Sequential Circuits Inc. (SCI) was a California-based synthesizer company that was founded in the early 1970s by Dave Smith, and sold to Yamaha Corporation in 1987. Throughout its lifespan, Sequential pioneered technologies and design principles that have served as a foundation for the development of modern music technology. Sequential was also pivotal in the planning, design, and support of 1982's groundbreaking innovation in electronic music, MIDI.
Products and innovations 
Sequential's first synthesizer, the brainchild of Dave Smith and John Bowen, was the very successful Prophet-5, released in 1978. This was the first affordable, fully programmable polyphonic analog synthesizer, which enjoyed considerable popularity in the early 1980s. By combining full microprocessor control with low-cost synthesizer module chips (initially by Solid State Music and later Curtis Electromusic), Sequential was able to produce a relatively low-cost synthesizer with five voices of polyphony. The then-revolutionary principle of combining five analog voices with easy editing and programming within a compact format established a new standard for polyphonic instruments. With the addition of patch storage (an innovation by Dave Smith with assistance from E-mu's Dave Rossum)  the user was able to cycle through up to one-hundred sounds at the push of a button. These were viewed as remarkable developments in the synthesizer industry, especially from a previously unknown company that operated out of a California garage. Dave Smith designed, programmed, and built the Prophet 5 in less than eight months. The Prophet-5 was in production from 1978–1984, and sold approximately 8,000 units.
Sequential followed up this successful debut with a ten-voice version of the Prophet 5, the Prophet-10, in 1980. The Prophet 10 was packaged in a massive dual-manual format, with a digital sequencer and the ability to play two different timbres simultaneously. When equipped with the optional sequencer, it sold for $9,000 USD. Fewer than 1,000 were produced.
In 1981, Sequential released a low-cost monophonic version of the Prophet 5, the Pro-One. The Pro-One became particularly popular, with sales of over 10,000 units.
Sequential was also instrumental in developing MIDI, and introduced the first MIDI synthesizer: the Prophet 600 in 1982. At the January, 1983 NAMM convention, this instrument successfully communicated with a Roland Jupiter-6 synthesizer in the first public demonstration of the MIDI protocol. At the Goodman Music Exposition in 1983, Hybrid Arts and Robert Moore demonstrated Atari's first personal computer, the Atari 800 (8 bit with 48k RAM), communicating with a MIDI equipped Prophet 600. This was made possible through the MidiMate hardware interface and MidiTrack program, developed by Moore and his partner, Paul Rother. The MidiMate holds the distinction as the first MIDI to personal computer interface, and physically connected the Prophet 600 to the Atari 800 computer. The virtual interface was achieved through the MidiTrack program, which included a 16 track MIDI recorder/sequencer, full graphic user interface (GUI), and a programmer/editor/librarian. Moore designed the graphics, and programming was accomplished using a light pen. The software application displayed the controls of the Prophet 600 on the Atari's CRT, and made it possible to store over 1,000 sound patches from the Prophet 600 onto a single floppy disk.
1983 also saw the release of the Prophet-T8, which featured a 76-note wooden keyboard (uneven A to C), and implemented a basic MIDI interface. It had an eight-voice architecture that was similar to that of the Prophet 5. The weighted, velocity and aftertouch-sensitive keyboard was so well received that New England Digital adopted it for use in their Synclavier workstations. Dave Smith himself keeps a T8 unit as the main controller keyboard in his home studio.
The Six-Trak was released in 1984. It was one of the first multitimbral synthesizers, and featured MIDI and an on-board six-track digital sequencer. It was designed as an inexpensive and easily portable 'scratch-pad' for writing arrangements. Its larger six voice brother, the MultiTrack would be released that following year.
Sequential released the six voice, six-way multitimbral Max in 1984, which was heavily dependent upon the accompaniment of an external computer for editing duties. Thus began the ill-fated leap into the new realm of computer-based editing and sequencing, which would prove to be a relatively unsuccessful experiment that set the stage for further financial troubles.
Sequential also released two drum machines during the mid-1980s, Drumtraks (1984) and TOM (1985).
