[sdiy] Chris Synths polysynth

Tue Oct 26 09:42:52 CEST 2021

I'm not knocking Behringer, I'm just talking how tough are Behringer's 
limits with respect to BOM cost and parts selection.
I mean first things you get to know are that BOM must not exceed $x.xx 
and you must use this and this part, never that or that part.
B. is working with engineers and designers from all over the world, I 
think everyone was contacted by B. at some point.
That sort of resembles what Cheater Cheater tries to make for the last 
couple of years.

Roman

W dniu 2021-10-25 o 17:07, Mike Bryant pisze:
> Yes I was addressing the original poster's request - and did mention the whole lot needs to be in a shielded box.  And most MCUs offer spread spectrum which would also be needed.
>
> But I really wouldn't knock Behringer like you seem to be doing.  Some of the very best audio engineers in the industry work for Music Tribe.
> Though quite why those engineers remain doing so given some of the things rumoured to happen there is of course another question entirely.
>
>
> -----Original Message-----
> From: Roman Sowa [mailto:modular at go2.pl]
> Sent: 25 October 2021 13:49
> To: Mike Bryant; synth-diy at synth-diy org
> Subject: Re: [sdiy] Chris Synths polysynth
>
> I think there's some misunderstanding here. I'm talking only about using something like "PWM decoder" instead of "PWM filter/smoother". The guy already plans 100 I/O pins dedicated for PWM, generating a hell of EMI storm with nanosecond rise-fall slopes spread around huge area needed to fit at least one R and C per PWM line.
>
> PWM decoder does not need a micro, the example from EDN was using it only as part of bigger solution described in article.
>
> Sure, not all CVs need to be fast and ripple-free, so those can be done with passive RC filters. But pitch CVs and punchy envelopes would benefit from 1-cycle PWM decoding IMHO.
> Another thing - with a decoder the actual PWM frequency may be much lower, as low as 200Hz if you want to reproduce response of vintage polysynths.
>
> BTW it's not 1/3 of 4053, it uses all 3 switches, that's why I chose it, to minimize parts count. It can be done with transistors and NOR gates, but then the parts count is higher. One 4053 is just 3.5x2.5mm in SOT763
>
> But anyway if something really needs 100 DACs and Behringer level cheap, then it's either a concept going wrong way, or indeed a contract work for Behringer.
>
> Roman
>
>
> W dniu 2021-10-23 o 18:47, Mike Bryant pisze:
>> If you’re doing 100 DACs then it will always be extra micros.  Very
>> few have more than about 16 PWMs.  Adding in the extra opamp, 1/3 *
>> 4053 and PCB area that’s still about 30cents per DAC which is a lot
>> more than 10cents for little added performance in most use-cases.
>> Also discharging the capacitor every cycle like that will create extra
>> EMI which will have to be dealt with.
>>
>> I’m not saying it’s a bad design but it really depends what the
>> application is.
>>
>> *From:*Roman [mailto:modular at go2.pl]
>> *Sent:* 23 October 2021 15:42
>> *To:* synth-diy at synth-diy org; Mike Bryant
>> *Subject:* ODP: RE: [sdiy] Chris Synths polysynth
>>
>> But the micro is not needed at all. It was there because it was part
>> of the design anyway, so it could also do selfcalibration. But the
>> idea in its core does not need any micro. Like I said, 2 opamps, 4053,
>> about 10 pasives. Or a bunch of transistors and 74HC02 instead of
>> 4053. When treating PWM with traditional LPF, one needs a couple of
>> similiarly priced and sized components anyway.
>>
>> Roman
>>
>>
>>
>> ---- Użytkownik Mike Bryant napisał ----
>>
>> 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
>> <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 <mailto: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 <tel:2021-10-22> o15:36, 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
>> <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 <mailto: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 <tel: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-circui
>>
>>   >> t
>>
>>   >> s/index.html <http://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 <mailto: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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