[sdiy] Simple DSP as VCO substitute (was Re: CEM3340 reissue)

Tim Ressel timr at circuitabbey.com
Tue Jun 14 19:47:07 CEST 2016

Two DAC output yields 2 waveforms. I'm thinking sine and triangle. A 
digital out pin can make squares and PWMs, or a comparator can handle 
it. If you make triangles rather than sawtooths then the DAC holds 
together better at higher frequencies. It is a simple matter of 
converting a tri to a saw externally.

I don't see 12 bit ADCs as much of a limitation. It should do V/Oct out 
to 10 octaves.


On 6/14/2016 10:40 AM, Tom Wiltshire wrote:
> On 14 Jun 2016, at 16:44, Tim Ressel <timr at circuitabbey.com> wrote:
>> Speaking of doable, it should be possible to take a DSP chip and mimic the function of a 3340. Yes I know, the 3340 has "that sound". But it also has "that price". I'm thinking the DSPIC33FJ128GP802. It comes in a 28 pin dip and has built-in audio DACs. Might be interesting.
> The idea had occurred to me too, for the same reasons - the dsPIC 33FJ128GP802 is about the right size and format to make a good analog oscillator substitute. It has two DAC channels, so maybe even a dual oscillator would be possible.
> There are definitely limitations with the chip (only six 12-bit ADC inputs when using the DAC, for example), and moving to a digital chip implies doing certain things differently (you won't get multiple waveform outputs in parallel, for instance). It's a good idea, and one I've played with quite often, but never been happy enough with the result. That won't stop me trying again though.
> In some ways, I think it might make more sense to go in the direction of my 8-bit PIC VCDO and give up with pure virtual analog and accept that you can do a better job if you don't try and be something you're not. Perhaps it should be a voltage-controlled digital oscillator chip, doing a few of the interesting things you can do digitally instead of trying to do anti-aliased analog waveforms with hard sync and PWM and so forth.
> Tom

--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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