The Prophet VS vector synthesizer, Sequential's only digital synthesizer, was released in early 1986. Boasting a scheme described as vector synthesis, it combined a digital waveform generator and vector joystick to the proven analog Curtis filters, which resulted in a unique instrument with a very distinct sound. The VS is still revered today despite its reliance on rare custom components with high failure rate.
Sequential Circuits' final release was the Studio 440. This expensive ($5000 USD) unit was constructed like a drum machine, and combined a sampler with a sequencer to make a music composition workstation. This preceded the release of the popular Akai MPC sequencers by several years, which were designed by Roger Linn, a good friend and frequent collaborator of Dave Smith.
In 1987, Sequential was finalizing development of a 16-bit sampler called the Prophet 3000, but went out of business and was acquired by Yamaha. Yamaha liquidated the few completed Prophet 3000 samplers at a substantial discount. Like most of the Sequential line, this sampler contained features that were far ahead of its time, such as automatic pitch detection, key-mapping, a remote control interface, and facilities for easily looping and trimming sampled sounds. Many of these technologies were later included in Yamaha's A-series samplers.
Support for Sequential Circuits instruments is now provided by Wine Country Productions, which is directed by ex-sequential employee Dave Sesnak. After a short stint at Yamaha to develop the SY22 and TG33 vector synthesizers, several members of the Sequential team became part of the Korg R&D department. Dave Smith consulted with Korg during that time, and the powerful and memorable Korg Wavestation synthesizer was born. This synthesizer borrowed design elements from the Prophet VS, but broke new ground in making full use of rapidly evolving digital technology.
Sequential's legacy products are popular targets for simulation or emulation in software synthesizers, with developers such as Native Instruments and SonicCore offering virtual instruments inspired by Sequential designs. Various analog modeling synthesizers also include presets to emulate Sequential's signature sounds. For example, the Clavia Nord Lead includes a preset patch bank which contains faithful recreations of the Prophet 5's original factory sounds.
Today, Dave Smith operates Dave Smith Instruments, which manufactures, among other things, the Prophet '08 (an update of Sequential Circuits' Prophet line), and the Tempest analog drum machine.
Sequential Circuits products 
- Model 600 (1974) – analog sequencer 
- Model 800 (1975) – digital sequencer 
- Model 700 (1977) – programmer 
- Prophet-5 (1978) – 5-voice analog synthesizer with patch memory
- Fugue (1979) – ensemble keyboard, designed by SCI and built by Siel 
- Prophet-10 (1980) – essentially two Prophet-5s in one large enclosure
- Pro-One (1980) – basically a monophonic version of the Prophet 5
- Pro-FX (1982) – the first programmable modular effects units 
- Prelude (1982) – 4 section orchestral synthesizer, manufactured by Siel; a silk screen variation of Orchestra 2 and OR400 
- Prophet 600 (1982) – the first MIDI synthesizer
- Prophet Remote (1982) – remote keyboard for Prophet-5 
- Prophet T8 (1983) – The first "piano action" MIDI synthesizer
- Six-trak (1984) – the first multi-timbral synthesizer
- Drumtraks (1984) – programmable drum machine with MIDI
- MAX (1984) – 6 voice synthesizer controlled by external PC
- Multi-Trak (1985) – Sequential's second multi-timbral synthesizer 
- Split-8 / Pro-8 (1985) – 8-voice synthesizer
- TOM (1985) – advanced programmable drum machine
- Prophet 2000 (1985) – 12bit sampler
- Prophet VS (1986) – the first vector synthesizer
- Studio 440 (1986) – sequencing/sampling drum and music production machine 
- Prophet 3000 (1987) – 16bit sampler, the final Sequential Circuits product
- Julian Colbeck, Keyfax Omnibus Edition, MixBooks, 1996, p. 123
- Peter Forrest, The A-Z Of Analogue Synthesisers, Short Run Press Ltd, p. 107
- http://www.vintagesynth.com/sci/seqpro1.php Pro-One
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- Joseph Akins, An Overview of Electronic Music History
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