[sdiy] Discrete MOS 6581 (Commodore C64 SID chip) schematics / analysis

Tue Aug 18 19:24:58 CEST 2026

See here.

https://6502.org/forum/viewtopic.php?f=8&t=4150

It uses a PLL.

I think some of the recreations use a crystal, and others use PLL as well.

However, a PLL made in an antiquated late 70s process is not going to
be nearly as stable as a PLL using modern processes of today. There is
no comparison.

On Tue, Aug 18, 2026 at 7:16 PM Tom Wiltshire <tom at electricdruid.net> wrote:
>
> How does the SID generate its phase accumulator compared to how the recreations do it?
>
> I'm curious now. I only know one way to run an NCO, hardware, software, it's all the same.
>
> Thanks.
>
> > On 18 Aug 2026, at 17:18, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
> >
> > I personally think the oscillators on the recreations are too stable
> > in pitch. The real SIDs have more instability which makes the unison
> > sound more lively. The recreations sounded too exactly tuned and ended
> > up in lock step. Which is well explained given the difference in how
> > the SID generates its phase accumulator vs how the recreations do it,
> > they're just worlds apart. So maybe a "crappier", discrete or even
> > analog, phase accumulator would be better, and then a small FPGA to
> > recreate the rest of the logic.
> >
> > I would personally love an extended range SID that can do higher
> > pitches, and has finer envelope control and so on.
> >
> > Like Rainer said, the SID is meant as a voice, not as a whole
> > instrument. It's funny that the oscillators in the recreations are so
> > stable that they remind me of my SQ-80, which generates all the voices
> > on a single chip.
> >
> > On Tue, Aug 18, 2026 at 4:05 PM cheater cheater
> > <cheater00social at gmail.com> wrote:
> >>
> >> That's a good recap, thanks Rainer. Bear in mind no one here argued
> >> that the emulations don't sound like a specific single acceptable SID
> >> sound. I specifically just said they "don't sound good" and also they
> >> "don't sound right". But that doesn't have to be emulating a specific
> >> kind of SID. The demos you provided reinforce my opinion :) In fact,
> >> even though Noah had me curious whether just the oscillator section
> >> recreations were good, now I am firmly back in the camp of believing
> >> that they aren't good... The demo renders have obvious differences,
> >> and the way the recreations differ are imo strictly for the worse.
> >>
> >> I think we all know there were different versions of SIDs. Yes, "there
> >> was manufacturing spread" and "there were different models" are
> >> classic arguments when discussing recreations of SIDs, 303s, 808s,
> >> CS80s, Hammond B3s, beer, whisky, pastel paint, artisan paper, Nissan
> >> Skylines, Ferrari F40s, Faberge eggs, Gutenberg bibles, etc. Someone
> >> has to say it, so it can be out of the system and doesn't have to be
> >> brought up again. Thank you for performing that indispensable role for
> >> our benefit.
> >>
> >> I think the reason for comparisons of SID to DOC are that the DOC is
> >> the next chip by the same designer. So the assumption is that given
> >> enough time and enough space on silicon he would have made the SID
> >> more like a DOC, even if not exactly like it.
> >>
> >> Nice tunes all in all btw. I think we need more musicians like that.
> >> Thanks for the links.
> >>
> >>
> >> On Tue, Aug 18, 2026 at 3:09 PM Rainer Buchty <rainer at buchty.net> wrote:
> >>>
> >>> On Tue, 18 Aug 2026, Noah Vawter via Synth-diy wrote:
> >>>
> >>>> there is not much remarkable about the SID except for its context, how
> >>>> it is programmed with wavetables and the filter.
> >>>
> >>> Which in this very context unfortunately also means modeling all the
> >>> design quirks the various music routines relied upon.
> >>>
> >>> 1) 6581/8580 differences: combination-waveform output (leading to more
> >>> useful combi-waveforms on the 8580), DC-offset removal (leading to quite
> >>> silent §d418 samples on the 8580), and of course the general gain and
> >>> cleaner sound (giving the 8580 more "punch")
> >>>
> >>> 2) phase accumulator and internal DAC behavior (or else the 8-bit sample
> >>> technology won't work -- which it IIRC didn't with SID emulations
> >>> predating the original "Fanta/SounDemoN" routine that brought us
> >>> 3-channel full-filterable and envelopeable 8-bit samples using a mere
> >>> SID chip).
> >>>
> >>>> It is 100% digital plus the pseudo analog filter. most of the
> >>>> microcontroller implementers are lazy about the filter.
> >>>
> >>> Which is a shame, because with ReSID there's a proven-working
> >>> state-of-the-art C++ implementation (making VICE a faithful, authentic
