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The Prologue 21 (プロローグ21) is a "multimedia karaoke system" released by Sega in Japan during the 1990s. It is an exceedingly rare device outside of Japan and so very little is understood about it in the English speaking world.
It is not thought that Prologue 21s were sold directly to consumers, but rather karaoke venues and similar outlets. An entire Prologue 21 system consists of four separate parts; the "Amplifier" (SKA-3000), the "Commander" (SKC-1000), the "Changer" (SKA-2000A) and the "Wireless Microphone Receiver" (SKN-xxx) for Sega-branded SKN-5000 microphones. Manufacturing was actually handled by Columbia, who is credited on the back of the system. Also offered were rechargable "Song Navigators" (SKH-4000A) manufactured by NEC, which are glorified remote controls, and the SKY-4100 Remote.
The system would be "locked" until "credits" were bought directly from Sega - Sega made its money by demanding repeat payments to keep Prologue 21s running, and entirely "unlocked" versions are a great deal more expensive. There is, however, thought to be support for Sega Saturn games, with controls delegated to the Sega Prologue Control Pad (although any Saturn controller will do).
A successor was released as the Super Prologue 21.