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

Tue Aug 18 09:59:28 CEST 2026

What Noah brought up about component tolerances and how everyone's C64
would sound a bit different is interesting. Perhaps DSP dropin replacements
should come with essentially a set of random parameters from the factory so
that each one does sound a bit different, within the limits of what a C64
can sound like.

Another thing to think about is noise that comes from interference from
other parts of the system. When making a digital replacement, one's impulse
might be to make sound output that is as clean as possible, right? Well I'm
reminded of the time when the Gameboy emulator SaneBoy added an option to
mix emulated interference noise into the audio. For example, every time the
CPU sleeps and then wakes up, a small click is introduced to the audio,
coupled through the power rails into the audio amplifier. This typically
happens every frame so you get effectively a 60 Hz PWM with duty cycle
controlled by the amount of CPU use, droning in the background. Same thing
with the graphics, you get interference matching the vertical refresh rate,
and it's influenced by the screen contents as well. When this feature was
added, a user commented that it was the first time an emulator felt "real"
for them.

Perhaps the same is true when people say a DSP SID emulator doesn't sound
real. Maybe the "tell" that it's not real is that interference noise like
that is missing, and nothing to do with the filter response and exact
distortion levels of the chip itself.

/Didrik

On Tue, 18 Aug 2026, 09:15 cheater cheater via Synth-diy, <
synth-diy at synth-diy.org> wrote:

