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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 waveshaper s 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.
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.
When microprocessors first appeared on the scene in the early 1970s, they were costly and difficult to apply. The first practical polyphonic synth, which was also the first to fully apply a microprocessor as a controller, was the Sequential Circuits Prophet-5 (1977). For the first time, musicians had a practical polyphonic synthesizer that allowed all knob settings to be saved in computer memory and recalled by pushing a button. The Prophet-5 was also physically compact and lightweight, unlike its predecessors. This basic design paradigm became a standard among synthesizer manufacturers, slowly pushing out the more complex (and more difficult to use) modular design.
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.
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.
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