[sdiy] Roland DCOs

[Tom Wiltshire]

On 26 Feb 2008, at 08:50, Florian Anwander wrote:

> Hi Tom
>
>> Thanks for your help, but I'm afraid I don't understand this. The   
>> ramp wave is produced by linearly charging a capacitor, which is  
>> then  discharged by a reset pulse from the counter. What can be   
>> assymetrical about this? How can the waveshape be altered?
> This is not a real digital controlled oscillator, but half voltage  
> controlled, half counter controlled. The voltage determines how  
> fast the capacitor of the VCO is charged. In a normal VCO a  
> comparator would check the load of the capacitor and if it reaches  
> a certain voltage the comparators output would switch the  
> transistor that shortens the capacitor. This part is missing here.  
> So if there is no signal from the counter, the voltage at the  
> capacitor will rise until the maximum and stay there.
> The trigger from the counter acts like a sync signal in a normal  
> VCO, where it unloads the capacitor. Now if the cap in the DCO is  
> charged "too fast" it will keep the maximum and the voltage will  
> stay there "flat" until the reset from the counter comes.
>
> So the waveform will be more like a spike than a saw, if the cv is  
> wrong.

Ah, yes, now I get it. For some reason my brain would only see the  
case where the CV wasn't enough, in which case you get a nice saw,  
but too quiet. Obviously too much CV will distort the saw (and  
consequently change PW on the pulse wave too). Thanks Florian.

>
>> I'd also be interested to know more about the Cross Mod, as this  
>> is  the part of the circuit that I understand least. The input  
>> marked   _METAL is a mystery to me. The cross mod seems to use  
>> DCO1's output  (which could be noise) to amplitude modulate the  
>> output of DCO2. This  will give an FM-like effect, but with fewer  
>> sidebands. Is there more  to it than this?
> I have to admit, I never really understood the metal crossmod.

Anyone else want to have a look at the circuit diagram and try and  
tell us what's going on? Please?

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

Regards,
Tom