[sdiy] Chris Synths polysynth

[Mike Bryant]

Some (very rough !) back of the envelope calculations to get you started :


The RP2040 has 8 PWMs and you can make another 8 with the PIOs.

Drive it from USB, SPI or I2S from a central controller, presumably a Pi CM4 or Pi4 which can drive up to 16 (USB), 6 (SPI) or lots (I2S) of RP2040s.

System clock to the counters is 133Mhz.

You don't want a sampling freq in the audio range so let's choose 20kHz.

Ratio is 133MHz/20kHz = 6650 which is about 12.5 bits (12 bits = 4096).

You can get more bits by summing in a second PWM.  In theory this could give you 2 * 12.5 = 25 bits but that assumes perfect resistors and zero noise.  In practice with careful design you may get to 16 bits, nothing more.

You'll need to kill the sampling rate by at least 60dB.

So with a simple RC at 6dB/octave the filter needs to start at 10 octaves below 20kHz = 16Hz so this is the fastest rate you can change voltages.

With a 12dB/octave Butterworth you'll need 5 octaves below 20kHz = 625Hz.

Or add a notch at the sampling rate and you'll get a lot better, depending on the depth of the notch.

You could gain a bit more with software tricks by warping the noise into higher frequencies but I'd just keep it simple if I were you.

RP2040 = $1, Rs/Cs/opamp = about 20cents. PCB space is about 10cents.  Connector cost is about another 20 cents.  So roughly 10 cents per channel.  

I don't think you'll manage much cheaper than that.

Hope that helps.  

Mike


-----Original Message-----
From: cheater cheater [mailto:cheater00social at gmail.com] 
Sent: 21 October 2021 04:34
To: Mike Bryant; synth-diy
Subject: Re: [sdiy] Chris Synths polysynth

On Wed, Oct 20, 2021 at 12:37 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> If you really want cheaper DACs consider using PWM followed by RC smoothing.  And not an FPGA but something like the new Pi 2040 MCU which has lots of I/O programmability.  I suspect you could get about 20 to 24 8 bit resolution voltages for about a $1.  And when if you need more resolution combine two PWM outputs with different summing Rs.  12 or 13 bits linearity is easily attainable this way, maybe a little more.
>
> I use such an arrangement for creating low-jitter audio PLLs.
>
> Best to keep the MCU and smoothing RCs on a different PCB and shield it from everything else, and use a separate voltage regulator as well to keep crap off the supply.
>
>

That's an interesting idea - why do you think the gpio pins only allow up to 8 bit resolution? I assume this is a very popular chip. I wonder if anyone has made any good analysis of what bit depth can be squeezed out of the chip using various software tricks? The RP2040 datasheet (section 4.5 "PWM") wasn't forthcoming on the attainable bit depth.

Best regards

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of 
> cheater cheater via Synth-diy
> Sent: 20 October 2021 11:17
> To: Roman Sowa
> Cc: synth-diy
> Subject: Re: [sdiy] Chris Synths polysynth
>
> This is the situation I find myself in - trying to squeeze in as many cheap DACs and ADCs into an inexpensive FPGA as possible...
> unfortunately I have to do all the learning from ground up :D
>
> On Tue, Oct 19, 2021 at 11:15 AM Roman Sowa <modular at go2.pl> wrote:
> >
> > This is pretty amazing. And inspiring. Makes me wonder about the 
> > meaning of life.
> > I would love to see the inside video, showing all the boards and wiring.
> > It might be easier then to guess who that is.
> > This person is probably on this list giggling now quietly.
> >
> > As for DACs, this is always designer's choice and I will not argue 
> > about that whatever was used. My personal view is that is comes much 
> > cheaper in the long run to use good speced part with true linearity 
> > than using various tricks to make $1 DAC with 64 LSB INL work in semi decent way.
> >
> > Do all CVs need good linearity? Sure they don't but since you have 
> > already good DAC and S&H MUX to make pitch CVs, then why not use it 
> > also for all other stuff, including LCD backlight dimmer?
> >
> > Roman
> >
> > W dniu 2021-10-19 o 09:56, cheater cheater via Synth-diy pisze:
> > > https://sonicstate.com/news/2021/10/17/impressive-diy-poly-synth-i
> > > n-
> > > progress/
> > >
> > > This looks particularly cool, and the fact that Chris diy'ed it 
> > > all really speaks to me. Some great sounds in the demos.
> > >
> > > What do you think of his choice of CV DACs? Do all synth CVs 
> > > really need something as good as DAC7554 or even AD5676?
> > >
> > > I love the fact that the front panel is just a piece of white 
> > > paper from a laser printer...
> > >
> > > That bare metal "aircraft panel" style construction really looks 
> > > good to be honest.
> > > _______________________________________________
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