Théorie

Additive synthesis is a technique of audio synthesis which creates musical timbre. The timbre of an instrument is composed of multiple harmonics or partials, in different quantities, that change over time. Additive synthesis emulates such timbres by combining numerous waveforms pitched to different harmonics, with a different amplitude envelope on each, along with inharmonic artefacts. Usually, this involves a bank of oscillators tuned to multiples of the base frequency. Often, each oscillator has its own customizable volume envelope, creating a realistic, dynamic sound that changes over time.

ADSR envelope. The contour of the ADSR envelope is specified using four parameters:
Attack How quickly the sound reaches full volume after the sound is activated (the key is pressed). For most mechanical instruments, this period is virtually instantaneous. However, for some popular synthesized “voices” that don’t mimic real instruments, this parameter is slowed down. ‘Slow attack’ is commonly part of sounds called ‘pads’.
Decay How quickly the sound drops to the sustain level after the initial peak.
Sustain The “constant” volume that the sound takes after decay until the note is released. Note that this parameter specifies a volume level rather than a time period.
Release How quickly the sound fades when a note ends (the key is released). Often, this time is very short. An example where the release is longer might be a percussion instrument like a glockenspiel, or a piano with the sustain pedal pressed.
While ADSR envelopes are a useful first approximation to the volumes of real instruments, they are not a complete substitute. Woodwind and brass instruments give the player the ability to vary the sound arbitrarily throughout a note, for instance. Many synthesizers, therefore, offer more flexible facilities for volume control which can be used if desired. On older synthesizers, such as the Korg MS-20, a common variation on the ADSR was ADSHR (attack, decay, sustain, hold, release). By adding a “hold” parameter, the system allowed notes to be held at the sustain level for a length of time before decaying.

Analog synthesizer is a synthesizer that uses analog circuits to generate sound electronically. A very common circuit component in analog synthesizers is the operational amplifier (op-amp), a kind of integrated circuit; most of analog synthesizers contain many of them; earlier models were built using transistors instead of microchips. Another common component is a potentiometer (pot, or variable resistor), which is used to adjust the traits of the sound that is produced. Some common components that consist of multiple parts are low-pass filters and high-pass filters. The earliest synthesizers used a variety of valve and electro-mechanical technologies. While some electronic instruments were produced in bulk, such as the Ondioline, the Hammond organ, the Trautonium, many of these would not be considered synthesizers by the standards of later instruments. However, some individual studios and instruments certainly achieved a high level of sophistication and untapped promise, such as the Mixturtrautonium of Oskar Sala, the Electronium of Raymond Scott. Early analog synthesizers used technology derived from electronic analog computers and laboratory test equipment. They were generally “modular” synthesizers, consisting of a number of independent electronic modules connected by patch cables. Synthesizer modules found in early analog synthesizers included voltage-controlled oscillators, voltage-controlled filters, voltage-controlled amplifiers, envelope generators, low-frequency oscillators, ring modulators. Famous makers of all-in-one analog synthesizers included Moog, Arp, Roland, Korg and Yamaha. Because of the complexity of generating even a single note using analog synthesis, most synthesizers remained monophonic. A second generation of analog synthesizers emerged later, with limited polyphony, typically supporting four voices. Oberheim was a notable manufacturer of analog polyphonic synthesizers. The Polymoog was an attempt to create a truly polyphonic analog synthesizer, with sound generation circuitry for every key on the keyboard. However, its architecture resembled an electronic organ more than a traditional analog synthesizer, and the Polymoog was not widely imitated. In 1978, the first microprocessor-controlled analog synthesizers were created by Sequential Circuits. These used microprocessors for system control and control voltage generation, including envelope generation, but the main sound generating path remained analog. The MIDI interface standard was developed for these systems. This generation of synthesizers often featured six or eight voice polyphony. Also during this period, a number of analog/digital hybrid synthesizers were introduced, which replaced certain sound-producing functions with digital equivalents, for example the digital oscillators in synthesizers like the Korg DW-8000 (which played back PCM samples of various waveforms) and the Kawai K3 (waveforms constructed via additive synthesis). With the falling cost of microprocessors, this architecture became the standard architecture for high-end analog synthesizers. Analog synthesizers were mostly replaced by digital synthesizers and samplers over the early-mid 1980s. In the mid 1990s a fashion emerged for “retro” analog synthesizers with their proponents claiming that the “analog sound” of old analog synthesizers could not be accurately replicated using samplers or digital synthesizers. This led to increased demand for used analogs (such as the Roland TR-808 drum machine and Roland TB-303 bass synthesizer) the construction of a new generation of analog synthesizers (including modern-day modular synthesizers) and the development of a variety of analog modeling synthesizers which emulate analog VCOs and VCFs using samples, software, or specialized digital circuitry.

