Battle of the Bits

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battle of the bits

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The MOS Technology 6581/8580 SID (Sound Interface Device) was the built-in Programmable Sound Generator chip of Commodore's CBM-II, Commodore 64, Commodore 128 and Commodore MAX Machine home computers. The SID is a mixed-signal integrated circuit, featuring both digital and analog circuitry. All control ports are digital, while the output ports are analog. The SID features three-voice synthesis, where each voice may use one of at least five different waveforms: square wave (with variable duty cycle), triangle wave, sawtooth wave, pseudo-random (but not white) noise, and certain complex/combined waveforms. Each voice may be ring-modulated with one of the other voices, i.e. the frequency spectrum is convolved 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. Each voice may be passed through a common, digitally controlled analog filter with variable cut-off frequency and resonance, which is constructed with the aid of capacitors external to the circuit. An external audio in port enables external audio to be passed through the filter. The sound chip, SID, had three channels, each with its own ADSR envelope generator, and with several different waveforms, ring modulation and filter capabilities.

There are two versions of the SID chip. The first version was the MOS Technology 6581, which is to be found in all of the original "breadbox" C64s, and early versions of the C64C and the Commodore 128. It was later replaced with the MOS Technology 8580 in 1987. The sound quality was a little more crisp on the 6581 and many Commodore 64 fans still prefer its sound. The main difference between the 6581 and the 8580 was the voltage supply: the 6581 uses a 12 volt supply, while the 8580 required only 9 volts. A voltage modification can be made to use a 6581 in a C64C board (which uses 9V).

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