Roland's first digital synthesizer model, introduced in 1987. Designed specifically to compete with Yamaha's DX-7, the D-50 used a rather odd hybrid voice architecture. It appears that Roland may have considered a rompler architecture, but memory was too expensive at the time. So they devised an approach called "Linear Arithmetic"; this involved ROM samples of short attack transients, which the synth spliced together with waveforms from a sort of virtual analog wave generator. This then went through a digital lowpass filter algorithm and digital VCA in conventional fashion. The synth was 16 voice polyphonic, and up to four-way layering was possible.
The D-50 was a sales success, and one of the few models to compete successfully with the DX-7 in the late 1980s. It was noted for its tuned percussion and atmospheric sounds; some of the factory patches were widely used in movie soundtracks, and became emblematic of the era. The D-50 also set the style for Roland models up into the late 1990s; the all-black look was a big change from the charcoal gray panels and colorful pushbuttons of earlier models.
The D-50 was a keyboard model; the same electronics were available as a rackmount synthesizer called the D-550. The panels contained a limited set of controls, but Roland produced a programmer called the PG-1000, which made most of the patch parameters available as sliders.
Subsequent models included the D-10 keyboard and D-110 rackmount, which were smaller and multitimbral and had an improved MIDI implementation, and the D-20, which was the final model in the series. A model D-70 was produced, but this was a pure rompler and did not have the Linear Arithmetic architecture.
Also in 2017, Roland released a VST plugin version of the D-50. via their Roland Cloud website.