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

[cheater cheater]

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.