reSID

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reSID is a reverse engineered software emulation of the MOS6581 SID (Sound Interface Device). This chip was used in the Commodore 64 computer. reSID is free software, published under the GNU General Public License.

reSID is a C++ library containing a complete emulation of the SID chip. This library can be linked into programs emulating the MOS6510 MPU to play music made for the Commodore 64 computer. reSID has been linked into VICE (a Commodore 64 emulator), SIDPLAY (a SID tune player), and GoatTracker (a tracker).

The emulator engine is cycle-based, emulating the internal operations of the SID chip. SID's audio filter is modeled as an actual two-integrator-loop biquadratic filter. The engine has been developed based on available information on SID, sampling of the OSC3 and ENV3 registers, filter theory, and testing.

Due to the manufacturing process and engineering decisions, the filter of the 6581 family of chips was significantly nonlinear. The distortion appears at first as gradual shifting of the specified center frequency upwards in spectrum — this effect may change the center frequency upwards by one octave — until some type of hard limit is reached at which point the output from the chip becomes nearly discontinuous. The filter was revised on several later revisions of the 6581 chip. Revision 4 is quite commonly found these days. The linear filters of reSID do not even attempt to emulate the nonlinear character of 6581 emulation, and some features such as the OptimiseLevel setting further degrades filter quality by limiting the filter upper frequency to mere 4 kHz (genuine chips can specify center frequencies up to 20 kHz). Additionally, some kind of systematic error has probably^[who?] crept into reSID's measurement of the CF (Center Frequency) value, a number from 0 to 2047, and physical Hz mapping.

As of April 2008, the current version is 0.16, which was released 11 June 2004.