Sid

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. It was one of the first sound chips of its kind to be included in a home computer prior to the digital sound revolution.

Together with the VIC-II graphics chip, the SID was instrumental in making the C64 the best-selling computer in history, and is partly credited for initiating the demoscene.

The SID has U.S. Patent 4,677,890 , which was filed on February 27, 1983 and issued on July 7, 1987. The patent expired on July 7, 2004.

Design process

The SID was devised by engineer Robert "Bob" Yannes, who later co-founded the Ensoniq digital synthesizer company. Yannes headed a team that included Yannes, two technicians and a CAD operator running Applicon (now a part of the UGS Corp.), who designed and completed the chip in five months' time in the latter half of 1981. Yannes was inspired by previous work in the synthesizer industry and was not impressed by the current state of computer sound chips. Instead, he wanted a high-quality instrument chip, which is the reason why the SID has features like the envelope generator, previously not found in home computer sound chips.

I thought the sound chips on the market, including those in the Atari computers, were primitive and obviously had been designed by people who knew nothing about music.

– Robert Yannes, On the Edge: The Spectacular Rise and Fall of Commodore

Emphasis during chip design was on high-precision frequency control, and the SID was originally designed to have 32 independent voices, sharing a common oscillator. However these features could not be finished in time, so instead the mask work for a certain working oscillator was simply replicated three times across the chip surface, creating three voices with a unique oscillator for each voice. Another feature that was not incorporated in the final design was a frequency look-up table for the most common musical notes, a feature that was dropped because of space limitations. The support for an audio input pin was a feature Yannes added without asking, even though this had no practical use in a computer, although it enabled the chip to be used as a simple effect processor. The masks were produced in 7-micrometer technology in order to gain a high yield: the current state-of-the-art at the time was 6-micrometer technologies.

The chip, like the first product using it (the Commodore 64), was finished in time for the Consumer Electronics Show in the first weekend of January 1982. Even though Yannes was partly displeased with the result, his colleague Charles Winterble said: "This thing is already 10 times better than anything out there and 20 times better than it needs to be."

The specifications for the chip were not used as a blueprint. Rather, they were written as the development work progressed, and not all planned features made it into the final product. Yannes claims he had a feature-list of which three quarters made it into the final design. This is the reason why some of the specifications for the first version (6581) were accidentally incorrect. The later revision (8580) was revised to match the specification. For example, the 8580 expanded on the ability to perform a logical AND between two waveforms, something that the 6581 could only do in a somewhat limited and unintuitive manner. Another feature that differs between the two revisions is the filter: the 6581 version is far away from the specification.

Features

* three separately programmable independent audio oscillators (8 octave range, approximately 16 - 4000 Hz)
* four different waveforms per audio oscillator (sawtooth, triangle, pulse, noise)
* one multi mode filter featuring low-pass, high-pass and band-pass outputs with 6 dB/oct (bandpass) or 12 dB/octave (lowpass/highpass) rolloff. The different filter-modes are sometimes combined to produce additional timbres, for instance a notch-reject filter.
* three attack/decay/sustain/release (ADSR) volume controls, one for each audio oscillator.
* three ring modulators.
* oscillator sync for each audio oscillator.
* two 8-bit A/D converters (typically used for game control paddles, but later also used for a mouse)
* external audio input (for sound mixing with external signal sources)
* random number/modulation generator

Technical details

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.

Upon setting the main output volume/gain on the 6581's 4 bit volume control, a click noise would be produced on the output. This was proportional to the volume difference, both positive and negative, and giving an opportunity to play 4-bit digitized sounds on the commodore 64 (The digitalization/sampling had to be done using external means, typically a simple 8 bit sampler circuit attached to the 8 bit parallel part of the parallel/serial communications port). This flaw was used in several games (probably first on Ghostbusters, whereby pressing the space key everything would stop, but the theme-word "Ghostbusters!" would be played) and in many demos. The sounds produced was typically some words, but most often percussion instruments like drums and the like - the amount of sound possible to store on a fraction of 64 kilobytes was very limited. Also, it was hugely CPU intensive - one had to output the samples very fast (in comparison to the extreme slowness of the 6510 CPU), and one could not do much other activity while a sample was playing. This "defect" was partially corrected in the 8580 used in the Commodore 64C and the Commodore 128DCR. This made digitized sound (samples) very quiet. Fortunately, the volume level could be mostly restored with either a hardware modification, or a software trick involving the Pulse waveform. The software trick generally renders one voice temporarily unusable, although clever musical compositions can make this problem less noticeable.

The 6581 and 8580 differ from each other 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 simple hardware modification can be added to 6581-based computers to remove most of the noise, but this involves disabling the Audio-In function.

A HMOS-II version of the 6581 was produced, the 6582. Like the 8580, this chip used 5 volts and 9 volts for its power supplies. It was never shipped in new Commodore 64s, however due to its lower voltage requirement, Creative Micro Designs later used it in their SID Symphony expansion cartridge.

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. In reality, the final output isn't exactly a logical AND of the selected waveforms. On the 6581, one notable exception is that all 6581 revisions appear to be capable of using the Sawtooth + Pulse waveform combination, the output from which is actually a logical AND of the Sawtooth and Triangle waveform oscillator values, resulting in a very complex final waveform, even though the Triangle waveform was not explicitly requested. The filter is also different between the two models, with the 8580 being closer to the designers' actual specification.

Despite its documented shortcomings, many SID musicians prefer the flawed 6581 chip over the corrected 8580 chip. The main reason for this is that the filter produces strong distortion that is sometimes used to produce simulation of instruments such as a distorted electric guitar. Also, the highpass component of the filter was mixed in 3 dB attenuated compared to the other outputs, making the sound more bassy. In addition to nonlinearities in filter, the D/A circuitry used in the waveform generators produced yet more additional distortion that made its sound softer and smoother.

Revisions

No instances reading "6581 R1" ever reached the market. In fact, Yannes has stated that "[the] SID chip came out pretty well the first time, it made sound. Everything we needed for the show was working after the second pass." High-resolution photos of Charles Winterble's prototype C64 show the markings "MOS 6581 2082", the last number being a date code indicating that his prototype SID chip was produced during the 20th week of 1982, which would be within 6 days of May 14, 1982.

These are the known revisions of the various SID chips:

* 6581 R2 - Will say "6581" only on the package
* 6581 R3 - Will say "6581 R3" or "6581 CBM" on the package. Produced until early 1986
* 6581 R4 - Will say "6581 R4" on the package. Produced from early to mid 1986
* 6581 R4 AR - Will say "6581 R4 AR" on the package. Produced from mid 1986 until at least 1990
* 6582 A - Will say "6582 A" on the package. Typically produced around 1992
* 8580 R5 - Will say "8580R5" on the package. Produced from 1987 to 1992

Some of these chips are marked "CSG" ("Commodore Semiconductor Group") and the Commodore Logo, while others are marked with "MOS". This includes chips produced during the same week (and thus, receiving the same date code), indicating that at least two different factory lines were in operation during that week. Generally, chips produced through the middle of 1989 were marked "MOS", while those produced after that point may show either marking.

Game audio

The majority of games produced for the Commodore 64 made use of the SID chip, with sounds ranging from simply clicks and beeps to complex musical extravaganzas or even entire digital audio tracks.

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. Other noteworthies include Jeroen Tel (Cybernoid and Myth) and Chris Hülsbeck, whose composition career started with the SID but has spanned nearly every kind of computer music and other synthesizers since.

(from http://en.wikipedia.org/wiki/MOS_Technology_SID)