1980s Yamaha DX-21 FM Keyboard Synthesizer1980s Yamaha DX 21 FM Keyboard Synthesizer Essentially a miniature DX7, the Yamaha DX21 is a digital controlled bi timbral programmable digital FM synthesizer with a four operator synth voice generator which was released in 1985. It uses sine wave based frequency modulation (FM) synthesis. It has two FM tone generators and a 32 voice random access memory (RAM), 32 user voices and 128 read only memory (ROM) factory preset sounds. As a programmable