DCOs (was Re: [sdiy] Active VCO temp compensation)

[Tom Wiltshire]

On 22 Dec 2008, at 15:32, Amos wrote:

>
> On Mon, Dec 22, 2008 at 9:00 AM, Tom Wiltshire  
> <tom at electricdruid.net> wrote:
> After all, I haven't heard anyone slating the Prophet for not being  
> analogue enough, or sounding "weak" or "thin" or any of the other  
> rubbish people used to say about DCOs...
> err... I have.  I've heard "Sounds like a Juno without the chorus"  
> or "thin like an 80's Roland" said about the new Prophet, by more  
> than two people.  Nonetheless I am all in favor of new ideas in  
> analogue/hybrid VCOs... for example, it seems like there's a lot  
> more that could be done in the realm of analogue waveshaping  
> applied to a digital-timing-controlled analogue core.  Something  
> with the good bits of the P08's hybrid design, but not limited by  
> the "all on one chip" architecture of that oscillator, could be  
> really special.

Ryan Williams did some nice stuff a little while ago (dsPIC VCDO,  
15th Aug) with analogue waveshaping after a NCO triangle core. I  
thought the use of a triangle output rather than the usual counter- 
output-to-DAC ramp was very clever. Whereas the harmonics of the ramp  
only drop off at 1/h, the triangle wave's drop off at 1/h^2, which  
makes the output triangle far less affected by aliasing - nice trick.  
Once you've got your triangle, you can use the usual analogue  
waveshapers.

I've also been talking to Fredrik Carlqvist about his Minod Vorga  
synth DCO design (http://www.minod.com/products/minodvorga/ 
saw_generator.gif   - posted here in 2006). That's a more typical  
ramp generator with reset pulses coming from the uP, like the P08.  
I'm yet to get one lashed up on the breadboard, but it's definitely  
something to try.

Regards,
Tom