[sdiy] Digital VCO

[Paul Maddox]

Eric,

> There are 20-pin PSoCs available for only few cents more than the 8-pin 
> version. If there are enough features to use the extra I/O then it would 
> be simple to upgrade to the larger devices. I just set the 8-pin chip as a 
> goal to see how much I could squeeze the design down. Plus, using an 
> inexpensive 8-pin chip allow you to get a whole lot of 'em on one board 
> for that 'cloud of oscillators' effect.

hehe, fair comments and an admirable goal!

> Yeah - AVRs run a lot faster than PSoCs. The PSoC has a max CPU clock of 
> 24MHz (12MHz recommended) and the average instruction takes 6 cycles - 
> 2MIPs.

ouch, that's a big shame..
Have you considered over clocking?
I've heard of 16Mhz AVRs running at 48Mhz...

> Cool idea. I was really more interested in how much sh*t I could stuff 
> into one little box. The engineering trade-offs are part of the fun. Now 
> if the result ends up being useful too then that's even better.

Assuming that if you're going for a 'synth' that a ramp generater ought to 
be easy enough to do without causing too much problem. Also if you're 
carefull with the code 'switching waves' when you get to a high frequency 
could, let you go even higher.
For example if, say upto 5Khz you use 256byte samples of waves, you could 
switch to a 128byte wave at 5.1Khz, and also halve your control value (half 
the number of samples = half the speed required).

It'd require a bit of software jiggery pokery, but it's a common enough 
trick in DSP (I forget what it's called), though usually you'd do some cross 
fading of waveforms, use one with less harmonics and keep the frequency the 
same.
Then just post filter the wave at your sample rate (62Khz or whatever you go 
for) and you should get a pretty reasonable VC-DDS :-)

Paul