[sdiy] Electrosmith Eurorack DSP platform

Mike Bryant mbryant at futurehorizons.com
Fri Jan 28 20:39:01 CET 2022


You can get 18 to 20 bits out of a 1 bitter.  Sony did that in the early 90s.    It's only the 24 and 32(sic) ones that need multibit convertors.

It all depends on how sharp you can make the low frequency cutoff HP filter in the sigma-delta convertor.  They originally solved the cutoff problem using FIR filters which gave a huge delay (which doesn't matter for a CD player, etc, but is a killer for live music), then as semiconductor nodes shrunk gradually migrated to the current IIR filters that have massive cut-offs, almost no delay, and noise figures on a par with the background noise of the universe .... if you pay enough (and believe their specs :-)

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
Sent: 28 January 2022 18:57
To: Eric Brombaugh
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Electrosmith Eurorack DSP platform

So you're getting SNR that's like 16 bit at 48 kHz? Really? That's really, truly impressive. I heard 1 bit DACs had some magic in them but this really brings the point to bear.

On Fri, Jan 28, 2022 at 5:38 PM Eric Brombaugh via Synth-diy <synth-diy at synth-diy.org> wrote:
>
> Because I'm using a sigma-delta approach to generating the PDM 
> signals, there is some noise shaping going on and this system performs 
> better than merely 12MHz/48kHz bits. I'm actually getting SNR that's 
> approaching 16-bit performance.
>
> I found the FPGA-only ADC approach fiddly because it suffers from a 
> lot of the same nonlinearity issues seen in the PDM DAC methods, is 
> very sensitive to layout and component selection and requires a fair 
> bit of post-filtering to reduce noise. It's kinda like the singing dog 
> - amazing that it works but not what you'd call "good".
>
> Eric
>
> On 1/28/22 08:54, cheater cheater wrote:
> > Thanks. 12 MHz 1-bit is equivalent to 48 kHz at 8 bit. What do you 
> > think would be necessary to bump this up to add 4 more bits? Seems 
> > like using a 1-bit approach you'd need an FPGA able to output at 200 
> > MHz... at which point layout becomes a problem. Do cost effective 
> > FPGAs with that capability even exist? (never mind availability)
> >
> > What made the ADCs fiddly?
> >
> > On Fri, Jan 28, 2022 at 4:24 PM Eric Brombaugh via Synth-diy 
> > <synth-diy at synth-diy.org> wrote:
> >>
> >> On 1/27/22 23:20, cheater cheater wrote:
> >>> On Thu, Jan 27, 2022 at 11:00 PM Eric Brombaugh via Synth-diy 
> >>> <synth-diy at synth-diy.org> wrote:
> >>>>
> >>>> I was seeing harmonics down slightly better than 80dB with all 
> >>>> the mitigation measures active. Without it was more like 60-65dB. 
> >>>> Not quite as good as even an inexpensive sigma-delta I2S audio 
> >>>> DAC, but a few SMT passive parts beat that for cost & simplicity 
> >>>> and I2S DACs don't support 12MHz sample rates.
> >>>>
> >>>> Eric
> >>>
> >>> Thanks for the write-up, that's some great info. The github 
> >>> repository was very helpful learning a bunch more about your 
> >>> approach. Really nice of you to share, and those are some 
> >>> interesting results. I've had a few questions if you don't mind.
> >>>
> >>> Regarding the results - really impressive. Is that 12 MHz at 1 bit?
> >>
> >> Yes.
> >>
> >>> What was the performance at DC?
> >>
> >> Didn't measure that - I don't see any reason it wouldn't be 
> >> equivalent to the AC performance though. The back end is fully DC-coupled.
> >>
> >>> How many channels can you run on that ICE5LP4K? Assuming the data 
> >>> comes from somewhere else and you want DC+Audio rates at enough 
> >>> bit depth for a synth signal (pre-VCA/ENV which reduces required 
> >>> bit rate but, say, it could be post filter, or any other module)
> >>
> >> This particular FPGA (iCE40UP5K in the QFN48 pkg) doesn't have a 
> >> lot of I/O pins, so you'd be limited in that regard. Two pins / 
> >> channel and roughly 32 uncommitted I/O so 16 chls. For more you 
> >> could go to other families with larger pincounts - the cheapest 
> >> ECP5 parts are available with 197 pins free, so 98 channels max, 
> >> but then you'd likely be limited by the internal resources and 
> >> would need to bump up to higher-capacity devices.
> >>
> >>> Do you think the same method could be used with a comparator or 
> >>> with a saw oscillator to create a similarly inexpensive and good ADC?
> >>
> >> I've tried a few different types of FPGA-based ADCs - either with 
> >> sigma-delta techniques or with PDM or charge-timing methods. So far 
> >> I've found them to be fairly fiddly and not performing as well as 
> >> inexpensive dedicated devices like the SPI ADC I used in the 
> >> oscillator project. It may be possible to get good performance from 
> >> such approaches but it's a trade-off in R&D time. For me it was simpler/easier to just buy something.
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