Existing SID models differ significantly regarding the used manufacturing technology and the actual chip design. Early 6581 models used NMOS which imposed certain design limitations which where resolved in later 8580R5 models that used HMOS-II instead.

One of the limitations affected the design of "high-gain op-amps" which were essential to a resonant filter. Early models here used FETs - but these where prone to significant variances between different production lots (over time Commodore also varied the quality of the used silicon to make things more interesting). This lead to different lots of chips having different cutoff frequency characteristics. Commodore then also used varying quality filter capacitors during final board assembly (whatever the factory happend to have in stock). To "correctly" emulate the resulting flaws of the hardware is quite a challenge and is where work remains to be done in the WebSid emulation.

For its analog filter the SID chip uses a classic multi-mode (state variable) VCF design. It provides simultaneous low-pass, band-pass and high-pass outputs that can be combined (e.g. a notch filter can be implemented by enabling the high-pass and low-pass outputs simultaneously). Each of the SID's three voices can then be routed either through or around the filter - while all the filtered voices share the same filter configuration.

SID voices - blue border signifies that this voice is filtered using:

Here a selection of songs that had been designed to work with the 6581 SID: Another_Tune_for_Joanne, Stormlord, RoboCop, Lightforce, IK+, Street_Hawk, Wizball, Chordian, Digi-Piece_for_Telecomsoft, Turbo_Outrun, Cybernoid_II, Myth, Last_Ninja_3 (it would have strongly depended on the production lot of the used 6581 SID what these songs actually sounded like in the wild).

Use of the filter became much more popular on the 8580R5 SIDs - since actual audio results became so much more predictable. Here some examples: Curved_Note, Title_Needed, Wings_of_Icarus, Al-Risah, Blend_with_Green, Markanoid, Mario_is_Dead, Metamorphosis, Mekanix, Andropolis, Perfect_Symmetry, Trick_n_Treat, Broken_Beauty, Kirmestechno, One_Riff_Loader, Running_on_Empty, Glacial_Blues, One_Little_Wish_tune_2 (as can be seen in the filter-status display use of the filter has become much more dynamic in the more recent songs).


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