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Synthesizer

The term "synthesiser" is also used to mean frequency synthesiser, an electronic system found in communications. This article is about the musical instrument.

Synthesizers create sounds through direct manipulation of electrical currents (as in analog synthesizers), mathematical manipulation of discrete values using computers (as in software synthesizers), or by a combination of both methods. In the final stage of the synthesizer, electrical currents are used to cause vibrations in the diaphragms of loudspeakers, headphones, etc. This synthesized sound is contrasted with recording of natural sound, where the mechanical energy of a sound wave is transformed into a signal which will then be converted back to mechanical energy on playback (though sampling significantly blurs this distinction).

The term "speech synthesizer" is also used in electronic speech processing, often in connection with vocoders.

Table of contents

1 Sound basics 2 Overview of popular synthesis methods 3 Synthesizer basics 4 The start of the analog synthesizer era 5 Electronic organs vs. synthesizers 6 Microprocessor controlled and polyphonic analog synthesizers 7 MIDI control 8 FM synthesis 9 Samplers and sampling 10 The physical modeling synthesizer 11 The modern digital synthesizer 12 Software-only synthesis 13 Commercial synthesizer manufacturers 14 Classic synthesizer designs 15 See also 16 Source 17 External links

Sound basics

When natural tonal instruments' sounds are analyzed in the frequency domain, the spectra of tonal instruments exhibit amplitude peaks at the harmonics. These harmonics' frequencies are primarily located close to the integer multiples of the tone's fundamental frequency.

Percussivess and rasps usually lack harmonics, and exhibit spectra that are comprised mainly of noise shaped by the resonant frequencies of the structures that produce the sounds. The resonant properties of the instruments (the spectral peaks of which are also referred to as formants) also shape the spectra of string, wind, voice and other natural instruments.

In most conventional synthesizers, for purposes of resynthesis, recordings of real instruments can be thought to be composed of several components.

These component sounds represent the acoustic responses of different parts of the instrument, the sounds produced by the instrument during different parts of a performance, or the behaviour of the instrument under different playing conditions (pitch, intensity of playing, fingering, etc...) The distinctive timbre, intonation and attack of a real instrument can therefore be created by mixing together these components in such a way as resembles the natural behaviour of the real instrument. Nomenclature varies by synthesizer methodology and manufacturer, but the components are often referred to as oscillators or partials. A higher fidelity reproduction of a natural instrument can typically be achieved using more oscillators, but increased computational power and human programming is required, and most synthesizers use between one and four oscillators by default.

One of the most salient aspects of any sound is its amplitude envelope. This envelope determines whether the sound is percussive, like a snare drum, or persistent, like a violin string. Most often, this shaping of the sound's amplitude profile is realized with an "ADSR" (Attack Decay Sustain Release) envelope model applied to control oscillator volumes. Each of these stages is modelled by a change in volume (typically exponential). The attack is the initial run-up of the sound level. The decay is the run down immediately after the attack. Sustain is the volume when the note is held. The release is the volume profile when the note is released. Exponential rates are commonly used because they closely model real physical vibrations, which usually rise or decay exponentially.

Although the oscillations in real instruments also change frequency, most instruments other than the viol-family and human voice can be modelled well without this refinement. This refinement is necessary to generate a vibrato.

Overview of popular synthesis methods

One of the easiest synthesis systems is to record a real instrument as a digitized waveform, and then play back its recordings at different speeds to produce different tones. This is the technique used in "sampling." Most samplers designate a part of the sample for each component of the ADSR envelope, and then repeat that section while changing the volume for that segment of the envelope. This lets the sampler have a persuasively different envelope using the same note. See also: Sample-based synthesis.

Synthesizer basics

There are two major kinds of synthesizers, analog and digital.

There are also many different kinds of synthesis methods, each applicable to both analog and digital synthesizers. These techniques tend to be mathematically related, especially frequency modulation and phase modulation.

