From Yamaha Wiki
The Yamaha DSP-1 was a revolutionary piece of early home theater Surround sound equipment, produced in 1985 by the Yamaha Corporation. The DSP-1 (referred to by Yamaha as a Digital Sound field Processor) allowed owners to synthesize up to 6-channels of surround sound from 2 channel stereo sound via a complex Digital signal processor (DSP). Much like today's home theater receivers the DSP-1 offered sixteen "sound fields" created through the DSP including a jazz club, a cathedral, a concert hall, and a stadium. However, unlike today's receivers, these sound field modes were highly editable, allowing the owner to customize the effect to his or her own personal taste. The DSP-1 also included an analog Dolby Surround decoder as well as other effects such as real-time echo and pitch change.
Oddly, most of the DSP-1's controls are on the unit's remote control. This can make it difficult for collectors to find a complete functioning unit.
Yamaha has kept the DSP prefix for many of its home DSP and audio receiver products.