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

Tom Wiltshire tom at electricdruid.net
Tue Aug 18 19:16:08 CEST 2026 

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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