[sdiy] Roland DCOs

[Florian Anwander]

Hi Tom

 >> 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
This one is missing the DCO2. And the metal happens there.

I made a oversight in the following file: 
http://fa.utfs.org/diy/jx3p/schemo/jx3p_DCOs.jpg

There you can see: there are two control signals coming from the CPU: 
one named "metal/sync" the other named only "sync". sync additionally 
enables that the clock from counter1 resets counter2. metal/sync enables 
that counter1 troggers TR107.

The problem is, that I do not know whether the CV for oscillator2 
changes in the metal mode.

Florian

Tom Wiltshire wrote:

> 
> 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
> 
> 

-- 
Florian Anwander                  |ConSol
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