[sdiy] DCOs

[Richie Burnett]

Interestingly, I think the "DCO chip" in the Roland Alpha Juno series synths 
(and MKS-50) is actually an entirely digital chip that uses NCO 
(phase-accumulator) techniques.

I've had discussions with Tom Wiltshire about this, and although the Roland 
documentation calls it a DCO, there are no integrator capacitors anywhere 
around the "DCO" chip on the PCB, or any other signs of analogue integrators 
operating under digital control.  There are, however, a few other things 
that point towards an entirely digital solution...

1. Playing sawtooths at extremely low pitches reveals stair-stepping in the 
waveform.  This wouldn't happen with an analogue integrator in a classic DCO 
arrangement.
2. Playing sawtooths at very high pitches reveals traces of aliasing that 
are at a level consistent with digital synthesis at a 6 MHz sample rate.
3. The weird wave-shapes offered by the alpha-juno oscillators are 
relatively easy to generate digitally by combining various bits of the NCO 
with logic gates.  e.g. the serrated sawtooths, or the famous "hoover" PWM 
sawtooth waveform.

Just thought this might be an interesting addendum to the discussion on 
DCOs.

-Richie,