Synthesizer glossary: tech terms

Experienced players, musicians and sales staff probably take it for granted that buyers understand many of the technical terms involved in a synthesizer, but if you’re just getting started, how can you decide what is going to be the right instrument for you? Make sure you don’t miss out on an essential feature or ability, by brushing up on some common terms used in synthesizer tech.

Sound synthesis: oscillators, waveforms and filters

Sound is oscillation. Your speakers oscillate. Your eardrums oscillate. If you’re fortunate enough to know one, a contented Ocelot oscillates, a frequency between 20-50Hz. But not all oscillators are the same. You can go crazy trying to understand it scientifically, so here’s an unscientific guide to help you understand similarities, as well as the differences, in synthesier specifications.

The pitch you hear is down to the frequency of an oscillation – music is made up of loads of frequencies at once, with some that complement each other, all mixed up, but even a single note from an instrument as you hear it consists of many frequencies.

Analogue synthesizers – basic waveforms

Synthesizers use a waveform – a shape that creates subtle variations in the electrical pulse that ultimately, drives a loudspeaker and moves air to your eardrum – to define which groups of frequencies – harmonics – you’ll hear at a given pitch. People perceive this as the tone, or timbre, or colour of an instrument; it’s why a square-wave sounds bright and harsh, and a sine wave sounds clean, but muted by comparison for the same given frequency.

If you look at the output of a synthesizer visually (for example, with Multiscope in Cubase), you’ll see the shape of the waveform is repeated more the higher the pitch you play. The waveforms actually represent the amount of energy – the ‘loudness’ of the wave – before it’s fed through the other parts of the synthesizer. What you hear from a typical programmed sound will look more complex than these basic forms, and will change shape over time. The art of the synthesizer programmer is in controlling how the basic waveform blends with other aspects to create the finished sound, intended instrument tone, or ‘patch’.

A square wave has a hard transition from loud to quiet/silent. Notice that the ‘square’ wave generated by Roland’s SH-101 plug-in is not exactly square – it’s the subtle, tiny variations in waveshape generated by analogue electronics that gives many synthesizers a unique character.

A ramp wave builds, then drops suddenly; a sawtooth is opposite, going to full energy almost instantly, then tailing off. Notice the controls for the synthesizer here? The square wave has been turned off, and the ramp wave turned all the way up.

A bended waveform is the result of using the synthesizer’s controls to mix more than one together, and it’s the first step in creating a more complex sound than a constant tone. For example, on the SH-101 you may mix between a pulse wave (a square wave is 50%, you can make different tones by making it high for a shorter duration) and a ramp wave, and then a pulse wave one or two octaves below, all generated by one oscillator.

Here you can see the effect of blending the square wave with the ramp wave. You get a sudden step in the shape, which alters the harmonic behaviour of the tone.

(Behringer chose to add a triangle waveform generated by the VCO and unused in Roland’s original design when making the MS-1).

You will find references to, and combinations of, pulse, square, ramp, sawtooth, and triangle waves on most synthesizers. They’re there whether they are old or modern, analogue, digital, or even software. These are the basic building blocks, and they are representations of the natural behaviour of instruments, such flutes, woodwinds (reeds), brass and strings.

These differing waveforms excite different harmonics in the note, which the synthesizer can then shape further to produce a final sound; you perceive the ‘fundamental‘; the original pitch. It is the harmonics above it, which may fade at a different rate for each cycle of the oscillator and occur at different frequencies above, that bring life and character to an otherwise boring beep.

There is one exception – the sine wave. This smoothly varies over time, and generates a ‘pure’ pitch, at least before the other electronics and factors get involved. There’s a lot more to this than there’s room to cover here, but what matters when picking your synthesizer is how you can use those waveforms to make the sound you want.

Combining waveforms: the start of a synthesizer patch, or tone

In the examples of basic waveforms above, you can see the two shapes the Roland SH-101 oscillator generates, pulse/square and ramp. These can generate two different types of harmonic. To make a more complex waveform – which sounds different, even though the pitch is the same – you mix the shapes together, exciting a different set of harmonic frequencies again.

Synthesizers with more than one oscillator let you mix the oscillators together to make a more complex waveform, but most oscillators generate more than one kind of waveform in the first place – and you can use them in different ways.

• Switch: Switch an oscillators waveform on or off
• Select: Choose a waveform for the oscillator
• Blend: Smoothly sweep between waveforms
• Mix: Mix an oscillators waveforms together.

Having more oscillators allows even more complex waveforms and even more sophisticated ranges of frequency to be controlled – there’s no good or bad technology here, just different approaches; in the old days, you wouldn’t get a dual-oscillator synth for single-oscillator money, which is why so many low-cos early monosynths were single-oscillator, but could use tricks like generating two waveforms from the same source, then mixing them.