• Subtractive synthesis
• Additive synthesis
• Granular synthesis
• Wavetable synthesis
• Frequency modulation synthesis
• Physical modelling synthesis
• Digital sampling

In the 1950s, RCA produced experimental devices to synthesize both voice and music. The Mark II Music Synthesizer (1958) was only capable of producing music once it had been completely programmed; that is, the system had to be completely re-set for each new piece. A wide paper tape was punched with holes that controlled pitch sources and filters, similar to a mechanical player piano but with far greater control over timbre.

In 1958 Daphne Oram at the BBC Radiophonic Workshop produced a novel synthesizer using her "Oramics" technique, driven by drawings on a 35mm film strip. This was used for a number of years at the BBC. Hugh Le Caine, John Hanert, Raymond Scott, Percy Grainger (with Burnett Cross), and others built a variety of automated electronic-music controllers during the late 1940s and 1950s.

By the 1960s, synthesizers were developed which could be played in real time but were confined to studios because of their size. Modularity was the usual design, with standalone signal sources and processors being connected with patch cords or by other means, and all controlled by a common controlling device.

Early synthesizers were often experimental special-built devices, usually based on the concept of modularity. Donald Buchla, Hugh Le Caine, Raymond Scott and Paul Ketoff were among the first to build such instruments, in the late 1950s and early 1960s. Only Buchla later produced a commercial version.

The first playable modern configurable music synthesizer was created by Robert Moog and displayed at the Audio Engineering Society convention in 1964. It took hours to set up the machine for a new sound. The Moog synthesizer was at first a curiosity, but by 1968 it had caused a sensation.

Among the first music performed on this synthesizer was the million-selling 1968 album Switched-On Bach by Walter Carlos (now Wendy Carlos). Switched-On Bach was one of the most popular classical-music recordings ever made. During the late 1960s, hundreds of other popular recordings used Moog synthesizer sounds. The Moog synthesizer even spawned a subculture of record producers who made novelty "Moog" recordings, depending on the odd new sounds made by their synthesizers (which were not always made by Moog) to draw attention and sales.

Moog also established standards for control interfacing, with a logarithmic 1-volt-per-octave pitch control and a separate pulse triggering signal. This standardization allowed synthesizers from different manufacturers to operate together. Pitch control is usually performed either with an organ-style keyboard or a music sequencer, which produces a series of control voltages over a fixed time period and allows some automation of music production.

Other early commercial synthesizer manufacturers included ARP, who also started with modular synthesizers before producing all-in-one instruments, and British firm Electronic Music Systems.

One major innovation by Moog was in 1970, when they made a synthesizer with a built-in keyboard and without modular design--the analog circuits were retained, but made interconnectable with switches in a simplified arrangement called "normalization". Though less flexible than modularity, it made the instrument more portable and its use much easier. This first prepatched synthesizer, the Minimoog, became very popular, with over 12,000 units sold. The Minimoog also influenced the design of nearly all subsequent synthesizers.

In the 1970s miniaturized solid-state components let synthesizers become self-contained and movable. They began to be used in live performances. Soon, electronic synthesizers had become a standard part of the popular-music repertoire, with Giorgio Moroder's "Son of my Father" the first #1 hit to feature a synthesizer (Shapiro, 2000).

Electronic organs vs. synthesizers

All organs (including acoustic) are based on the principle of additive or Fourier synthesis: Several sine tones are mixed to form a more complex waveform. In the original Hammond organ, built in 1935, these sine waves were generated using revolving tone wheels which induced a current in an electromagnetic pick-up. For every harmonic, there had to be a separate tonewheel. In more modern electronic organs, electronic oscillators serve to produce the sine waves. Organs tend to use fairly simple "formant" filters to effect changes to the oscillator tone--automation and modulation tend to be limited to simple vibrato.

