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

[Richie Burnett]

I'd be more inclined to go down this route:

Analogue CV inputs for: Pitch, Pulse-width, Saw level, Tri level, Sqr level, 
Sin level.

One analogue audio output for "combined oscillator mix".

If you want to model a second instance of the oscillator then you could use 
the second audio output of the dsPIC for the other oscillator.  Then you can 
do an analogue mix of the two oscillators, ring modulate them together, send 
them through different filters, different VCAs, whatever....

Some other random comments I picked up on about DSP modelling of analogue 
oscillators:

1. Don't use PWM to output analogue audio.  It's evil.  Really it is.  It's 
great for outputting CV signals but you get horrible intermodulation when 
outputting signals that change quickly (audio.)
2. Don't try to output square waves on a single digital I/O pin.  You need a 
proportional analogue output (DAC) to do the anti-aliasing properly because 
there will be samples at intermediate levels.  Otherwise you have to run at 
an incredibly high sample rate to get away with just writing 0's and 1's to 
a digital port pin if it is to sound any good at high pitches and the pitch 
isn't to be unduly quantised.

-Richie,



-----Original Message----- 
From: Tom Bugs
Sent: Wednesday, June 15, 2016 10:21 AM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Simple DSP as VCO substitute (was Re: CEM3340 reissue)

I've only skim read the rest of the messages as I never really look into
such areas, but a quick thought  ----

two outputs - quadrature triangles.

>From there you can generate pretty much anything externally.

I'd think it a good thing to keep the core simple (but effective),
allowing designers to put whatever wrappers around it.


On 14/06/2016 18:40, 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
>
>
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