MOS Technology SID

From Academic Kids

Missing image
MOS Technology SIDs: The right image shows a 6581 from MOS Technology, at the time they were known as the Commodore Semiconductor Group (CSG) and the left image shows an 8580 from MOS Technology.

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 last sound chips to be made for any home computer prior to the digital sound revolution.

The SID was created by engineer Robert Yannes, who later founded the Ensoniq digital synthesizer company. 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 demo scene.



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 random (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.

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 6581 had a quirk in that changing volume levels on a channel would result in a slight "pop". Eventually this bug was found to be useful for producing a fourth voice of percussive sounds and even digitized speech. Unfortunately, this "defect" was partially corrected in the 8580 used in the C64C and the C128. This made digitized sound (samples) very quiet. Fortunately, a hardware modification could be made to the 8580 to restore the volume to 6581 levels.

The 6581 and 8580 are different in several ways. The original 6581 was manufactured using the older NMOS process, which used 12V DC to operate. The 8580 was made using the HMOS-II process, which required less power (9V DC), and therefore made the IC run cooler. The 8580 was thus far more durable than the 6581. Additionally, a better separation between the analog and the digital circuits made the 8580 chip's output less noisy and distorted.

A HMOS-II version of the 6581 was produced, the 6582. It was never shipped in new C64s.

The original manual for the SID mentions that if several waveforms are enabled at the same time, the result will be a logical AND between them, but only the 8580 actually has this functionality: on the 6581 some waveform combinations will only yield silence or be close to inaudible depending on the chip revision. The filter is also different between the two models, with the 8580 being closer to the actual specification.

Game audio

The C64 had agonizingly slow tape and disk drive protocols, taking minutes to load its 64K into RAM. As a result, while the tape cassette or disk loaded it was common for game companies to put up a graphics display and play music, in what was sometimes called a "loader". The combination of slow loading and an excellent sound chip may be why composers for Commodore game music have received somewhat more attention compared to composers for other game platforms.

Well known composers of game music for this chip are Martin Galway, known for many titles, including Wizball, and Rob Hubbard, known for titles such as ACE 2, Delta, International Karate, IK+, and Monty on the Run.

Modern developments

  • In 1997, an electronic musical instrument utilizing the SID chip as its synthesis engine was released. It is called the SidStation and is produced by Swedish company Elektron. As the SID chip had been discontinued for years, Elektron allegedly bought up almost all the remaining stock. In 2004, Elektron released the Monomachine pattern-based sequencer with optional keyboard. The Monomachine contains several synthesis engines, including a single 6581 oscillator with ring modulation. Unlike the Sidstation, the Monomachine simulates a 6581 oscillator using a DSP.
  • Also in the 1990s, a sound card for IBM PC compatibles called HardSID was released. The card contains one or two SID chips and allows a PC to utilize the sound capabilities of the chip directly instead of by emulation via generic sound cards (e.g. SoundBlaster).
  • In June 1998, a cycle-based SID emulator engine called reSID became available. The all-software emulator, available with C++ source code, is licensed under the GPL by the author, Dag Lem.
  • The Catweasel from German company Individual Computers, a PCI + Zorro multiformat floppy disk controller and digital joystick adapter for PCs, Macs, and Amigas, includes a hardware SID option, i.e. an option to insert a real SID chip in a socket for use when playing .MUS files.
  • The MIDIbox SID is a MIDI controlled synthesizer which can contain up to four SID chips. It is a free open source project using a PIC microcontroller. Control of the synthesizer is done either via software or via a control panel with knobs, LEDs etc., mounted on e.g. a keyboard-less C64 body.
  • The Prophet 64 ( is a newly developed series of synthesizer software that mimics a set of old machines like the TB-303. Through a clever utilization of the user port, the Prophet 64 can be synchronized to MIDI-instruments via DIN-Sync standard (SYNC 24).


A .SID file, colloquially known as "a SID", is a sound data file (akin to NSF files) typically copied directly from the memory of a C64 game or demo. Several utility cartridges allowed the user to locate and save any part of C64 RAM by "freezing" the computer: stopping the running program in its tracks, allowing the user to examine and change memory, save parts of it, and restart the program. The High Voltage SID Collection contains several thousand such extracted SID-tunes. These are very compact as the file actually consists of a 6502 machine code program and a set of music patterns – all actual sound is eventually produced by the SID chip. The SID files have the MIME media type audio/prs.sid


  • Appendix O, "6581 Sound Interface Device (SID) Chip Specifications", of the Commodore 64 Programmer's Reference Guide (see the C64 article).

See also

External links

Template:VG Music Sound Formatsda:SID de:MOS Technologies SID sv:SID fi:MOS 6581


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