Some call it Analog: How subtractive synthesizers work

Some call it Analog: How subtractive synthesizers work

18 October, 2007 | 2.51PM

• Section: Music News Topics: Guide To Synthesis, Technology
• Written by: francispreve

The term ‘subtractive synthesis’ refers to the sound generation method used by many modern software synths, as well as all analog synthesizers – new or old.

The fundamental principle consists of taking a sound source, called an ‘oscillator’, then feeding that signal into a filter.

The filter then subtracts frequency content from the oscillator signal, allowing it to be made brighter, duller, squeakier, and so on.

After the filter, the overall volume of the resulting sound can be adjusted to create percussive sounds, long swells, and so forth.

If this sounds too complex to grasp, fear not.

The secret to understanding subtractive synthesis lies in comparing it to other, more familiar, concepts from the DJ world. 

A good analogy would be a classic DJ rig.

Two turntables (the sound sources), feed a mixer that contains an EQ (which correlates to the filter section), the result then feeds an amplifier.

If you have any experience with funky guitars, you can think of it this way: A guitar generates the music, which is then processed by a wah-wah pedal (which is in fact, a filter), and the wah-wah output is amplified.

This, gentle readers, is the underlying concept behind every analog synth.

The components for subtractive synths share familiar terminology, so here’s a summary of the most common components, along with a basic description of their associated functions.

As this series progresses, we’ll dig much deeper into each module.

Oscillators

The oscillator is where the sound generation journey begins.

Most analog hardware and software models rely on two (sometimes even three or four) oscillators to generate the initial sound.

These oscillators are capable of generating an array of different waveforms, each with its own unique sonic fingerprint.

For example, sawtooth waves have a bright, buzzy character that’s great for in-your-face basses, pads and leads.

Square waves have a more hollow character that evokes both woodwind instruments and old school video games.

Other waveforms, like triangle and sine, have their own unique sound as well.

Since there are at least two oscillators involved, each one can be tuned independently and assigned its own waveform, then mixed together to create an even more complex texture.

Common combinations include using the same waveform on both oscillators, then detuning each slightly for thick chords and trance-like leads.

Another increasingly fashionable option relies on tuning each oscillator to a different musical interval, like a fifth, then playing both notes from a single key.

The main riff from the D.Ramirez remix of ‘Yeah Yeah’ is a perfect example of this approach [listen to the track in the player below].

Arturia’s impressive Jupiter 8v softsynth [right] faithfully recreates Roland’s original dual-oscillator design.

Filters

The filter section modifies the mixed oscillator sound.

The most common type of filtering is called ‘low pass’ because it allows frequencies below a set frequency to pass unmodified, while frequencies higher than the set frequency — called the ‘cutoff’ — are attenuated, or lowered, in volume.

Another parameter, called resonance, interacts with the cutoff frequency, creating squeaky/squelchy effects that are the hallmark of both TB-303 acid riffs and old school funky synth basslines.

Other filter modes include high pass (which reduces low frequencies), band pass (kind of like a sweepable mid EQ) and notch.

Each mode has specialized characteristics that are useful for different types of sounds. We’ll explore these in more depth in future tutorials.

Ableton’s Sampler instrument [right] includes an extensive array of subtractive synthesis amenities like graphically adjustable filters and envelopes.

Envelopes

In addition to filters and oscillators, classic analog synths also include tools like envelopes and LFOs — often called ‘modulators’ — that allow the oscillator and filter characteristics to be modified dynamically as you play a sequence of notes.

Envelopes are modulators that are triggered when a key is played, thus shaping the character of each individual note.

For example, an amplifier envelope governs how the overall loudness of a sound changes when a key is pressed and held.

Some envelope settings create percussive sounds, other settings create long sustaining sounds, and so on.

The majority of modern synths include dedicated envelopes for the filter, amplifier, and oscillator pitch, though it’s sometimes possible to route the envelopes to alternate destinations for even more exotic results.

LFOs

The acronym LFO stands for Low Frequency Oscillator, meaning an oscillator that operates below the range of human hearing (which incidentally, is 20 Hz to 20 kHz, for the supergeeks out there).

LFOs can be routed to control the most common synth parameters like pitch, amplifier, and filter cutoff, as well as other useful destinations like panning and oscillator mix.

Since the rate of oscillation is so slow, it can be used to create repeating effects like vibrato (a quiver in pitch), tremolo (a quiver in loudness), or wah-wah (a repeating change in filter character).

Like audio oscillators, LFOs also offer a selection of different waveforms, each with its own rhythmic properties.

Sine and triangle waves are commonly used for vibrato and tremolo, whereas sawtooth and square waves are terrific for pulsating or echo-like effects.

Also included in Ableton’s Sampler are three LFOs [right] that can be synchronized to tempo for creating powerful rhythmic effects.

MIDI Control

MIDI control is another critical component for creating interesting, evolving textures that build to a peak and/or transform in other ways over the course of a mix.

One way to control that evolution is to have the various controllers on a MIDI keyboard affect different aspects of the synthesis engine.

Options such as note velocity (how hard you hit the keys), modulation and pitch wheels (knobs or levers at the far left end of a keyboard), or even foot pedals, can allow specific parameters in the filter or envelope sections to morph as you adjust these controllers.

This really helps to make a repeating sequence come to life and give your tracks a lot more complexity.

With a little experimentation, you’ll find a better understanding of how the various components of a synth interact.

I strongly urge readers who do not already own a sequencer or digital audio workstation software to head over to www.propellerheads.se and download the demo version of Reason.

Doing so will allow you to follow along with these lessons, experimenting as you go.

In the next lesson, we’ll go much deeper into the specifics of oscillators.

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