelopes to shape each note and an LFO to add movement. The Minimoog put that layout into one instrument in 1970, and most subtractive synthesisers since have kept it.
Every part of this voice has had its own chapter. The oscillator produces a tone with many harmonics. The filter removes some of them, and the amplifier sets the level. Those three are the signal path, the route the audio takes to the output. The keyboard, the envelopes and the LFO are not in the signal path. They produce control signals: the keyboard sets the oscillator's pitch and sends a gate, one envelope moves the filter's cutoff for each note, the other controls the amplifier, and the LFO adds vibrato.
The tool below is that voice with its connections fixed, as on a keyboard synthesiser. Play it with the keys on screen, or click the keyboard and use your computer's keys, A to K for the white notes and W, E, T, Y and U for the black ones. The diagram above the keys shows the level at each part as you play.
A classic voice
A monophonic synthesiser runs an oscillator through a filter and an amplifier, with a filter envelope, an amplifier envelope, vibrato and glide. The presets set every control at once.
Listen for With the Bass preset each note starts bright and darkens almost at once. With Lead, play a new key before letting go of the last one to hear the glide, and hold a note to hear the vibrato. The Pad preset opens and fades slowly, so play it with long, overlapping notes.
The Minimoog
Modular synthesisers in the 1960s, including those made by the engineer Robert Moog's company, were large cabinets of separate modules connected with patch cables. The Minimoog, which Moog's company began selling in 1970, put a fixed selection of those modules into a portable keyboard instrument and connected them internally. It has three oscillators and a noise source feeding a mixer, then a 24 dB per octave lowpass filter and an amplifier, with one envelope, which Moog called a contour generator, for the filter and another for loudness. Its modulation comes from the third oscillator or the noise source, with the amount set by a wheel beside the keys.
Because the connections are fixed, the Minimoog can be played like any other keyboard instrument, without patching. The order it uses has been repeated by a great many subtractive synthesisers since, including the Roland SH-101 of 1982 and the Juno-106 of 1984, and by software synthesisers that model them. Each adds or rearranges details, such as more envelopes, a highpass filter before the lowpass, or a modulation matrix in place of fixed routing, but the oscillator, filter and amplifier stay in the same order.
Why this order
The order follows from what each part does. The oscillator comes first because it produces the sound, and the filter and the amplifier change a signal they receive. The filter comes next so that it shapes the full harmonic content of the oscillator. The amplifier comes last so that when it closes between notes it silences everything before it, including any noise or resonance from the filter. The envelopes and the LFO can be sent to other settings as well, and the next chapter builds the voice one part at a time to show what each connection adds.
Monophonic, glide and the keyboard
This voice, like the Minimoog, is monophonic: it plays one note at a time. Pressing a second key while holding the first moves the same oscillator to the new pitch without starting a new gate, so the envelopes carry on rather than restarting. Glide, also called portamento, makes the pitch slide to the new note over a set time instead of jumping. In this tool, as on many monophonic synthesisers, the pitch glides only when notes overlap, and a note played after a gap jumps straight to its pitch. Letting go of the second key returns to the first if it is still held.
Common questions
What is a subtractive synthesiser?
A subtractive synthesiser starts from an oscillator with many harmonics and shapes the sound mainly by filtering some of them out. The standard layout is an oscillator, then a filter, then an amplifier, with envelopes and an LFO controlling them.
What is the signal path of a synthesiser?
The signal path is the route the audio takes from the oscillator to the output. In a classic subtractive voice it runs from the oscillator to the filter to the amplifier. Envelopes, the LFO and the keyboard sit outside the signal path and produce control signals that change settings along it.
Why is the Minimoog so important?
It put the oscillator, filter and amplifier, with their envelopes and modulation, into one portable instrument with fixed internal connections, so it could be played without patch cables. Most subtractive synthesisers that followed used the same layout.
What is glide or portamento?
Glide, also called portamento, is a slide in pitch from one note to the next instead of a jump. The glide time sets how long the slide takes. Many monophonic synthesisers glide only when one key is pressed before the previous one is released, and some offer a switch to glide between every note.
What does monophonic mean?
A monophonic synthesiser plays one note at a time. It has one voice, so pressing a second key while holding the first changes the pitch of the same voice. A polyphonic synthesiser has several voices and can play chords.
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