DCOs vs VCOs

A recent poll went up on The MATRIXSYNTH Lounge asking whether DCOs are analog or digital. DCO stands for Digitally Controlled Oscillator and in many cases they are actually analog. They are just digitally controlled. I remember the debate coming up frequently on the Analogue Heaven mailing list so I did a quick search and found the following from the late Jürgen Haible. He was an extremely respected synth DIY designer who's work is frequently featured on the site to this day.  I thought I'd share it here on the site as well.  Note the bits preceded with  > are from someone else.  JH is the rest.

">
>>There are different types of DCOs.
>>An analog oscillator, whose frequency is synced to a digital clock. Thus
>>the tuning is perfectly, but the waveform is smooth.
>>Example: Oberheim Matrix 1000, Matrix 6/6R
>
>My M-6 can really get out of tune sometimes (not often though). Perhaps it
>only syncs when you use the "tune" function?

I always thought it's the (resonating) *filter* that is tuned on request.
The DCOs are locked to clocks as said in the original mail.

>>A digital circuit, which aproximates analog waveforms as step functions in
>>hardware, the bit resolution gets worse the older the gear is.
>>Example: Korg Poly-800, Bit-99
>
>The DCO's in these should rather have been labled DO's by the manufacturers
>then, as the sound is digitally generated.

IMO, there are quite a lot of nuances of VCO-DCO. Let me try and make the whole chain from "analogue" to "digital" (Though I wouldn't say strictly from "good" to "bad" ;->)

(1) free running analogue oscillator, controlled by analogue voltage divider keyboard. (Minimoog, ...)

(2) free running analogue oscillator, controlled by digital via a D/A converter without correction, and analogue modulation (Oberheim n-voice (?) )

(3) free running analogue oscillator, controlled by digital via a D/A converter, with autotune, scale correction and stuff, but still analogue modulation (Sequencial Circuits Prophet5 rev. 3)

(4) free running analogue oscillator, controlled by digital via a D/A converter, with autotune, scale correction and digital LFO / Sample&Hold / Glide modulation, but with analogue ADSR->VCO modulation (Oberheim OB-8).

(5) free running analogue oscillator, controlled by digital via a D/A converter, with autotune, scale correction, and all modulations including ADSR via DAC (Sequential Prophet 600).

(6) free running analogue oscillator, controlled by digital via a D/A converter, with autotune, scale correction, and all modulations including ADSR via DAC, plus automatic autotune in the background from time to time (Oberheim Matrix12 (?)).

(7) Simple Ramp-Oscillator that is synced by a counter every cycle, but with different clock/divider tree for both DCO's in a voice (Oberheim Matrix 6, EDP Wasp)

(8) Simple Ramp-Oscillator, that is synced by a counter every cycle, but with one single clock/divider tree for both DCO's in a voice (Oberheim Matrix 1000)

(9) Staircase Waveform stuff (as described in different recent mails)

(10) Wavetable stuff (also as described in recent mails)

These are the different types I know of. Maybe there are still others in between. Speaking in the word's original sense, (2) - (6) would be something like "DCVCOs" ("digital controlled VCOs") in the sense of a digital word forms an analogue voltage that controls an oscillator. (7) and (8) would be real "DCOs", cause they are still oscillators, only under the iron grip of digital clock, which makes them sound thin and lifeless. (9) and (10) shouldn't be called "controlled oscillators" anymore, perhaps just call them "DOs" ("digital oscillators") or even closer to the point, "DDs" (digital dividers) or "DS's" (digital scanners). Well, but that would go too far then.

Oh, I forgot another interesting variation:

Analogue oscillators with digital dividers / waveforms:

(11) analogue VCO with dividers (some ARP-synths - thank You Joachim, for the schematics! - and of course the Suboscillators in various VCO- based synths)

(12) 12 analogue VCOs for the 12 top-octave-semitones and digital dividers (Farfisa VIP 245, Korg PS-3x00 series)

JH."