SID 8580

MOS Technology 8580 SID

Overview

The MOS Technology 8580 is the revised successor to the original 6581 SID chip, introduced in 1987 to coincide with the launch of the redesigned, cost-reduced Commodore 64C. Rather than being an entirely new sound chip, the 8580 represents a re-engineering of the same fundamental SID architecture, three voices, ADSR envelopes, multiple waveforms and a multimode analogue filter, built using a newer, more efficient manufacturing process.

Where the original 6581 was manufactured using an older NMOS process requiring a 12-volt power supply, the 8580 was built using the newer HMOS-II process, needing only 9 volts to operate. This lower power requirement made the chip run considerably cooler and proved far more durable in the long run, addressing one of the most notorious weaknesses of the original chip: the 6581's well-documented sensitivity to static discharge, which frequently damaged its delicate analogue filter circuitry and left a large proportion of surviving 6581 chips with non-functional filters today.

Beyond the voltage and manufacturing changes, MOS also used the 8580 revision as an opportunity to correct several long-standing quirks of the original design. The 6581's waveform generator suffered from unwanted bit-mixing effects when multiple waveforms were selected on the same voice simultaneously, producing combined waveforms that deviated noticeably from the chip's original design specification. The 8580's more stable waveform generators bring these combined-waveform outputs much closer to the intended behaviour, described in the chip's specification as a straightforward binary AND of the selected waveforms.

The analogue filter, arguably the SID's most musically important and most idiosyncratic feature, was also substantially reworked. The 6581's filter is often described as sounding moodier, dirtier and more unpredictable, with resonance having comparatively little audible effect, while the 8580's filter behaves in a cleaner, more precise and more predictable manner, generally regarded as technically superior even though many Commodore 64 enthusiasts still express a strong preference for the rawer, more characterful sound of the original 6581.

Technical characteristics

• Manufacturer: MOS Technology (Commodore Semiconductor Group)
• Model: 8580 (successor to the 6581)
• Channels: 3, each with independent ADSR envelope
• Waveforms: Triangle, sawtooth, pulse, noise; combined waveforms behave closer to true binary AND than on the 6581
• Filter: Multimode analogue filter (low-pass, band-pass, high-pass, notch); cleaner, more precise response than the 6581
• Manufacturing process: HMOS-II
• Supply voltage: 9V DC (versus 12V DC on the 6581)
• Filter capacitors: 22nF (versus 470pF required by the 6581)
• Durability: Significantly more resistant to static damage than the 6581
• Computers: Commodore 64C (short-board revision), Commodore 128
• Related chip: MOS 6582, a later replacement chip sonically very similar to the 8580

Composing and the "sample bug"

One of the most celebrated quirks of the original 6581 was a flaw in its master volume register that allowed programmers to play crude digitised audio samples through the chip by rapidly modulating volume at high speed, a technique often referred to as the SID "sample bug." This trick was used to striking effect in a number of well-known Commodore 64 games, producing sampled speech and sound effects that seemed far beyond what the hardware should have been capable of.

The 8580 largely corrected this flaw as part of its broader stability improvements, which meant that software relying on the 6581's volume-register sample-playback trick often sounded noticeably different, or failed to work as intended, when run on 8580-equipped machines. This became one of the more practically significant differences between the two chips for programmers and musicians working across both hardware revisions, since compatibility with older sample-playback tricks could not always be assumed on the newer chip.

For composers writing more conventional SID music, the 8580's cleaner and more predictable filter behaviour made it, in many respects, an easier chip to work with in a controlled, studio-like sense, while the 6581 continued to be prized by many musicians specifically for its unpredictable, characterful "grit," a sonic quality actively sought out rather than considered a defect by a large part of the SID music community.

SID 8580 vs SID 6581: side-by-side comparison

Further reading

• MOS Technology 6581/8580 overview (Wikipedia)
• MOS Technology SID technical comparison (Synth DIY Wiki)
• C64 SID Chip: MOS 6581 & 8580 (commodore-64.eu)