
8-Operator FM synthesizer that seems to use 12-bit D/A
More on the Yamaha FVX-1 in German and Google translated. Also on 120 Years: "YAMAHA used FM synthesis in their Electones, too. HX electones and FVX-1 module were 8-Operator FM synthesizer that seems to use 12-bit D/A, and it could have been the evolution of DX7 I series." Via Sequencer.de.
Update via Loscha in the comments:
Link to Manual, and "no-one ever mentioned this in the YamahaBlackBoxes group!!!"
Update: One more site sent my way via brian comnes: link

- very rare programmable 8-operator FM synthesizer expander released in 1988
- 64 editable parameters - 22 direct + 42 with DOS based editor
- DSP - 11 effect algorithms Stereo-Chorus, Stereo-Pan, Symphonic, Phaser, Flanger, Delay, Tremolo, Digital hall, Celeste and Wah.
- 5 LED dsplays + levelmeter
- 4 x 8 voice blocks (4 part multitimbral)
- 248 ROM, 248 RAM presets, 16 performances
- 2x MIDI IN (front and back), 2x MIDI OUT (front and back), MIDI THRU, Headphone OUT, Aux OUT Stereo, Main OUT Stereo, Main In Stereo Cinch, Main Out Stereo Cinch, Aux In Stereo Cinch, Aux Out Stereo Cinch, Pack Port
- 14 Buttons + 1 Coarse, 4 endless controllers, 1 Main Out volume knob, 1 Aux Out volume knob, 1 headphone Out volume knob