The italian company Jen Electronica of Pescara released a very basic and low-cost monophonic synth in 1978.
The concept is very basic, but not found in this useful combination that often:
1 oscillator with little V/Hz characteristics that drive a digital oscillator with built in keyboard logistics and note frequenc generator. Maybe very close to a top octave divider in a single chip.
Analog waveshaping (saw, square, variable pulse).
Easy to calibrate
If this is a DCO is something debatable.
There are no digital numbers that control it. There is some CV analog pulse that drives the main chip M110. And the chip get the note name pressed and on other inputs the octave in which the key is pressed. This is all done as pure analog switches. So this is not digital was normally defined.
Pink or white noise selectable with its own volume control
1 LFO with triangle wave only. Routable to PWM mod, oscillator and filter
1 VCF lowpass made from four op-amp based filters in a row. This should be the equivalent to a 4 pole filter = 24 dB/oct! Not a 12 dB filter as often stated on internet pages.
1 ADSR envelope for the filter
1 ADSR for the VCA
The interfacing is very easy:
mains in via a build in power cord
It takes 6 screws to open the Jen. And it is build very easy and efficiently. But you need to undo the keyboard action to reach the M110 chip that does the oscillator frequency.
The top left board is the noice PCB
The biggest board seen here is the oscillator board
The main chip (M110) is hidden under the keyboard action.
The upper board is the LFO and filter board
Below is the envelope and VCA board