[sdiy] Roland DCOs

[Tom Wiltshire]

This seems like a entirely reasonable approach, JP. I've only got one  
question: how are you going to control the current source from the PIC?

I'd wondered about using the PWM module as a simple DAC for this job,  
like I did for my LFO/ADSR projects, but I don't think it will  
respond quick enough.

Regards,
Tom


On 26 Feb 2008, at 13:42, jpdesroc at oricom.ca wrote:

> Here is my next DCO (or DVCO..?) project.. hope this could give some
> ideas..
>
> I want to build the DVCO around a 20mhz Microchip PIC.
> I'm gonna read an external DC control voltage (1v/oct) using
> the PIC inner 12 bits ADC, use the ADC read value to read a lookup
> table
> period value that will generate a narrow pulse (16bits inner counter)
> and discharge a continually charging cap
> at the needed time period.
> This will give a saw wave shape. Ok.
> But the amplitude wil vary according to the wave period time.
> So to correct that I'll charge the capacitor with a controlable
> current source that will keep the amplitude the same for
> all the DVCO range. A simple formula will calculate
> the correct current to charge the cap with according to the
> instantaneous
> frequency period. Doing so I get a clean (no alliasing) very
> temp/freq stable saw waveshape
> of several octave range. The only thing that could vary with
> temperature
> is the wave amplitude but very little. I can do a wave shaping
> further on
> for all other wave shapes.
> I hope I'm clear enough here.
> Thanks for your time.
> JP
>
>
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