Time to review some fundamental synthesis concepts and how they relate to synthio.
There are many different ways to approach audio synthesis -- additive synthesis, subtractive synthesis, frequency modulation, phase distortion, waveshaping, physical modeling, wavetables, and more.
We'll look at the building blocks of how subtractive synthesis works, since it is the most similar to the synthio approach, as well as wavetable synthesis, as this is another feature of synthio.
Audio Oscillator, a.k.a VCO
Arguably the most important piece of the subtractive synthesis puzzle -- the audio rate oscillator. This is a wave that oscillates at an audible rate -- anywhere from ~20Hz to 20,000Hz. Ultimately, this electrical signal is used to vibrate a speaker cone which disturbs the air, creating sound waves that can reach your ear.
The rate of the oscillation determines the pitch we hear. In many synthesizers the rate/pitch is controlled via voltage signals, hence the name Voltage Controlled Oscillator, or VCO. These pitch CV signals typically come from a keyboard or step sequencer.
There is a secondary pitch modification control on most oscillators called pitch bend which can nudge the key that's currently being played up or down. Many keyboards have a dedicated pitch bend wheel used for this purpose.
Wave Shapes/Wavetables
The shape of the oscillator's wave determines the timbre or "character" of the sound, based on the different harmonic content of the sound above and beyond the fundamental frequency.
Common wave shapes include sawtooth, triangle, square, and sine.
Some oscillators (including synthio) have an extra trick up their sleeve -- instead of being confined to a single wave shape, they can use buffered single-cycle waveforms of any shape imaginable, and can morph between two or more of these shapes! This is called wavetable synthesis, and it gives huge flexibility in sound design.
Patch Symbols from PATCH & TWEAK by Kim Bjørn and Chris Meyer, published by Bjooks, are licensed under CC BY-ND 4.0 The included symbols are available free of use under the following terms and license.
Filter, a.k.a. LPF, HPF, BP, notch
Most oscillator waveforms include a lot of rich harmonics that give them their distinct characters, such as the "buzziness" of a sawtooth wave. A filter can be used to cut off some of those harmonics above or below a certain frequency.
Low-pass filters can be set to reduce or remove sound above a certain cutoff frequency you chose, which can give your audio a sort of muffled sound.
High-pass filters can be set to reduce or remove sound below a certain cutoff frequency, which can give your sound a sort of bright or "tinny" character.
Band-pass filters reduce or remove sound both above and below their cutoff frequency.
Notch filters work like an inverted band-pass filter, by reducing or removing sound within a min-max cutoff frequency
Many filters also include a control called resonance (or 'Q'), which allows you to emphasize the harmonics right at the cutoff frequency. This is the little hump seen in the first two diagrams shown here. Strong resonance loops can start to feedback on themselves and even self-oscillate, which can be an interesting sounding effect.
The particular cutoff frequency of a filter is often modified with incoming control voltage, sent from another modulation source such an LFO (more on this below).
Amplifier/Attenuator, a.k.a. VCA
An oscillator that drones on forever could get pretty tiresome. In order to play sounds for fixed durations we need a ways to turn the volume up and down!
An amplifier (a misnomer we live with in the synth world, because it is technically a loudness attenuator) is used to adjust the volume of the audio oscillator. It will generally be "off" until a CV signal tells it to open up to partial or full volume, allowing the sound to pass through.
Again, this signal that opens and closes the amplifier varying amounts is a control voltage (often coming from an envelope generator as discussed below), hence the name Voltage Controlled Amplifier, or VCA.
Envelope Generator, a.k.a. EG
Envelope generators are used to "shape" the loudness of a sound over time, so that we don't live with simple ON-OFF attenuation of the volume.
The envelope generator, or EG, is what tells the VCA how loud or quiet to get during the different stages of a note's lifetime.
A typical set of stages on an EG are:
- Attack - how long it takes from the onset of a note until it reaches it's full volume
- Decay - how long it takes for the volume to drop from full attack level to the sustain level
- Sustain - the level to stay at while a note is held
- Release - how long to take to drop from the sustain level to fully "off" after the note is told to stop playing
For brevity, this type of parameter set is usually referred to as an ADSR envelope generator.
The EG sends control voltage (CV) to the VCA that varies as the EG moves through its stages, thus attenuating the loudness of the VCO signal traveling through the VCA.
But what tells the EG when to start and stop? This is a simple ON-OFF signal called a Gate. A typical scenario is a key being pressed on a keyboard sends a high Gate signal to the EG, and when the key is released on the keyboard a low Gate signal is sent to the EG.
Subtractive Synth Signal Flow
Using the above building blocks here is the signal flow for a typical subtractive synth:
Here's how it works:
- You press a key on a keyboard, which sends 1V/Oct pitch CV to the sawtooth oscillator, and a Gate signal to the ADSR envelope generator
- Oscillator sends audio to the filter, where certain harmonic above a frequency are reduced
- Filter sends the audio to the amplifier. The envelope generator tells the amplifier to attenuate the audio loudness according to its ADSR stages/settings
- The audio then heads off to an audio output device, e.g., speakers/headphones/amp, etc. (optionally, it first passes through a mixer where it can be combined with other audio)
Low Frequency Oscillator, a.k.a., LFO
At any step of the synth signal chain where a knob can be twiddled by hand to modulate the signal, a Low Frequency Oscillator, or LFO, can automate the twiddling.
LFOs are very similar to audio oscillators, they just have much slower rates, running anywhere from just below audible rates to many seconds or even minutes to complete a single cycle.
A typical use for an LFO is to send CV signal to the cutoff frequency input of a filter so that it varies over time.
Ring Modulation
Ring modulation is a method of multiplying the oscillator frequency (called the "carrier") by a second frequency (called the "modulator"), and then passing along only the sideband frequencies.
Ring mods can produce special effects (Dalek voice from Dr. Who is a classic example) and metallic sounds, since the harmonics produced tend to not be very musical sounding ones.
Multiple Voices / Polyphony
The examples above represent a single synthesizer voice, sometimes referred to as a monophonic synth. These are great for basslines and sometimes leads, but what about when you want to play a chord?
The good news is, you simply add more voices! synthio is a truly polyphonic synthesizer, meaning it can play multiple full voices/notes at once. Each note has a full signal chain, so you can adjust all of the parameters (wave shape, pitch, filter, envelope) individually per voice.
These voices/notes can be sent through the audio mixer to combine them on their way to the audio output.
Two Voices
This is an example of two voices/notes, running through an audio mixer on their way to the audio output.
LFOs Added
We've added a couple of LFOs for modulation of filter cutoff on one voice and pitch bend on another.
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