Most analog synthesizers produce their sound using subtractive synthesis. In this method, a waveform rich in overtones, usually a sawtooth or pulse wave, is produced by an oscillator. The signal is then passed through filters, which preferentially remove some overtones to obtain a sound which may be an imitation of an acoustical instrument, or may be a unique tonality not existing in acoustical form. An ADSR envelope generator then controls a VCA (voltage controlled amplifier) to give the sound a loudness contour.

Other circuits, such as waveshapers and ring modulators, can change the tonality in non-harmonic ways or create distortion effects which are often not found in natural sound sources. In spite of the popularity of modern digital and software-based synthesizers, the purely analog modular synthesizer still has its proponents, with a number of manufacturers producing modules little different from Moog's 1964 circuit designs, as well as many newer variations.

Microprocessor controlled and polyphonic analog synthesizers

Early analog synthesizers were always monophonic, producing only one tone at a time. A few, such as the Moog Sonic Six, ARP Odyssey and EML 101, were capable of producing two different pitches at a time when two keys were pressed. Polyphony (multiple "voices", each having its own pitch, thus allowing the playing of chords), was only obtainable with electronic organ designs at first. Popular electronic keyboards combining organ circuits with synthesizer processing included the ARP Omni and Moog's Polymoog and Opus 3.

By 1976 the first true music synthesizers to offer polyphony had begun to appear, most notably in the form of the Yamaha CS-80 and Oberheim Four-Voice. These early instruments were very complex, heavy, and costly. Another feature that began to appear was the recording of knob settings in a digital memory, allowing the changing of sounds quickly.

MIDI control

Synthesizers became more usable with the invention in 1983 of MIDI, a standardized digital control interface, and later with the creation of all-digital synthesizers and samplers. MIDI interfaces are now almost ubiquitous on music equipment, as well as personal computers.

The so-called General MIDI (GM) standard was devised in the late 1980s to serve as a consistent way of describing the set of synthesized tonalities available to a PC for playback of musical scores. For the first time, a given MIDI preset would consistently produce an oboe or guitar sound (etc.) on any GM-conforming device. The file format .mid was also established and became a popular standard for exchange of music scores between computers.

FM synthesis

John Chowning of Stanford University is generally considered to be the first researcher to conceive of producing musical sounds by causing one oscillator to modulate the pitch of another. This is now called FM, or frequency modulation, synthesis. Chowning's early FM experiments were done with software on a mainframe computer.

In a nutshell, FM uses digitally-generated sine-wave oscillators (called operators). Each operator's audio output may be fed to the input of another operator, via an ADSR or other envelope controller. The first operator modulates the pitch of the second operator, in ways that can produce complex waveforms. FM synthesis is fundamentally a type of additive synthesis and the filters used in subtractive synthesizers were typically not used in FM synthesizers until the mid-1990s. By cascading operators and programming their envelopes appropriately, almost any subtractive synthesis effect can be simulated. FM is also well-suited for making sounds that subtractive synthesizers have difficulty producing, such as complex plucked-string and bell timbres.

Chowning's patent covering FM sound synthesis was licensed to giant Japanese manufacturer Yamaha, and made millions for Stanford during the 1980s. Yamaha's first FM synthesizers, the GS-1 and GS-2, were costly and heavy. Their second version, the DX-7 (1983), was about the same size and weight as the Prophet-5, was reasonably priced, and depended on custom digital integrated circuits to produce FM tonalities. The DX-7 was a smash hit and may be heard on thousands of pop recordings from the 1980s. Yamaha later licensed its FM technology to other manufacturers. By the time the Stanford patent ran out, almost every personal computer in the world contained an audio input-output system with a built-in 4-operator FM digital synthesizer -- a fact most PC users are not aware of.

Samplers and sampling

One kind of synthesizer starts with a binary digital recording of an existing sound, which is then replayed at a range of pitches. This is called a sampler.

Sampling can also be used in combination with other synthesizer effects. Some popular software synthesizers take a sampled sound and process it with software-based filters, reverbs, ring modulators and the like.