Envelope: A synthesizer’s envelope is a way to tailor a “voice” (the technical term is timbre) for the synth, sometimes to help it sound more like a mechanical instrument. A quick attack with little decay helps it sound more like an organ; a slower decay helps it sound more like a guitar. While envelopes are most often applied to volume, they are also commonly used to control other sound elements, such as filter frequencies or oscillator pitches.

Envelope generator: this circuit, on receiving a trigger pulse and/or gate, produces a voltage which changes in a predictable, repeatable manner over time. It can be triggered either manually (for example, by an electronic keyboard) or automatically (for example, by a sequencer). This envelope is most commonly used to control the amplitude of the sound, by routing it to the control input of the VCA or the timbre of the sound, by routing it to the control input of the VCF. This technique can be used to create (synthesize) the volume and timbre contour of naturally decaying sounds such as a piano. The patchable structure of the synthesizer makes it possible to use the envelope generator to modulate other parameters such as the pitch of the VCO, the width of the pulse wave on the VCO and so on. The most common form of envelope contour is the ADSR, although some electronic instruments, such as those produced by Buchla & Associates or the Wiard Synthesizer Company use the simpler AD or AR contour which can be mixed to form an ADSR (Attack, Decay, Sustain, Release) envelope.

Filter: common components of a synthesizer are low-pass filters, high-pass filters, band-pass filters and notch. Filters attenuate frequencies below (high-pass), above (low-pass) or both below and above (band-pass) a certain frequency.

Fingerboard synthesizer is a synthesizer with a ribbon controller or other fingerboard-like user interface used to control parameters of the sound processing. A ribbon controller is similar to a touchpad, however a ribbon controller only registers linear motion. Although it could be used to operate any parameter that is powered by control voltages, a ribbon controller is most commonly associated with pitch control or pitch bending. Old types of fingerboard were resistor-based with the long wire pressed to the resistive plate. The modern ribbon controller has no moving parts. Instead, a finger pressed down and moved along it creates an electrical contact at some point along a pair of thin, flexible longitudinal strips whose electric potential varies from one end to the other. Different the fingerboards instruments – or ribbon controllers – were developed like the Ondes Martenot, the Hellertion, the Trautonium, the Electro-Theremin, the Fingerboard-Theremin, The Persephone, the Continuum, etc. Ribbon controller is used as an additional controller in the Yamaha CS-80, the Korg Prophecy, the Kurzweil synthesizers, etc.

Frequency modulation synthesis (or FM synthesis) is a form of audio synthesis where the timbre of a simple waveform is changed by frequency modulating it with a modulating frequency that is also in the audio range, resulting in a more complex waveform and a different-sounding tone. For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency modulation increases, the sound grows progressively more complex. Through the use of modulators with frequencies that are non-integer multiples of the carrier signal (i.e., non harmonic), bell-like dissonant and percussive sounds can easily be created.

Granular synthesis is a sound synthesis method that operates on the microsound time scale. It is often based on the same principles as sampling but often includes analog technology. The samples are not used directly however, they are split in small pieces of around 1 to 50 ms (milliseconds) in length, or the synthesized sounds are very short. These small pieces are called grains. Multiple grains may be layered on top of each other all playing at different speed, phase and volume. The result is no single tone, but a soundscape, often a cloud, that is subject to manipulation in a way unlike any natural sound and also unlike the sounds produced by most other synthesis techniques. By varying the waveform, envelope, duration, spatial position, and density of the grains many different sounds can be produced. The result is usable as music, sound effects or as raw material for further processing by other synthesis or DSP effects. The range of effects that can be produced include amplitude modulation, time stretching, stereo or multichannel scattering, random reordering, disintegration and morphing.

Low-frequency oscillation (LFO) was first introduced in the modular synths of the 1960s and 70s. Often the LFO effect was accidental; so myriad were the number of configurations that could be ‘patched’ by the synth operator. LFOs have since appeared in some form on almost every synthesizer. More recently other electronic instruments, such as samplers and software synthesizers, have included LFOs to augment their sound alteration capabilities. The primary oscillator circuits of a synthesizer are used to create the audio signals. An LFO is a secondary oscillator that operates at a significantly lower frequency (hence its name), typically around or below the threshold of human hearing (which is approximately 20Hz). This lower frequency or control signal is used to modulate the audio signal, changing it without introducing another sound-signal source. Like a standard oscillator, this usually takes the form of a periodic waveform, such as a sine, sawtooth, triangle or square wave. Also like a standard oscillator, LFOs can incorporate any number of waveform types, including user-defined wavetables, rectified waves and random signals. Using a low-frequency oscillation signal as a means of modulating another signal introduces complexities into the resulting sound, such that a variety of effects can be achieved. The specifics vary greatly depending on the type of modulation, the relative frequencies of the LFO signal and the signal being modulated, et cetera. Electronic musicians use LFO for a variety of applications. They may be used to add simple vibrato or tremolo to a melody, or for more complex applications such as triggering gate envelopes, or controlling the rate of arpeggiation. Differences between LFO rates also account for a number of commonly heard effects in modern music. A very low rate can be used to modulate a filter’s cutoff frequency, thereby providing the characteristic gradual sensation of the sound becoming clearer or closer to the listener. Alternatively, a high rate can be used for bizarre ‘rippling’ sound effects (indeed, another important use of LFO would be for various sound effects used in films).

