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

[sleepy_dog at gmx.de]

Square wave on the DAC?

Would it not be enough to just do that with a harwdare PWM channel?
It did sound nice enough at first "glance" when I tried it, but I'm not 
sitting on a pile of signal theory to know what I'm doing there ^^
I mean including PWM. I.e. directly using a PWM timer output swinging at 
the pitch you want to play, LFO-modulated in the firmware - sure, the PW 
resolution varies depending on pitch, but it sounded nice enough to me. 
Between the low end and 7 kHz. I have not looked at the spectrum or 
would know how that of a typical synth pwm should look like, though.

But if you could do that, then you'd have 3 waveforms on the dac, plus 
PW-variable square.

Is the F303 + needed parts & PCB still cheaper than the other options, 
though? (I have no idea what the old chips cost)

Steve


Am 14.06.2016 um 20:00 schrieb Vladimir Pantelic:
>
> STM32F303 has 3 12-bit DACs and a ton of 12 bit ADC channels. so saw, 
> square, triangle and sine via an external shaper for extra grit :)
>
> On Jun 14, 2016 7:54 PM, "Tim Ressel" <timr at circuitabbey.com 
> <mailto:timr at circuitabbey.com>> wrote:
> >
> > 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.
> >
> > --TimR
> >
> >
> > On 6/14/2016 10:40 AM, Tom Wiltshire wrote:
> >>
> >> On 14 Jun 2016, at 16:44, Tim Ressel <timr at circuitabbey.com 
> <mailto: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 <mailto:timr at circuitabbey.com>
> >
> >
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