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(Acronym for "Sound Interface Device") A synthesizer-on-a-chip integrated circuit created by MOS Technology (which at the time was a division of the Commodore computer company) , and famously included in the Commodore 64 line of microcomputers. Bob Yannes designed the initial version of the SID in 1981, after surveying existing large-scale integrated synth ICs and finding nothing that met both his personal standards, and the power and space constraints of the Commodore 64's packaging. Yannes set out to design a device that would have more capability and better sound quality than competing devices such as the Atari POKEY.

The SID was essentially a three-voice paraphonic synth, with a single multimode filter. Each of the three VCOs could output sawtooth, triangle and pulse waves. Each VCO had an accompanying VCA and ADSR envelope generator. There was also a noise source. There was no LFO capability, so all modulation had to be done by the controlling software. A mix of analog and digital circuitry was used; waveforms could be combined in the digital domin to produce ring modulation and other types of intermodulation effects. There was also an external audio input that could be mixed with the internal waveforms, which the Commodore computers that used the SID left unconnected.

An odd quirk of the original design was that, due to the limitations of the IC fabricating techniques of the day, it was impossible to get the output to not have a DC offset. Sound designers quickly exploited this in an odd way: the SID had an additional VCA by which software controlled the overall output level. By modulating this VCA from software with sample data, the SID could be made to play back samples, in a crude and distorted way. Game designes use this at first to produce speech, and then when later versions of the computer came with more memory, they could play back complex sampled music clips. The sample playback intermodulated with any audio being produced by the SID itself, and designers used this to create unique timbres and effects which came to be regarded as characteristic of the SID.

Two versions of the SID were produced. The first was numbered by MOS as part number 6581, as a complement to the 6502 CPU also designed by MOS and used by Commodore. An improved version was produced using two different numbers; 8580 was for use in Commodore products, and 6582 was for sale to external customers and hobbyists.

Commodore went bankrupt in 1994, taking MOS Technology with it. MOS management bought the company back during the bankruptcy proceedings, but they never succeeded in restarting production of the SID. In 1997, the synthesizer manufacturer Elektron acquired a large stock of 6581 SIDs and used the IC as the basis for a synthesizer called the SidStation. Using a microprocessor tied to hardware knobs and switches, they brought out the capabilities of the SID to a panel for direct tweaking, something that had never been done by Commodore. The SidStation also had MIDI for external control (it had no keyboard). Elektron claims to now have used up its supply of 6581's, although they have several times issued additional runs of SidStatoins using 6581's that they claimed to have found in the international spot market.

The supply of both 6581s and 8580s has been nearly exhausted now, and some people have resorted to locating old Commodore computers and scrapping them to get SIDs for repairs or new designs. Several other synth products and computer sound cards have been produced to take advantage of the SID. SID emulations have been produced both as software, and using programmable DSP circuits.

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