MIDI controller is used in two senses. In one sense, a controller is hardware or software which generates and transmits MIDI data to MIDI-enabled devices. In the other more technical sense, a MIDI controller is an abstraction of the hardware used to control a performance, but which is not directly related to note-on/note off events. A slider assigned to open and close a low-pass filter on a synthesizer may be assigned to controller 18, for example. Changes in the position of the slider are transmitted along with “18” so that they are distinguished from changes in the value of other controllers.

MIDI (Musical Instrument Digital Interface) is an industry-standard electronic communications protocol that enables electronic musical instruments, computers and other equipment to communicate, control and synchronize with each other in real time. MIDI does not transmit an audio signal or media — it simply transmits digital data “event messages” such as the pitch and intensity of musical notes to play, control signals for parameters such as volume, vibrato and panning, cues and clock signals to set the tempo. As an electronic protocol, it is notable for its success, both in its widespread adoption throughout the industry, and in remaining essentially unchanged in the face of technological developments since its introduction in 1983.

Modular synthesizer
The modular synthesizer is a type of synthesizer consisting of separate modules which must be connected by wires (patch cords) to create a so-called patch. These synthesizers are very flexible. Every output generates an electric current of variable voltage. All inputs expect a voltage so that almost any combination of connections between the modules is allowed and valid. There exist many different modules and even modules with the same function have different inputs and outputs on various models. Modules of different brands may be combined. More and more manufacturers are bulding eurorack format (Doepfer format) modules that are compatible (Doepfer, Analogue Solutions, Analogue Systems, Plan B, LiveWire, Cwejman…).
Some standard modules found on almost any modular synth are VCO – VCF – VCA – LFO – Ring modulator – Mixer – Sample&Hold –
Sequencer and Slew Limiter.

Mixer: combines multiple signals into one.

Pulse-width modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load.

Ring modulation is an effect in electronics, related to amplitude modulation or frequency mixing, performed by multiplying two audio signals, where one is typically a sine-wave or another simple waveform. It is referred to as “ring” modulation because the analog circuit of diodes originally used to implement this effect took the shape of a ring. This circuit is similar to a bridge rectifier, except that instead of the diodes facing “left” or “right”, they go “clockwise” or “anti-clockwise”. It can be used to create bell-like sounds. On lower modulation frequencies, the ring modulation is perceived as a tremolo effect Ring modulators are mostly used in synthesizers. They combine or heterodyne two waveforms, and output the sum and difference between the two. This process of ring modulation, which also amplitude modulation, produces a signal rich in overtones, suitable for producing bell-like or otherwise metallic sounds.

Sample-based synthesis is a form of audio synthesis that can be contrasted to either subtractive synthesis or additive synthesis. The principal difference with sample-based synthesis is that the seed waveforms are sampled sounds or instruments instead of fundamental waveforms such as the saw waves of subtractive synthesis or the sine waves of additive synthesis

Sample and hold: takes a “sample” of the input voltage when a trigger pulse is received and “holds” it until a subsequent trigger pulse is applied.

Sequencer was originally any device that recorded and played back a sequence of control information for an electronic musical instrument. More recently the term has come to refer to the feature of recording software which allows the user to record, play back and edit MIDI data. This is distinct from the software features which record audio data. Early analog music sequencers used control voltage/trigger interface, but were replaced by digital hardware- or software-based MIDI sequencers, which play back MIDI events and MIDI control information at a specified number of beats per minute. As computer speeds increased in the 1990s, audio recording, audio editing, and sample triggering features were added to the software. Software so enhanced is called a digital audio workstation (DAW). DAWs almost always include sequencing features but, strictly speaking, go beyond what a sequencer is. Many sequencers have features for limited music notation, or are able to show music in a piano roll notation.

Slew Limiter: smooths off the peaks of voltages. This can be used to create glide or portamento between notes. Can also work as a primitive low-pass filter.

