[sdiy] Polyevolver internals

[Tom Wiltshire]

Antti,

>> No idea how well it would work to have both DCOs and digital  
>> oscillators running simultaneously though. Could end up with some  
>> IRQ contention if the DCO hardware needs much babysitting.
>
> I don't see much point in using DCOs when you can simply add more dsp
> oscillators and mix them in software.

There are various reasons; DCOs generate genuine analogue ramps,  
which is important to some people; Related is the fact that DCOs can  
easily generate sync waveforms without aliasing, which is *hard* to  
do with digital oscillators; DCOs have a relatively low processor  
overhead, since you only have to deal with a timing reset pulse once  
per wavecycle instead of dealing with generating a new sample at x  
KHz. That leaves you time to do modulation processing on the same chip.

I doubt Dave Smith would have got two voices on one dsPIC if he'd  
used digital oscillators not DCOs in the Prophet 08. I also doubt  
he'd have sold as many if he couldn't claim that it had 'an analogue  
audio path'. So the DCOs are essential to the design.

> dsPIC with integrated audio DAC should work as a pretty nice two  
> voice chip for building polysynths (or monosynths). Use SPI to  
> control N dspics and mix the separate voice outputs together in  
> analog. IIRC, they cost about 6e / part for the 28 pin dip  
> versions. I'm planning to try this early next year.

Let me know when you do. I've been working on a similar scheme.

Regards,
Tom