SID chip analog filter

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.

The various SID chip models that Commodore used in the C64 differ significantly regarding the used manufacturing technology and the actual chip design. Early 6581 models used NMOS which imposed certain design limitations that where resolved in the later 8580R5 models (which 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 filter 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.


SID chip voices (a red border signifies that this voice is filtered)

filter type 
resonnance 
cutoff frequency 

SID volume setting


Here a selection of songs that had been designed to work with the 6581 SID (click a song name to play it on this page):

, , , , , , , , , , , , (It would have strongly depended on the revision and production lot of the used 6581 SID what these songs actually sounded like in the wild. And obviously composers found it quite frustrating when their song did sound quite differently on the customer's C64 than it had on their own machine. Though the gritty sound of the old models still has its fans.).

Due to the large differences in the behavior of the older 6581 type SID chip models these usually are further classified by their "revision" ("R1", "R2", "R3, "R4 AR", "R4") and/or production month. The idea being that the chips from the same revision might have more similar characteristics as compared to the chips from different revisions. Unfortunately this must be seen as a very rough approximation at best - since differences within the same revision often seem to be just as large as those to other revisions. You might try this standalone page here for additional background information and on how webSID models the filter of the old generation SID chips.

From a composers perspective use of the filter became much more popular on the 8580R5 SIDs - since audio results on different machines became so much more predictable. Here some examples:

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, , , , , , , , , , .


Note: The circuitry used in the C64 also adds an additional hard-wired "external" filter to post-process the output of the SID chip. It is intended to attenuate frequencies above 16kHz (probably meant to reduce the noise introduced by the C64's cheap ass power supply and/or other insufficiently shielded components). The effect of that filter is not reflected in the above graphs. It is funny that in direct comparisons with the real hardware, emulators are often exposed by the lack of random noise in their output singal - of which the real hardware has plenty.

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