Having two oscillators where you can select a different footage (octave, taken from pipe organ terms) means you can manipulate more of the audible spectrum with one voice, even if each oscillator can only contibute waveform. This is why the SH-101’s sub-oscillator is useful; it literally gives it a deeper voice.

What this really means when choosing your first synthesizer is that the ability to blend or mix oscillator, rather than switch, oscillator waveforms allows more subtle control of your foundation for a given sound – before you adjust how it evolves over time.

Time changes everything: envelope generators

Every instrument has a means of exciting or silencing a note – or both. Otherwise they would just keep making a constant tone and would get very annoying. This effect can be demonstrated on a synthesizer with free-running oscillators patched directly to the amplifier and output, and it is very annoying.

The invention of the envelope generator marks the point at which an audio-frequency oscillator becomes an instrument. If you think a straight square wave just switched on and off at different pitches doesn’t count as an instrument, you clearly missed the early years of rave music.

An envelope, in synthesizer terms, is the wrapping around a note. It controls the transition from silence, to tone, and to silence again, based on time and input from the musician, such as pressing a key or button.

AR: attack-release

The most basic form of envelope is a simple button to allow the amplifier to sound, usually a switch under a key that controls pitch. The sound’s start and end will be controlled electronically, and logically the first type of envelope generator allowed the programer of a sound to define the attack stage – after the key is pressed, and release stage – after the key is released.

Since the envelope applies to behaviour over time, we refer to a sound that starts off quiet, and becomes loud gently, as a slow attack, and one that fades once the key is released, a slow release. If the sound is at full intensity the moment the key is pressed, or cuts off the moment it is released, that is a fast envelope.

The purpose of the envelope is once again, to mimic the behaviour of an instrument. Something like a violin may have a slow attack as the energy builds up in the strings, and a relatively slow release if the player does not damp, or silence, the strings themselves. An organ will have fast attacks and delays as the valves open and close to allow air (or electrons) to flow. A drum may have a fast attack, but then a slow decay.

ADS, ADSR: a mid-step change

If you’re thinking about natural instrument sounds, you’re probably thinking ‘but they don’t just rise and fall, they make different sounds as the player strikes or plucks, and the note sustains’. Synthesizers naturally got more sophisticated envelope generators, but the most prevalent format that is still very much in use after fifty years of progress, is the ADSR shape.

ADSR means attack, decay, sustain, release. Some designs had a combined sustain/release function. The decay and sustain stages define the level at which a note is sustained, and how long it takes to fall from the maximum level, to the level it will be at while the note sounds. As an example, you could have a sharp, loud initial phase that drops slightly, mimicking the volume change as a violinist first contacts the string with a bow, and the tone produced as the bow is drawn over the string.

Attack and decay control the speed of a change, which may vary along a different curve depending on the design of synthesizer, and sustain controls the level of the held tone. Release, again, controls time for the note to return to silence.

Complex envelope generators: the computer age

Microprocessor control of circuits allowed cheap synthesizers to have the kind of features only dreamed of when the instruments were first invented. Multi-stage envelope generators allow a tone to rise and fall over time, usually based on other inputs such as velocity or a trigger from an LFO or clock signal. Rather than relying on capacitors and resistors, their transitions are calculated, and this allows advanced virtual analogue and softsynths the flexibility to change the curve of their transitions between levels, as well as having multiple transitions.

The theory remains much the same as ADSR, but it can be seen as ADSASDSR for example. Most synthesizers with such features just call the adjustment points stages, and fortunately the majority of synths where you’ll encounter this level of sophistication will have some sort of visual feedback for the shape of the transition from start to finish.

Addressing the envelopes: destination matters

From the point of view of programming a sound – what you see on the front panel of the instrument – you may only see one envelope. On a basic analogue synthesizer such as the Roland SH-101, that’s all you get, a single ADSR envelope generator. It can be assigned to the filter, the amplifier, or both.

Assigning the envelope just to the VCA means only the volume of your sound is affected. Assigning it to the filter means you can change the harmonics and higher frequencies by varying the amount the filter applies over time; this is particularly useful when using a noise source in a sound.

The more sophisticated the synthesizer, the more envelope generators you will find, but you may only see one set of ADSR controls on models that have preset storage – there will be a switch to flip between a VCF and VCA filter. In the case of an instrument such as the ARP Odyssey or its recreations, you get one ADSR and one AR filter, assignable to several destinations such as pulse width.