> >>> C64 emulator with all the bells and whistles, i.e., creatively used
> >>> design quirks of the original).
> >>>
> >>> You can tune ReSID to any 6581/8580 behavior to get the "right" sound
> >>> for the used software.
> >>>
> >>>> The ESQ-1 *is* the no-compromise SID synthesizer
> >>>
> >>> I honestly never understood this comparison.
> >>>
> >>> First of all, the SID is no synthesizer but a synthesizer voice :)
> >>>
> >>> Second, while both, DOC and SID use phase accumulation at heart, the SID
> >>> uses it directly, converting it into sawtooth (direct use), triangle
> >>> (XOR the lower bits based on the topmost one), and pulse (like with
> >>> analog designs, compare with pulsewidth value and either output max or
> >>> min). DOC uses the phase accumulator solely for sample-memory access
> >>> (and determining sync-out/osc stop).
> >>>
> >>>> and predictable mass-produced filters (curtis? CEM?).
> >>>
> >>> I wouldn't exactly call the CEM3379 "predictable", just check the
> >>> datasheet for the horrible spread in voltage and resonance CV.
> >>>
> >>> Hence, the ESQ1/SQ80 has software-based filter tuning (to at least get
> >>> the frequency response somewhat right) and limits the resonance setting
> >>> to a safe "will definitely not resonate" range.
> >>>
> >>>> For example, people only rarely use LFOs in sid music.
> >>>
> >>> Define "rarely". Most trackers do offer vibrato and PWM since the dawn
> >>> of time (i.e., Chris Hülsbeck's SoundMonitor that's now turning 40), as
> >>> it was paramount to let a single osc sound lively.
> >>>
> >>> There usually also is a software-based ADSR (or just AR) envelope for
> >>> filter modulation, sometimes also for PWM.
> >>>
> >>> The program that put it to the max was the "Modulator" published in the
> >>> German 64'er magazine back in (IIRC) 1985. It featured *eight* LFOs and
> >>> a software envelope, where via the modulation matrix even the LFOs
> >>> frequency and phase(!) could be modulated -- in addition to the obvious
> >>> targets (osc frequency, pulse width, filter frequency). But it ate so
> >>> many cycles that it could just about handle a keyboard scanner.
> >>>
> >>> As for "recreations sound inferior", I think this is not true.
> >>>
> >>> Here's Tuneful Eight (featuring an 8-SID stereo tune):
> >>>
> >>> Real SIDs (8580):
> >>>        https://www.youtube.com/watch?v=nhz3vHYX0E0
> >>>
> >>> Too bad LMAN removed his very own FPGASID rendering, so let's use these:
> >>>
> >>> UltiSID/FPGASID mix:
> >>>        https://www.youtube.com/watch?v=KnFin5dxWLg
> >>>
> >>> Pure UltiSID emulation:
> >>>        https://youtu.be/SwN7anqqDP8
> >>>
> >>> The FPGA-based ones IMO sound a tad harsher / cleaner / less "rounded",
> >>> but only in direct A/B comparison. Probably not noticeable if heard
> >>> alone. Adding a strategic tanh (approximation) here and there (or just
> >>> some mild LPF at the output) might fix that already.
> >>>
> >>> Considering the variances between the various 6581 revisions (and also
> >>> C64 board revisions where the filter capacitors where changed from
> >>> 2200pF to 470pF -- or was it vice versa?) with the particularly audible
> >>> change coming with the 6581(A)R4, not to mention the switch to 8580,
> >>> makes discussions about "the" SID sound and emulating it "correctly"
> >>> futile anyway, as there was never "the" SID.
> >>>
> >>> If people today talk about "the" SID sound, there's at least two: the
> >>> 8580 for the "later" sound, and the 6581(A)R4 as the golden reference of
> >>> the NMOS years.
> >>>
> >>> Having said this, here's some nice (YMMV :) examples of current SID
> >>> sound:
> >>>
> >>> "Beat SID" (closing the circle, as the original was probably sequenced
> >>> on a C64 using Steinberg's Pro16):
> >>>        https://www.youtube.com/watch?v=fRYrSVIIz2o
> >>>
> >>> "Robot - Not Human" (bringing its own video):
> >>>        https://www.youtube.com/watch?v=CKi_m0NhwE0
> >>>
> >>> "In Surgery" (way less minimalistic, yet nicely synced):
> >>>        https://youtu.be/YbiGbyHKqy4?t=18
> >>>
> >>> You want pure SID sample insanity? Ok, here's the latest work of Pex
> >>> "Mahoney" Tufvesson, showing that there's always some cycles left to
> >>> spare on other stuff (like the interleaved loader and decompressor plus
> >>> a little bit of eye candy):
> >>>        https://youtu.be/GqEI0T_WoAw?t=302
> >>>
> >>> Citing: "This is still your unmodified Commodore 64 from 1982: 0.001GHz
> >>> CPU, 0.000064GB RAM, 0.0002 billion transistors in 8000 nanometer
> >>> technology, 0.0000002TB storage."
> >>>
> >>> Cheers,
> >>>        Rainer
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