Subtractive audio synthesis
Subtractive synthesis is easy to learn and yet powerful audio synthesis capable of creating all kinds of instruments from soft to ear piercing leads, fat sounding baselines, pads as well as aggressive hoover sounds.
Subtractive synthesis setup
Core of analog subtractive synthesis voice consists of single or a mix of voltage controlled oscillators generating a tonal harsh wave form with lots of overtones patched to an analog resonant filter to soften it to a certain degree, also exaggerate some of the harmonics using resonance. The resulting signal is then patched to voltage controlled amplifier to mute the sound when the note is no longer playing.
1 Volt per octave pitch control for both VCO and VCF
Usually both oscillator tone and filtering level are connected via same 1Volt per octave pitch control signal so the filter would move with the note that is being played creating similar sounds across many octaves. Filter is then controlled with a second envelope that modulates it while the note is pressed.
Such synthesis is excellent for producing basslines, leads, pads and other tonal sounds. Character of subtractive voice depends greatly on the initial waveform, type of filter used, also resonance and many other parameters to sculpture a unique audio voice.
Oscillator waveforms
In subtractive synthesis square and sawtooth waveforms usually work best as they have the most amount of overtones for filter to work with.
Single oscillator waveform + filter produces very clean sound which is good for sub bass or 303 like acid bassline but might not be rich enough for a truly roaring analog bass. Our VCO module has dedicated knobs and voltage control to mix available waveforms also to mix in sub oscillator waveform an octave below.
Mixing multiple oscillators
To enrich overtones 2 or more slightly detuned waveforms from separate oscillators can be mixed together and sent to the filter. This creates much richer sounding bass as harmonics from different oscillators are interacting with each other, when both waveform phases are positive they sum up, when they have different polarity they cancel out.
This creates harmonic movement and the sound appears more fat to the ear. The effect can be comparable to chorus when several human voices are singing the same note at the same time.
Voltage controlled filters
In subtractive audio synthesis filter type and quality is very important as they are the main component that sculptures the harsh sound.
Electronic filtering components
Analog filters work by by passing the sound though a precision capacitor that takes away some of the frequencies depending on the amount of current the sound is carried. The amount of current is carefully controlled so the filtering edge would correspond to the frequency of the note that is being played.
Due to physics one capacitor filters the sound with 6db per octave rolloff curve which is usually not enough for a good sounding bass therefore usually 2 or 4 capacitors are used in parallel to achieve steeper filtering curves of 12db/oct or 24db/oct.
Filter poles
Each capacitor filtering element in the filter module is called pole, 12db/oct would correspond to 2 pole filter which is used in Korg MS20, 24db/oct curve would correspond to 4 poles which are used in famous Moog and many other filters. More poles the filter has the deeper it sounds.
Filter resonance
Another very important analog filter feature is the resonance. Resonance is basically feedback when a portion of filtered output is sent to the input.
Resonance exaggerates frequencies at the filtering point creating very characteristic subtractive synthesis sound. The way frequencies are exaggerated depends on the amount of poles also if filter has clean resonance or if it is passed through analog distortion.
Types of filters
In subtractive synthesis low pass filter is traditionally used as it removes high pitch harmonics however other filter types are also useful to further tame the signal.
Band pass filter removes both high and low frequencies leaving only tones around the filtering point while high pass filter removes low frequencies only. Such filters can be used if we don’t want to achieve very bassy sound or simply to tighten the low end.
High pass filter can also be used to tighten the low end of the sound as it both filters the frequencies below the chosen point and exaggerates frequencies at filtering point using resonance.
VCA and audio post processing
Signal from the filter is usually patched to VCA or voltage controlled amplifier that mutes the sound once the note is no longer playing. VCA is generally controlled by envelope generator that controls attack and decay – how fast the sound kicks in when the note is pressed and how slow it decays when the note has ended. Different envelope generators often have more parameters such as decay and sustain to create quick volume jump at the beginning of the note sequence for more plucky sound.
Combination of VCO, VCF and VCA create a dry subtractive synthesis voice which is usually processed further to make it more appealing. Often distortion module is added between filter and amplifier. Distortion exaggerates harmonics making filter resonance sound brighter and heavier.
Making electronic music
Subtractive synthesis mainly produces harmonic tones because oscillator is tuned to produce tone with static harmonic that are then softened with the filter.
Depending on music style you are planning to make we recommend studying modern music theory to understand basics of harmony and how to implement it to make sequences.
Basslines are both rhythmic and harmonic often focused on first and 5th note of the chord. Many styles of electronic music have bassy sound that goes with a kick simply playing the root note with intricate rhythm that works well with the track. Over that usually there is another voice playing some kind of catchy repetitive sequence which might be harmonized or not depending on the artist choice.
Delay and reverb in subtractive synthesis
At the end of the signal chain the sound is also usually processed with delay or reverb or both. Reverb effects make bassline sound much better as it generates the sense of space as if the sound would be played in a church, room or any other cavity with unique properties. Delay repeats the sound in different ways merging it with the dry source adding complexity and often dreamy feeling to the sound.
We to save Eurorack synthesizer space we apply delay and reverb digitally using DAW and VST plugins as it is where digital technology shines. Analog reverbs such as spring reverb are very simple and noisy in comparison.
To fit well in the mix subtractive noise is also often side chained and processed with equalizer reducing certain frequencies in the sound to make space for drums and other voices. in the track.
Analog sequencers
We usually control analog voices using digital to analog converter module that takes midi data from de daw and creates it to 1Volt per octave pitch control, also gates and envelopes required for voltage controlled filter and amplifier.
There are other interesting ways to generate basslines, one of them us is by using classic step sequencer that receives clock signal and cycles through 8 or 16 notes. Each note in the cycle has its own dedicated knobs for pitch and other parameter control. There are also dedicated stand alone sequencer machines with many interesting possibilities to generate sequences. They tend to have pitch and other analog controls that can be patched directly to the oscillator and other modules in the signal chain.