Sampling started out as the purview of academic researchers with access to mainframe computers. The appearance of the Fairlight CMI in 1979, the first well-known digital instrument capable of sampling, started a revolution. The Fairlight was used on scores of popular recordings by artists such as Peter Gabriel and Art of Noise. The costly, complex and rare Fairlight (and an equally costly competitor, the New England Digital Synclavier) caused California company E-Mu to introduce their Emulator in 1981, a lower-cost sampling keyboard which could save sound recordings to floppy disk.

The physical modeling synthesizer

Physical modelling synthesis is the synthesis of sound by using a set of equations and algorithms to simulate a physical source of sound. When an initial set of parameters is run through the physical simulation, the simulated sound is generated.

Although physical modelling was not a new concept in acoustics and synthesis, it wasn't until the development of the Karplus-Strong algorithm, the subsequent refinement and generalization of the algorithm into digital waveguide synthesis by Julius O. Smith III and others, and the increase in DSP power in the late 1980s that commercial implementations became feasible.

Following the success of Yamaha's licensing of Stanford's FM synthesis patent, Yamaha signed a contract with Stanford University in 1989 to jointly develop digital waveguide synthesis. As such, most patents related to the technology are owned by Stanford or Yamaha. A physical modeling synthesizer was first realized commercially with Yamaha's VL-1, which was released in 1994.

The modern digital synthesizer

Most modern synthesizers are now completely digital, including those which model analog synthesis using digital techniques. Digital synthesizers use digital signal processing (DSP) techniques to make musical sounds. Some digital synthesizers now exist in the form of 'softsynth' software that synthesizes sound using conventional PC hardware. Others use specialized DSP hardware.

Digital synthesizers generate a digital sample, corresponding to a sound pressure, at a given sampling frequency (typically 44100 samples per second). In the most basic case, each digital oscillator is modelled by a counter. For each sample, the counter of each oscillator is advanced by an amount that varies depending on the frequency of the oscillator. For harmonic oscillators, the counter indexes a table containing the oscillator's waveform. For random-noise oscillators, the most significant bits index a table of random numbers. The values indexed by each oscillator's counter are mixed, processed, and then sent to a digital-to-analog converter, followed by an analog amplifier.

To eliminate the difficult multiplication step in the envelope generation and mixing, some synthesizers perform all of the above operations in a logarithmic coding, and add the current ADSR and mix levels to the logarithmic value of the oscillator, to effectively multiply it. To add the values in the last step of mixing, they are converted to linear values.

Software-only synthesis

The earliest digital synthesis was performed by software synthesizers on mainframe computers using methods exactly like those described in digital synthesis, above. Music was coded using punch cards to describe the type of instrument, note and duration. The formants of each timbre were generated as a series of sine waves, converted to fixed-point binary suitable for digital-to-analog converters, and mixed by adding and averaging. The data was written slowly to computer tape and then played back in real time to generate the music.

Today, a variety of software is available to run on modern high-speed personal computers. DSP algorithms are commonplace, and permit the creation of fairly accurate simulations of physical acoustic sources or electronic sound generators (oscillators, filters, VCAs, etc). Some commercial programs offer quite lavish and complex models of classic analog synthesizers--everything from the Yamaha DX-7 to the original Moog modular. Other programs allow the user complete control of all aspects of digital music synthesis, at the cost of greater complexity and difficulty of use.

Commercial synthesizer manufacturers

Notable synthesizer manufacturers past and present include:

• ARP
• Akai
• Electronic Music Studios
• E-mu
• Ensoniq
• Fairlight CMI
• Korg
• Moog
• New England Digital
• Novation
• Oberheim
• Roland Corporation
• Sequential Circuits
• WaveFrame Corporation
• Yamaha

• Minimoog (Yes, Emerson Lake and Palmer, Stereolab)
• EMS VCS3 (Roxy Music, Source | Copyright

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