[sdiy] Chris Synths polysynth

Mon Oct 25 17:58:10 CEST 2021

I don't think Roman is knocking Behringer, he's just saying they can
do stuff very cheaply.

Looking at a typical synth module, you have 4-16 in and 1-4 out plus
front panel controls, times a polyphony of 16, that's 128 in and 64
out. Take the amount of modules in a typical modular setup, and you
have say 40 modules. That's about 5000 DAC and 2500 ADC. Obviously a
$0.1 / channel DAC will be much more interesting than a $1 / channel
DAC but a balance has to be struck somewhere.

I thought you could only get TSSOP-16 4053s. However, Roman did
mention that they come as QFN, and indeed the 74LV4053BQ-Q100X from
Nexperia is way smaller, and also $0.15 at 3000, which is a quarter
better than what Mouser could do on the TSSOPs. And the power
dissipation is halved as well.

This also means with 5000 DAC you'd be looking at at least 5000*50
mm^2 = 0.1 square meters just for the 4053s, at an expense of ~ $750.
Of course, given the scale, $750 kind of drowns in other expenses.
That's a bearable expense. The power dissipation would be an
additional 25W (compared to 50W for TSSOP), which again doesn't seem
too bad.

On Mon, Oct 25, 2021 at 5:18 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> Have you actually calculated just how many ADCs and DACs you really need as there's an awful lot of finger in the wind here ?
>
>
> -----Original Message-----
> From: cheater cheater [mailto:cheater00social at gmail.com]
> Sent: 25 October 2021 12:54
> To: Mike Bryant
> Cc: Roman Sowa; synth-diy at synth-diy.org
> Subject: Re: [sdiy] Chris Synths polysynth
>
> That's a good consideration, Mike. While 100*10 cents per module is fine, 100*1 euro per module is not.
>
> On Sat, Oct 23, 2021 at 12:46 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
> >
> > Whilst the EDN circuit is obviously higher performing, the OP was after 100s of DACs so adding extra MCU ports and a S/H probably isn't going to meet his cost target, whereas a simple alteration to the PWM encoder may give higher performance for free.
> >
> > That said, I agree Arturia not doing something like the EDN channel for a single channel output on one of the most important functions of the keyboard is over cost-cutting.
> >
> >
> > -----Original Message-----
> > From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
> > Roman Sowa
> > Sent: 23 October 2021 08:06
> > To: synth-diy at synth-diy.org
> > Subject: Re: [sdiy] Chris Synths polysynth
> >
> > Or why not simply make sigma-delta modulator like in any audio DAC today?
> > Shouldn't take much of FPGA resources.
> >
> > This morning I came up with an idea of PWM decoder giving solid output after single PWM cycle using 2 opamps and 4053, but it looked so obvious that I thought it had to be done before, so did some googling and found this, which is basicaly the same concept, but overly complex:
> > https://www.edn.com/pwm-dac-settles-in-one-period-of-the-pulse-train/
> >
> > Roman
> >
> > W dniu 2021-10-22 o 15:36, Mike Bryant via Synth-diy pisze:
> > > Good links.  They also led to this one where the writer suggests bit-reversing the counter to give a simple form of PDM replacing PWM.  Definitely an idea I'll bear in mind.
> > >
> > > https://www.edn.com/three-paths-to-a-free-dac/
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf
> > > Of Steve via Synth-diy
> > > Sent: 22 October 2021 14:24
> > > To: synth-diy at synth-diy.org
> > > Subject: Re: [sdiy] Chris Synths polysynth
> > >
> > >
> > > I found these articles interesting, but have not tried in practise
> > >
> > > https://www.edn.com/cancel-pwm-dac-ripple-with-analog-subtraction/
> > >
> > > https://www.edn.com/fast-pwm-dac-has-no-ripple/
> > >
> > >
> > >
> > >
> > > Am 22.10.2021 um 12:15 schrieb Vladimir Pantelic via Synth-diy:
> > >> Arturia uses a dual PWM scheme in all of the their Keystep and
> > >> Beatstep products, basically following the circuit laid out here:
> > >>
> > >> http://www.openmusiclabs.com/learning/digital/pwm-dac/dual-pwm-circ
> > >> ui
> > >> t
> > >> s/index.html
> > >>
> > >>
> > >> it works nicely, BUT also introduces a >4ms slew on all CV outputs,
> > >> no biggie if driving an analog synth and it gets hidden in the
> > >> attack, but a real nuisance if it gets sampled by a digital module...
> > >>
> > >>
> > >> On 10/21/21 11:34 AM, Tom Wiltshire wrote:
> > >>> Here’s the correction:
> > >>>
> > >>> Imagine you have a DAC with 4-bit resolution. If we use one with a
> > >>> step-between-values of 1V, and then add the output of another with
> > >>> a step-between-values of 1/16th of a volt, we can make an 8-bit DAC.
> > >>> That’s basically what you do with the PWM. The process is helped
> > >>> by the very good linearity of digital-counter-based PWM as a
> > >>> method of producing a voltage.
> > >>>
> > >>> I’ve used a 3-bit PWM scaled to produce 1V per step and a 4-bit
> > >>> PWM scaled to produce 83mV per step to produce Pitch CVs. It
> > >>> worked very well and produced good CVs across the 8 octaves. The
> > >>> reduced bit depth of each PWM DAC means that the output frequency
> > >>> can be higher without the PWM clock frequency having to be
> > >>> super-high, and this improves the effect of filtering and the
> > >>> responsiveness of the CV to changes.
> > >>>
> > >>>> On 21 Oct 2021, at 09:54, cheater cheater via Synth-diy
> > >>>> <synth-diy at synth-diy.org> wrote:
> > >>>>
> > >>>> Correct me if I'm wrong, but I think summing two PWMs will just
> > >>>> add
> > >>>> 1 bit of depth.
> > >>>
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