> That's a great summary, thanks.
>
> I believe even for the purpose of using in playing SID music and
> reviving C64s, a SID recreation with an analog filter could be a great
> thing. The DSP versions will probably never sound just right. It seems
> like the part count is small, so it could probably fit within the
> footprint of an original SID (but may have to use stacked PCBs)
>
> Cheers
>
> On Tue, Aug 18, 2026 at 6:43 AM Noah Vawter via Synth-diy
> <synth-diy at synth-diy.org> wrote:
> >
> >
> > this topic has come up many times over the decades.
> >
> > there is not much remarkable about the SID except for its context, how
> it is programmed with wavetables and the filter.  outside of the
> retrocomputing, its sound is heavily out of place.  look how few songs - in
> any genre outside retro - have picked up on it.  but in a game, in a demo,
> you hear intensely creative and efficient intermingling of voices, sync,
> filtering, PWM, modulation, transposition, etc.
> >
> > It is 100% digital plus the pseudo analog filter. most of the
> microcontroller implementers are lazy about the filter.  they emulate the
> digital part just fine.  every few years another demo scene guy comes up
> with an improved DSP-based rendition of the sid filter.
> >
> > the designer was never ashamed of it.  he just wished he had more time
> to make the voices have a register bank.  so he had to simply duplicate the
> voice he made 3x.  and he got more time when he made the DOC chip at
> Ensoniq.  The ESQ-1 *is* the no-compromise SID synthesizer.  same feature
> set, more waveforms, more voices, more LFOs, envelopes fixed, and
> predictable mass-produced filters (curtis? CEM?).
> >
> > As mentioned by the designer, their NMOS process was more efficient than
> others, but it couldn't create high enough gain for a high Q value.
> >
> > plus, the caps in the SVF rarely matched so everyone's was slightly
> different and that detracted further from the Q.
> >
> > one interesting thing is those clusters of FETs forming the multiplier,
> etc modules have distortion, not unlike an MS-20 filter.  that is unusual
> and worth re-creating.
> >
> > but the chip has so many other weird quirks.  like the NCOs---all
> waveforms have different volumes and no mixer to control them.  that's why
> you rarely hear the gentle triangle waveform and if you do, it's usually
> not at the same time as the pulse waveforms--it gets drowned out.  same p2p
> mag as those blazing pulse waveforms.  sawtooth gets its share too, but
> it's not expressive like the duty cycle on the pulse waveform.
> >
> > the most important aspect though is the wave tables that combine
> waveform sequencing like a wave station with sync and ring mod in rhythms.
> almost any synth could be programmed like that, but no one ever does.  and
> if they did, to sound like a sid, they would have to e.g. turn the volume
> way down on the triangle.
> >
> > As for analog VCO versions of a SID:  look into the ASID format.  it was
> originally used by Elektron to send blocks of register changes to
> sidstations from a PC.  but you can have anything receive it.  You can make
> a sid register to MIDI/CV converter and drive all your synths from SID tune
> players via ASID!  but honestly, pulse is pulse, sawtooth is sawtooth, and
> tri is tri.  other than fixing the amplitudes, you won't get much unless
> you drive interesting synth voices that way.  and then the game/demo you're
> using won't sound like the original, though it might be fun...like playing
> a MIDI file with all the wrong voices!
> >
> > sure SID has a little aliasing, but since the phase accs are clocked so
> fast (2 MHz?) and only upper bits are used to drive the DACs, not much
> comes through.  I once calculated it.  a little does.
> >
> > another quirk is the absurd envelope choices.  the ADSRs use linear and
> exp haphazardly.  and only having a few lengths - 16 and most are not
> musically relevant - leads to some limited music choices.  people could
> program their synths like this, but most choose not to bc they have
> "better" aesthetic choices.  There's also some kind of ADSR env bug that I
> don't totally understand.  and also typically the chip is so noisy people
> turn off the voice completely when no note is coming out of it!  sidstation
> has a specific option for this.
> >
> > i'm not trying to discourage anyone.  just trying to set reasonable
> expectations and point to where the magic is and isn't.  It's not that the
> sid is magic, it's not that constraints force you into an aesthetic regime
> you wouldn't normally hold yourself to.  Most normal people don't make
> rhythms by alternating sync and ring mod controls in 1/8th and 1/16th note
> steps  (mp-4 kind of does that and it's super crazy).
> >
> > And furthermore, the context in which it's used - 8-bit assembly
> programs - lends itself way more to rhythmic step tables than continuous
> musical gestures.  For example, people only rarely use LFOs in sid
> music...maybe a little vibrato here and there or PWM because the chip
> itself doesn't facilititate it (except one readback register, but that's an
> exception).  programming an LFO is much more of a pain - especially when
> you have to scale it in volume or in pitch - remember, there's no voltage
> control paradigm, just digital registers - that most people don't do it.
> the 6502 has no multiply instruction.  and even if you make a multiply it's
> overkill and you sitll have to scale it to the ranges in question...and
> typically you want exponents instead.  no sid editor that I know has a
> facility for it.  they all only just use little tables of values.
> >
> > so: sid programming = rhythmic tables of sync, ring mod and waveform
> changes + distorted low-Q filter, limited envelope choices.
> > normal people = LFOs. voices with adjustable volume, warm high-Q
> filters, continuously-variable exponential envelopes (or sometimes
> selectable).
> >
> > it's like tubes and transistors (oh no here come the pitchforks). you
> can make one sound like the other, but no one ever does.
> >
> >
> >
> >
> > Noah Vawter's futuristic electronic music instrument lab:
> youtube.com/diydsp
> >
> >
> > On Monday, August 17, 2026 at 11:23:03 PM EDT, cheater cheater via
> Synth-diy <synth-diy at synth-diy.org> wrote:
> >
> >
> > Stumbled upon some interesting info by people trying to emulate the
> > SID on component level, which could also be used to create a discrete
> > recreation of the chip.
> >
> > I think this could be an interesting development given that
> >
> > 1. SID chips are extremely scarce now
> > 2. even crappy examples go for $100+, like here
> > https://www.ami64.com/product-page/genuine-sid-chip-for-commodore-64
> > 3. even when SIDs weren't scarce, finding good ones wasn't easy since
> > so many were crappy
> > 4. digital recreations (by this I mean all those microcontroller based
> > ones that slot into the SID socket) just don't sound good at all
> >
> > Even when SIDs weren't scarce at all, it was really difficult to find
> > one that sounded right, didn't have annoying noise, etc.
> >
> > Plus, especially for something like a musical module, someone might
> > want to extend parameter control like cutoff, Q, etc to have finer
> > control.
> >
> > With the little electronics knowledge I have, it seems like the filter
> > is easily doable on component level (see link #4 for a component and
> > block level diagram).
> >
> > The oscillator is an NCO. The 3rd link goes deep into analyzing it.
> > The main issue would probably be recreating the glitches used for
> > playing back samples etc. The noise source is an LFSR and I think
> > that's what's being used for sample playback, but I'm hazy on the
> > details. Could be fun to also have a POKEY noise mode, since that's an
> > LFSR too.
> >
> > I think it would be interesting to hear what the SID might sound like
> > in such an approach. I also think it would be fund to hear what it
> > would sound like with VCOs replacing the NCOs.
> >
> > https://www.kvraudio.com/forum/viewtopic.php?t=532918
> > https://www.kvraudio.com/forum/viewtopic.php?f=1&t=526480
> > https://6502.org/forum/viewtopic.php?f=8&t=4150
> > https://www.kvraudio.com/forum/viewtopic.php?t=526071&start=45
> >
> > BTW here's the 6581 datasheet:
> > https://archive.org/details/mos-6581-sid-data-sheet
> >
> > I hope this inspires someone to build something around this idea.
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