Sound synthesis is any of a number of methods of sound generation that a piece of hardware or software may employ, e.g. subtractive synthesis or FM synthesis. Modern electronic keyboard instruments are based on digital sound synthesizers that create audio waveforms that sound like they came from a violin, for example, without an actual violin, because the sound is synthesized to emulate the violin waveform patterns.

Subtractive synthesis is a method of subtracting overtones from a sound via sound synthesis, characterised by the application of an audio filter to an audio signal. For example, taking the output of a sawtooth generator and using a low-pass filter to dampen its higher partials generates a more natural approximation of a bowed string instrument than using a sawtooth generator alone. Typically, the complexity of the source signal and the cut-off frequency and resonance of the filter are controlled in order to simulate the natural timbre of a given instrument. Subtractive synthesis is historically associated with analogue voltage controlled synthesizers such as the Moog synthesizer due to the simple circuitry required to generate the most common source signals: square waves, pulse waves, sawtooth waves and triangle waves. Modern digital and software synthesizers may include other, more complex waveforms or allow the user to upload arbitrary waveforms. Some synthesizers may use a form of pulse width modulation which dynamically alters the source for a richer, more interesting, more organic tone.

Synthesizer as used in music, is a term derived from a Greek word syntithenai (synthesis) and is being used to describe a device capable of generating and/or manipulating electronic signals for use in music creation, recording and performance. A synthesizer is capable of generating and manipulating audio tones such as for example musical notes. Synthesizers can create an electrical signals needed to drive and play audio amplifiers. The tone is generated by electrical circuits which may have adjustable parameters (as in analog synthesizers), and can perform mathematical manipulation of signal using a microprocessor and digital signal processing (as in digital synthesizers), or by a combination of both methods. Synthesized sounds may sometimes contrast with recordings of natural sound , though sampling synthesizers significantly blur this distinction. Music synthesizers sometimes include a keyboard, which makes them reminiscent of certain traditional musical instruments, like a piano or an organ. Various alternative or additional pitch controllers, such as “fingerboards” and “ribbons”, have been employed as well.

VCA
A voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage (often abbreviated CV).They have many applications, including audio level compression, synthesizers, and amplitude modulation.

VCF
voltage-controlled filter (VCF) is an electronic filter whose operating characteristics can be controlled by means of a control voltage applied to one or more inputs. A VCF allows its cutoff frequency and Q factor to be continuously varied; it usually gives a lowpass response, but may also be switchable to allow highpass, bandpass or even notch responses. The filter may offer a switchable slope which determines how quickly signals outside the pass band become attenuated – usually 12dB/octave (a ‘2 pole’ filter) or 24dB/octave (a ‘4 pole’ filter). In analog synthesizers, which are commonly used to make electronic music, VCFs are commonly positioned after the oscillator(s). The oscillator generates an audio waveform, which (except for noise waveforms) includes a fundamental pitch and a series of harmonic overtones. By varying the cutoff frequency (the maximum frequency passed by the filter), the synth operator can add or remove some of the overtones to create more interesting and textured sounds. In much electronic music, “filter sweeps” have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF (sometimes very slowly) to reveal or conceal the oscillator’s overtones. A VCF is an example of an active non-linear filter: however, if its control voltage is kept constant, it will behave as a linear filter.

VCO
A voltage-controlled oscillator or VCO is an electronic oscillator specifically designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation, or rate of repetition, is varied with an applied DC voltage, while modulating signals may be fed into the VCO to generate frequency modulation (FM), phase modulation (PM), and pulse-width modulation (PWM).

Waveforms: Oscillators generate a waveform at a certain frequency (most usually waveforms are a square wave or a sawtooth wave, but also be pulse, triangle and sine waves. The sawtooth wave (or saw wave) is a kind of basic non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw. The usual convention is that a sawtooth wave ramps upward as time goes by and then sharply drops. However, there are also sawtooth waves in which the wave ramps downward and then sharply rises. The latter type of sawtooth wave is called a ‘reverse sawtooth wave’ or ‘inverse sawtooth wave’. The 2 orientations of sawtooth wave sound identical when other variables are controlled. A square wave is a basic kind of non-sinusoidal waveform encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels, which may or may not include zero.

Wavetable synthesis is a technique used in certain digital music synthesizers to produce natural tone-like sounds. The sound of an existing instrument (a single note) is sampled and parsed into a circular sequence of samples or wavetables, each having one period or cycle per wave; a set of wavetables with user specified harmonic content can also be generated mathematically. Upon playback, these wavetables are used to fetch samples (table-lookup) in the same manner as in a Numerically-controlled oscillator to produce a waveform. However, in wavetable synthesis, the output waveform is not normally static and evolves slowly in time as one wavetable is mixed with another, creating a changing waveform. Looping occurs when the wavetable evolution is halted, slowed, or reversed in time.