via Analogue Realities where you'll find additional pics and details.
"The Duophonic Keyboard PCB is fully working and it is sweet. This was the first "module" I built when I originally started this project in the mid 1970s, so it is fitting it is effectively the last module in this [Steiner Synthasystem] resurrection project..."
On the features: "This particular module acts like a MIDI to Duophonic Keyboard converter, but there is no reason you can't just build the Duophonic electronics into a dedicated keyboard, or even an external module.The Duophonic Keyboard was pushing the technology at the time when most others were still using monophonic keyboards. The Duophonic was both a low note priority monophonic keyboard and a two note keyboard.
There are some interesting "features" of this particular keyboard. The upper voice tracks the lower voice when an upper key is not pressed; there is no sample/hold for the upper voice due to the way it's generated.
Next, you can either have a new trigger/gate on each low key press event, or only a new trigger/gate when the previous key is released before the next key is pressed. Nyle calls this legato or lift playing respectively. The upper voice does not generate a gate/trigger.
There is a pitch bend knob. I chose to make this a big knob with a finger spinner to make it easier to turn. Also, because the pot is not spring loaded, I chose one with a center detente to make is easier to feel when I was back to center. Nyle added an electrical dead spot in the Pitch Bend, too so if you miss the detente, you will still be OK.
There is a Portamento knob, and I added a jack for a normally closed foot switch so I could dial in a Portamento and then hit a foot switch to activate it.
There is also an Auxiliary Scale which can be tuned for other scales and it can be switched in and out.
There is a reverse switch reverses the keyboard, and a Range switch lets you shift the keyboard output up or down by octaves."
