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

Tue Aug 18 18:18:52 CEST 2026

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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