Not all envelope generators have to be linked to the start and end of a note, either. You may find they can be set to repeat, or be linked to a modulation source such as an LFO.

LFO: Low Frequency Oscillator

An LFO, or low frequency oscillator, is an additional waveform generator that generally does not operate at cycles fast enough to generate audio. Many LFOs can begin to approach audio rates, causing effects that change the pitch and timbre of a sound ‘as a note’ rather than ‘as a special effect’, however.

LFOs are usually used to control, or ‘modulate’, a stage in the synthesis of a sound. A simple example would be a sine wave assigned to an oscillator’s pitch, which used in a small amount would create a vibrato. Many designs of LFO offer a choice of waveform, and control over the speed, with more advanced designs allowing synchronisation of the LFO to an incoming clock or other sources, or a delay before the LFO affects the selected destination,

Typical destinations for an LFO’s output are oscillator pitch or filter frequency. An LFO may be free-running, retriggered by gate, or offer a variety of states depending on the synthesizer design – a free-running LFO, where its effect on a destination is your main form of control, is the simplest setup.

In some cases, an LFO may be one of the VCOs (voltage controlled oscillators) configured to go to subsonic, inaudible frequencies that influence the audible waveform generated – often, this will be achieved by allowing a VCO audio output to be routed to a non-audio control, such as cutoff frequency or resonance, to modulate it. This is frequently referred to as FM, or frequency modulation, but can also be called ‘cross-modulation’.

Frequency Modulation, or FM

FM isn’t just a band on an old analogue radio – it refers to the practice of generating one complex sound by using two simple waveforms interacting with each other, referred to as a carrier wave and a modulating wave.

The difference between frequency modulation and ‘using an LFO or other source’ to modulate, or control, a part of the synthesis is that the modulating waveform is also in the audible spectrum. Depending on the design of synthesizer you may be able to hear both sources through the mixer, or only the modulated output, but the result of an FM source being applied to a flat carrier would be an audible tone. You can hear this if you apply an FM waveform to a resonant filter, with no other audio sources – the speed of filter modulation creates a tone, though it may not be as loud as the output from a VCO.

Polyphonic or Monophonic?

Mono, Poly, Para, Duo… You’ll see these terms relating to all types of electronic instruments including virtual ones, and they relate to the number of voiced notes you can play at once. On a keyboard synthesizer, that’s easy to understand, on a modular system it gets a little more complex.

The two significant choices of instrument are monophonic or polyphonic. The former can produce one voice, the latter, more than one voice of the same structure. That does not mean you are limited to one note or pitch by a monophonic synthesizer, however, as models with more than one oscillator may be tuned at intervals.

If you can use a controller to alter the pitch of one oscillator relative to another, that is generally referred to as paraphonic. This assumes that for the duration of the note played, nothing else changes – you can simply control the interval between the oscillators by playing a keyboard or other performance controller.

A duophonic synthesizer has two voices. If you do not use filters or envelopes then controlling individual oscillators counts as duophonic playing, but for a true duophonic synthesizer you would need separate envelopes and filters per voice, which is a rare configuration.

Even so, many paraphonic synthesizers may be marketed as duophonic.

Where it gets interesting is when the synth designers let you ‘play’ the other oscillators. The most obvious way of doing this is to create a polyphonic synth. In that case, every ‘voice’ is a whole synthesizer with potentially its own osillator, amplifier envelope, filter, filter envelope and low frequency oscillators too.

That means if you play one note, the next note can rise, sustain, fall and evolve exactly as the first one did. Repeat for however many notes of polyphony you’ve got.

In a monophonic synthesizer you get one lot of articulation per note; one amp, envelope and filter stage. You might have more than one oscillator though, and can therefore play an interval. You could just tweak the second oscillator’s pitch control as you’re playing, but paraphonic synthesizers let you press more than one key so you can play relative pitches during a ‘note’ for each oscillator.

A good analogy would be hammer-on during a chord on a guitar – you don’t strum or pick, you just alter a string’s pitch in the chord and that new note fades with the others.

Paraphonic can cover as many oscillators as you’ve got – four oscillators? You could set them all up in one stacked note with detune, you could make a chord to play, or, you can play different intervals paraphonically. It’s how the Behringer Poly D works, and the Korg Mono/Poly.

Most confusion lies around duophonic – two oscillator synths. The Behringer/Korg/Arp Odyssey is paraphonic, but marketed as duophonic; there are ways of building a polyphonic synth with just two voices though, and the Odyssey isn’t this.

Ultimately, if it’s not polyphonic, and has more than one oscillator, it’s some form of paraphonic from the listeners’ perspective – how easy it is to control the pitch of the voices is another matter!

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