Games - MOS Technology SID

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	Games - MOS Technology SID

The MOS Technology 6581/8580 SID (Sound Interface Device) was the built-in sound chip of Commodore's CBM-II, Commodore 64 and Commodore 128 home computers. It was one of the first sound chips of its kind to be included in a home computer prior to the digital sound revolution.

Together with the VIC-II chip, the SID was instrumental in making the C64 the best-selling computer in history, and is partly credited for initiating the demoscene.

The SID has , which was filed on February 27, 1983 and issued on July 7, 1987. The patent expired on July 7, 2004.

Design process

The SID was devised by engineer Robert "Bob" Yannes, who later founded the Ensoniq digital synthesizer company. Yannes headed a team that included Yannes, two technicians and a CAD operator running Applicon (nowadays a part of the UGS Corp.), who designed and completed the chip in five months' time in the latter half of 1981. Yannes was inspired by previous work in the synthesizer industry and was not impressed by the current state of computer sound chips. Instead, he wanted a high-quality instrument chip, which is the reason to why the SID has features like the envelope generator, previously not found in home computer sound chips.

Emphasis during chip design was on high-precision frequency control, and the SID was originally designed to have 32 independent voices, sharing a common oscillator. However these features could not be finished in time, so instead the mask work for a certain working oscillator was simply replicated three times across the chip surface, creating three voices with a unique oscillator for each voice. Another feature that was not incorporated in the final design was a frequency look-up table for the most common musical notes, a feature that was dropped because of space limitations. The support for an audio input pin was a feature Yannes added without asking, even though this has no practical use in a computer. The masks were produced in 7-micrometer technology in order to gain a high yield: the current state-of-the-art at the time was 6-micrometer technologies.

The chip, like the first product using it, the Commodore 64 was finished in time for the Consumer Electronics Show in the first weekend of January 1982. Even though Yannes was partly displeased with the result, his colleague Charles Winterble said: "This thing is already 10 times better than anything out there and 20 times better than it needs to be".

The specifications for the chip were not used as a blueprint. Rather, they were written as the development work progressed, and not all planned features made it into the final product. Yannes claims he had a feature-list of which three fourths made it into the final design. This is the reason why some of the specifications for the first version (6581) were accidentally incorrect. The later revision (8580) was revised to match the specification. For example, the 8580 can make a logical AND between two waveforms, something that the 6581 could never handle. Another feature that differs between the two revisions is the filter: the 6581 version is far away from the specification.

Features

three separately programmable independent audio oscillators (8 octave range, approximately 16 - 4000 Hz)
four different waveforms per audio oscillator (sawtooth, triangle, pulse, noise)
one multi mode filter featuring (low-pass, high-pass and band-pass outputs with 6 dB/oct (bandpass) or 12 dB/octave (lowpass/highpass) rolloff. The different filter-modes are sometimes combined to produce additional timbres, for instance a notch-reject filter.
three attack/decay/sustain/release (ADSR) volume controls, one for each audio oscillator.
three ring modulators.
oscillator sync for each audio oscillator.
two 8-bit A/D converters (typically used for game control paddles)
external audio input (for sound mixing with external signal sources)
random number/modulation generator

Technical details

The SID is a mixed-mode integrated circuit, featuring both digital and analog circuitry. All control ports are digital, but the output ports are analog. The SID features three-voice synthesis, where each voice may use one of four different waveforms: square wave (with variable pulse width), triangle wave, sawtooth wave and a pseudo-random (but not white noise) wave. Each voice may be ring modulated with one of the other waves, i.e. the frequency spectrum is multiplied and output. The ring modulation, filter, and programming techniques for switching between different waveforms at high speed make up the characteristic sound of the SID.

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