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The word 'Synthesizer' didn't arrive on the scene until the 1950s with the RCA Synthesizer I and II, but it wasn't long before these room-sized pieces of engineering had been, themselves, 'synthesized' down into more acceptable components and 'modules' thanks to the pioneering work of visionaries like Dr. Robert Moog, Don Buchla, Harold Bode, Pete Zinovieff, and Dave Cockerell.
Moog is generally, and appropriately, credited for taking the synthesizer out of the university laboratory and putting it in the hands of musicians. Certainly from the time of Wendy Carlos' ground-breaking Switched On Bach recording (1968) to the release of the MiniMoog (1970) both musicians and the music-buying public became enamored by the sonic possibilities on the musical horizon.
Much like the player pianos before them, one of the major attractions of these new synthesizers was the ability to control them externally, not with piano rolls, but control voltages. Unfortunately, even though gates and voltages are relatively simply, there was never any real standard – some companies used a volt per octave, some used hertz per volt and there were even different implementations with voltages including -5 to 5v, 0 to 5v, 0 to 10v. But the concept of 16 rows of steps with various parameters can be found on many products from the Roland TR808 to the Yamaha Tenori-on to any number of current tablet based apps.
The popularity of synthesizers got a major boost in 1978 when microprocessor-based instruments began to appear, spearheaded by a new California company Sequential Circuits. The Prophet-5, though still hugely limited by today's standards, offered reasonable levels of playability, stability, and polyphony, albeit at a hefty price at the time (around $4000).
Soon Korg, Roland, and Yamaha's microprocessor-based offerings would slash prices in half, and by the turn of the decade the polyphonic synthesizer was firmly on the map for every self-respecting keyboard player from hobbyists to touring professionals.
Stability, playability, and polyphony continued to evolve in the early 1980s but compatibility remained a thorn in the side of manufacturers. The multifarious nature of synthesizer design meant that each manufacturer had been defining pitch and timing (Control Voltage and Gate) data in their own way. Once polyphonic, digital technology became available manufacturers they began to design unique digital interfaces that would, at the very least, allow you to connected several Korg, or Roland, or Yamaha synths together. Roland developed its DCB (Digital Communication Bus), Yamaha its Key Code Interface etc