SID 6581

MOS 6581 SID

Overview

The MOS 6581, commonly known as the SID (Sound Interface Device), is one of the most famous sound chips in computer history.

Developed by MOS Technology for the Commodore 64, the SID was introduced in 1982 and became one of the defining characteristics of the computer.

Unlike simple PSG chips, the SID combines programmable oscillators, analogue filtering, envelopes and modulation capabilities. Its distinctive analogue circuitry is responsible for much of the characteristic sound of the Commodore 64.

Technical characteristics

• Manufacturer: MOS Technology
• Model: 6581
• Common name: SID
• Channels: 3
• Oscillators: 3
• Waveforms: Triangle, sawtooth, pulse and noise
• Envelope: ADSR
• Filter: Analogue multimode filter
• Ring modulation: Yes
• Hard sync: Yes
• Typical computer: Commodore 64

Three oscillators

The SID contains three independent oscillators. Each oscillator can generate triangle, sawtooth, pulse or noise waveforms.

The oscillators can also interact through features such as ring modulation and hard synchronization. These functions allow composers to create complex metallic, percussive and evolving sounds.

ADSR envelopes

Each SID voice has an independent ADSR envelope, providing attack, decay, sustain and release control.

The envelope system allows a single oscillator to behave like a plucked string, percussion instrument, sustained synthesizer or slowly evolving pad.

Analogue filter

One of the most important characteristics of the 6581 is its analogue multimode filter.

The filter can operate in low-pass, high-pass, band-pass and combined modes. It provides resonance control and can dramatically change the character of the SID sound.

The analogue nature of the filter also means that individual SID chips can sound different from one another. Manufacturing revisions and component variations contribute to the distinctive character associated with different SID chips.

6581 versus 8580

The 6581 and 8580 are two major SID revisions. Although they share the same basic architecture, their analogue characteristics are noticeably different.

The 6581 is generally associated with a warmer, more distorted and sometimes more aggressive filter character, while the 8580 introduced several electrical and design changes that generally produce a cleaner response.

Commodore 64 music

The SID became the foundation of the Commodore 64 music scene. Composers such as Rob Hubbard, Jeroen Tel and many others developed techniques that pushed the three-channel hardware far beyond its apparent limitations.

Through rapid note changes, arpeggios, waveform manipulation, pulse-width modulation, filtering and software effects, composers were able to create music that sounded much more complex than the three physical voices suggested.

Tracker relevance

The SID is one of the most important chips in the history of tracker-style computer music.

Although early SID music systems did not necessarily use the same tracker format as Amiga or PC trackers, the pattern-based approach to controlling three hardware voices strongly influenced later music software.

Historical importance

The SID is widely regarded as one of the landmark sound chips of the 8-bit computer era. Its combination of digital control and analogue sound generation gave the Commodore 64 a distinctive musical identity.

The chip continues to influence modern synthesizers, FPGA recreations, software emulators and new music trackers.

Related soundchips

• MOS 8580 SID — Later SID revision used in the C64C and other Commodore systems.
• AY-3-8913 — Three-channel programmable sound generator.
• Yamaha YM2149 — Three-channel PSG used extensively in the Atari ST.
• Philips SAA1099 — Six-channel programmable sound generator.

Further reading

• Wikipedia — MOS 6581
• C64-Wiki — SID
• High Voltage SID Collection — One of the largest archives of Commodore 64 SID music.