[sdiy] Idea for DCO

[Grant Richter]

Star Instruments used this scheme for their large keyboard like  
sequencer, played with drumsticks.

A microprocessor controlled the reset timing, and also used a linear  
DAC to control an exponential current source to keep the sawtooth  
amplitude near constant.

They used cheap trimpots and I had to replace a couple before the  
unit came up, but it did so working perfectly and in tune.

Star is an overlooked company, their engineering was quite good IMHO,  
although cost reduction caused problems in the long term.

Plus their products were pounded on continuously by drummers and that  
is a difficult environment to design for.

On Jul 11, 2008, at 7:18 PM, Stewart Pye wrote:

> Hi,
>
> 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.
>
> Regards,
> Stewart.
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