From Wikipedia
The Roland TR-909 Rhythm Composer is a partially analog, partially sample-based drum machine built by the Japanese Roland Corporation in 1984. The brainchild of Tadao Kikumoto, the engineer behind the Roland TB-303, it features a 16-step step sequencer and a drum kit that, at that time, aimed for realism and cost-effectiveness. It is fully-programmable, and like its predecessor, the TR-808, it can store entire songs with multiple sections, as opposed to simply storing patterns. Only around 10,000 units were produced.
All drums except for the hi-hats and cymbals are synthetically generated; there is an oscillator circuit with a dedicated filter and envelope curve. The hi-hats and cymbals are 6-bit samples, compressed and combined with a volume envelope curve (and tuning) to allow slight modification. Thanks to the analog circuitry, various aspects of the drum sound can be modified (pitch, attack, decay).
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