The sound generator is usually a synthesizer, which produces sounds by processing audio signals in electronic circuits (which can be analogue or digital), or a sampler, which produces sounds by replaying a stored recording of the sound (and the storage again can be analogue or digital).
The sounds produced by the sound generator will normally include drums and instrument sounds.
The sequencer stores events like notes and controllers (like pitch bend), and then replays them into the sound generator, which then makes the music.
One of the earliest instruments to describe itself as a "music workstation" was the Roland W-30, released in the late 1980s, though the earlier Roland D-20 was a combined sequencer and synthesizer.
Although many music workstations have a keyboard, this isn't always the case. In the 1990s, Yamaha, and then Roland, released a series of portable music workstations (staring with the Yamaha QY-10). These are sometimes called Walkstations.
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