MOS Technologies SID

Image of two SIDs, the one above a 6581 from MOS Technologies at the time they were known as the Commodore Semiconductor Group (CSG) and the lower one an 8580 from MOS Technologies.

The MOS Technologies 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.

Table of contents

1 Features 2 Technical details 3 Game audio 4 Modern developments 5 Trivia 6 Notes 7 References 8 External links

Features

• three separately programmable independent oscillatorss (8 octave range, approximately 16 - 4000 Hz)
• four mixable waveforms per oscillator (sawtooth, triangle, pulse, noise)
• three mixable filterss (low-pass, high-pass, band-pass, notch-reject) with 12 dB/octave rolloff
• attack/decay/sustain/release (ADSR) control of each audio oscillator
• two ring modulators
• two 8-bit A/D converters
• external input (for sound mixing with external signal sources)
• random number/modulation generator

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 oscillators: square wave (with variable pulse width), triangle wave, sawtooth wave and a random (white noise) wave. Any 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 C128.

The 6581 and 8580 are somewhat different in several ways: the supply power for the 6581 is 5V and 12V DC, whereas the 8580 requires 5V and 9V DC. 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 it will only yield silence. 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 titles including WizBall, and Rob Hubbard, known for titles such as International Karate, IK+, Monty on the Run and Delta.

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 PCss, Macss, and Amigas, includes a hardware SID option, i.e. an option to insert a real SID chip in a socket for use when playing .SID 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, LEDss etc., mounted on e.g. a keyboard-less C64 body.

• Swedish rock/metal band Machinae Supremacy uses a SID chip in one of their instruments, making for quite a unique overall sound.

A .SID file, colloquially known as "a SID", is a sound data file (akin to NSF files) typically copied directly from the SID chip music/sound part of a C64 game or demo. Several utility cartridgess allowed the user to locate and save any part of C64 RAM containing SID sound data after "freezing" the computer. The High Voltage SID Collection contains several thousand such extracted SID-tunes. These are very compact as the file actually consists of a small 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

References

• The High Voltage SID Collection
• SID enthusiast homepage
• SIDplay emulator
• The dmoz SID directory
• remix.kwed.org – a collection of several hundred C64 SID remixes
• Slay Radio – plays exclusively C64 SID remixes
• Home of MIDIbox SID
• Elektron Sidstation
• Elektron Monomachine