[sdiy] Idea for DCO

[Stewart Pye]

I sent this a few days ago but it didn't seem to make it onto the list...

After some thought, and advice from people on the list I've come to the 
conclusion that there are some serious issues with this scheme. I have 
no doubt that it could be done with a PIC and 16 bit ADC and get the 
notes at the correct pitch. The problem arises when you want do 
portamento, LFO mod, and (the real downfall) FM.

I agree that this type of design would be more suited to a MIDI 
controlled oscillator. It would still suffer some of the same 
limitations though.

Thanks for all the advice. I think I'll leave this one alone for now! I 
built a MIDI to drum trigger unit using AVR about 5 years ago. My 
development kit/programmer is almost obsolete now, so I'm tossing up 
wether to go to PIC or stay with AVR. I have used both. Once I get back 
into that I might try a MIDI controlled oscillator.


Regards,
Stewart.





Magnus Danielson wrote:
> From: Stewart Pye <stewpye at optusnet.com.au>
> Subject: [sdiy] Idea for DCO
> Date: Sat, 12 Jul 2008 10:18:04 +1000
> Message-ID: <4877F83C.5040504 at optusnet.com.au>
>
> Stewart,
>
>   
>> I've been thinking about attempting to build a VCO that has the integrator capacitor reset by a microcontroller. For now lets assume the VCO design is similar to the ASM1. It would still have the expo converter, but it could be built using standard (cheap) transistors and no tempco resistor. The expo converter is just there to charge the integrator capacitor. The comparator would be omitted as the discharge FET would be triggered by the microcontroller. The linear CV would go into a 16 bit DAC and the micro would calculate time period required for the reset pulse.
>>
>> There would be a timer interrupt keeping track of the capacitor reset timing. In the background you sample the ADC, convert it to the value needed to be loaded into the count register and keep storing the value until it's time to reset the VCO. When it is time to reset the VCO you take the timer reload value of the last sample and put that into the count register. So the next cycle will have its period determined by the last ADC sample taken.
>>
>> Has anybody tried something like this before? Am I missing any obvious flaws in this scheme.
>>     
>
> I think the flaw or design problem lies in the microprocessor timing details
> and resolution of the CV ADC.
>
> Many DCOs have been built with the basic idea of having digitally controlled
> timing such as you described, but they usually build upon timer circuits such
> as the Intel 8253/8254. Just using the PIC (or whatever) for expo-conversion
> isn't very optimal, but doable. You can probably make it hit notes fairly
> well. Resolution problems will prohibit as smooth frequency as you would like.
> Thus, frequency sweeps would be "steppy".
>
> I still think normal design fits the generic fashion best. If you do a MIDI-
> jobbie then you are on the right track, but better timing setup may be needed.
>
> Cheers,
> Magnus
>
>