[sdiy] DCO

[Tom Wiltshire]

Jerry,

On 30 Dec 2009, at 15:41, Jerry Gray-Eskue wrote:

> For those out there that are wondering about this also..
>
> I see why I did not run across a DCO as described by Tom.
>
> I have done some looking on the web and found, listed by most  
> common use.
>
> (1) Digital Controlled Oscillator refers to a Square wave generated  
> by a
> programmable divider.

This square wave is often fed through a HPF to generate a reset pulse  
to a ramp integrator. This is the basic scheme of the Roland Juno106  
and some other similar synths.

> (2) Use of a Programmable Divider to step through a wave table,  
> generating
> an arbitrary wave form.
> 	In the Digisound this takes the form of a Voltage Controlled Digital
> Oscillator Module 80-21

This sounds like the PPG/Monowave scheme to me, and personally I'd  
call that a "digital oscillator", since the output is a number via a  
DAC. However, I don't doubt such a system has been called a "DCO" at  
least once in its life.

> (3) Various wave shapers that modify a Square Wave.

This probably comes up in many of the synths that use (1). The Juno  
follows its standard 16-bit counter chips (generating the basic  
squarewave) with custom "waveshaper" chips that include the ramp  
integrator, a PWM comparator, and a sub-octave flip-flop.

To be honest, what counts as a "DCO" is a real minefield, and not a  
discussion I can be bothered with. That said, I think you'll find  
that there is actually a lot of cross-over between the things you've  
listed above. What I described is a fairly common scheme, used by  
Roland (Juno-60, Juno-106, JX-3P, JX-8P, and JX-10) the Korg Poly-61  
and Poly 800, and some instruments by Akai and Kawai. I think some of  
the later Oberheims work this way too (Matrix 1000/ 6R era). The  
Prophet 08 is only the most recent example.

My own research into the Roland Juno DCOs is up on my website:

http://www.electricdruid.com/index.php?page=info.junodcos

There's also a pdf (1MB) of the JX8P DCOs. You can see the counter  
chips, outputting a square wave, which is used directly, and also  
used to reset a ramp integrator by passing it through a HPF.

http://www.electricdruid.com/images/dcos/JX8PDCO.pdf

I hope these help you see what others have done. Can we see your  
design? I find I'm fascinated by this stuff...
Regards,
Tom