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Skimming the article, it does seem like they've built a model (there has been much work over the years on opensource emulators,etc so the basic operation is fai

Skimming the article, it does seem like they've built a model (there has been much work over the years on opensource emulators,etc so the basic operation is fairly well researched) and then sampled a bunch of real chips to find variances and behaviours to get profiles that comes close to the real ones. As for being scientifically correct, Commodore was infamous for cutting corners so resistances (and prob other parts of fabrication) varied wildly so at best you can probably compare to specific chips (even if families of fabrication are closer,esp w.r.t to 8580 vs 6581 behaviours). Read the article again in that light (as those details are alluded to) or look at the analysis of the C-64 graphics chip colors (There is a nugget in the response from the Sid-chip designer at the bottom of this other article about Commodore inner workings). http://unusedino.de/ec64/technical/misc/vic656x/colors/ http://unusedino.de/ec64/technical/misc/vic656x/colors/

leptons 3y ago
>As for being scientifically correct, Commodore was infamous for cutting corners so resistances (and prob other parts of fabrication) varied wildly so at best you can probably compare to specific chips (even if families of fabrication are closer,esp w.r.t to 8580 vs 6581 behaviours). It would be really neat if Chipsynth were capable of emulating those different resistances, to tweak them a bit to get a 'warmer' sound or something different, to allow further tweaking of the sound. I'm not sure if Chipsynth is emulating at the silicon gate level, or the analog circuits within the SID. Would be cool if it were.
- ilkke 3y ago
  6581 units are so wildly different re filter behaviour that most game musicians of the time avoided using filters altogether! 8580 is basically a fix, and had way more consistency
jacquesm 3y ago
> I can tell you that the design was based on the principle that adding a sine wave of a particular frequency and amplitude to an inverted version of the same sine wave at a different amplitude produces a phase-shifted sine wave of the same frequency. The amount of phase shift is directly proportional to the amplitudes of the two sine waves. I can't follow that. Wouldn't it just lead to a lower amplitude sine wave? Where does the phase shift come from? My visual imagination is failing me here, mostly I don't get why the sine wave would shift forward or backwards if all that changes if the amplitude because that's symmetrical.
- chillingeffect 3y ago
  Likely bc the "adder" is not an adder. I've analyzed it. It's composed of FETs with nonlinear gate voltage-drain current relationship (that's normal). Often in radio circuits these are set up and operated in a small linear region. They work well enough in a small voltage range, but in SID they are used for larger amplitudes. Result can be modeled as a sum of Taylor series meaning fun, new partials like x^N get synthesized for free!
  - jacquesm 3y ago
    That makes more sense. You need something that has a temporal element to get a phase shift.
coldtea 3y ago
>varied wildly so at best you can probably compare to specific chips (even if families of fabrication are closer,esp w.r.t to 8580 vs 6581 behaviours). Or you could just have some toggle to switch from and offer all of their behaviors.

leptons 3y ago

>As for being scientifically correct, Commodore was infamous for cutting corners so resistances (and prob other parts of fabrication) varied wildly so at best you can probably compare to specific chips (even if families of fabrication are closer,esp w.r.t to 8580 vs 6581 behaviours). It would be really neat if Chipsynth were capable of emulating those different resistances, to tweak them a bit to get a 'warmer' sound or something different, to allow further tweaking of the sound. I'm not sure if Chipsynth is emulating at the silicon gate level, or the analog circuits within the SID. Would be cool if it were.

ilkke 3y ago
6581 units are so wildly different re filter behaviour that most game musicians of the time avoided using filters altogether! 8580 is basically a fix, and had way more consistency

jacquesm 3y ago

> I can tell you that the design was based on the principle that adding a sine wave of a particular frequency and amplitude to an inverted version of the same sine wave at a different amplitude produces a phase-shifted sine wave of the same frequency. The amount of phase shift is directly proportional to the amplitudes of the two sine waves. I can't follow that. Wouldn't it just lead to a lower amplitude sine wave? Where does the phase shift come from? My visual imagination is failing me here, mostly I don't get why the sine wave would shift forward or backwards if all that changes if the amplitude because that's symmetrical.

chillingeffect 3y ago
Likely bc the "adder" is not an adder. I've analyzed it. It's composed of FETs with nonlinear gate voltage-drain current relationship (that's normal). Often in radio circuits these are set up and operated in a small linear region. They work well enough in a small voltage range, but in SID they are used for larger amplitudes. Result can be modeled as a sum of Taylor series meaning fun, new partials like x^N get synthesized for free!
- jacquesm 3y ago
  That makes more sense. You need something that has a temporal element to get a phase shift.

coldtea 3y ago

>varied wildly so at best you can probably compare to specific chips (even if families of fabrication are closer,esp w.r.t to 8580 vs 6581 behaviours). Or you could just have some toggle to switch from and offer all of their behaviors.