[sdiy] Digital VCO

[Eric Brombaugh]

Paul Maddox wrote:
>> the 8-pin PSoC I'd like to use only has 2 unused pins, so the number 
>> of realtime control options are limited.
>
> 8pin?
> blimey, that's kinda limiting.
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.

Other options include:
* waveform control inputs
* I2C I/O for additional external peripherals
* MIDI I/O
* Additional analog I/O would allow use of the on-chip op-amps as part 
of the filtering system
* Additional I/O pins support isolation of the crystal oscillator for 
better stability
* Gain control inputs (the PSoC has optional multiplying DACs!)
* etc

> What I did with the monowave was to use a 12Mhz Atmel AVR (AT90s1200) 
> as an NCO, which drove the lower 8bits of an EPROM to give me 
> waveforms. as the sample rate of the AVR was around 900Khz, you 
> couldn't hear any aliasing.
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.
> One thing I had planned to do, if I still had my modular, was to build 
> a VCDO, based on a similar trick to the monowave, but break it down 
> into two parts, like this ;-
> 1st Micro does the NCO+Waveform lookup (the AVR family has got bigger 
> and better since)
> A second micro does the ADC conversion and feeds the correct value to 
> the NCO.
>
> The reasons for using two seperate micros are ;-
> 1) the NCO can JUST do the NCO, it need not do any LPF of 
> ADC/conversion/etc, so you can maintain a HIGH sample rate (>1Mhz 
> should be easily acheivable).
> 2) AVRs are pretty cheap :-)
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.

Eric