Chip Hall of Fame: MOS Technology 6581

A synthesizer that defined the sound of a generation

5 min read
Photo of the SID 6581 chip.
Christian Taube/Wikipedia

1982 was a big year for music. Not only did Michael Jackson release Thriller, the bestselling album of all time, but Madonna made her debut. And it saw the launch of the Commodore 64 microcomputer. Thanks to the C64, millions of homes were equipped with a programmable electronic synthesizer, one that's still in vogue.

The C64 became the bestselling computer of all time (some 17 million were sold) largely because it had graphics and sound capabilities that punched way above the system's price tag: US $600 on release, soon falling to $149. Like many machines from that era, the C64 has a devoted following in the retrocomputing community, and emulators are available that let you run nearly all its software on modern hardware. What's unusual is that a specific supporting chip inside the C64 has also retained its own dedicated following: the 6581 SID sound chip.

6581 SID

Manufacturer: MOS Technology

Category: Amplifiers & Audio

Year: 1982

The C64 was developed by MOS Technology in 1981. MOS had already had a hit in the microcomputing world with its creation of the 6502 CPU in 1975. That chip—and a small family of variants—was used to power popular home computers and game consoles such as the Apple II and Atari 2600. As recounted in IEEE Spectrum's March 1985 design case history [PDF] of the C64 by Tekla S. Perry and Paul Wallich, MOS originally intended just to make a new graphics chip and a new sound chip. The idea was to sell them as components to microcomputer manufacturers. But those chips turned out to be so good that MOS decided to make its own computer.

Creation of the sound chip fell to a young engineer called Robert Yannes. He was the perfect choice for the job, motivated by a long-standing interest in electronic sound. Although there were some advanced microcomputer-controlled synthesizers available, including the Super Sound board designed for use with the Cosmac VIP system, the built-in sound generation tech in home computers was relatively crude. Yannes had higher ambitions. “I'd worked with synthesizers, and I wanted a chip that was a music synthesizer," Yannes told Spectrum in 1985. His big advantage was that MOS had a manufacturing fab on-site. This allowed for cheap and fast experimentation and testing: “The actual design only took about four or five months," said Yannes.

On a hardware level, what made the 6581 SID stand out was better frequency control of its internal oscillators and, critically, an easy way for programmers to control what's known as the sound envelope. Early approaches to using computers to generate musical tones (starting with one by Alan Turing himself) produced sound that was either off or on at a fixed intensity, like a buzzer. But most musical instruments don't work that way: Think of how a piano note can be struck sharply or softly, and how a note can linger before decaying into silence. The sound envelope defines how a note's intensity rises and falls. Some systems allowed the volume to be adjusted as the note played, but this was awkward to program. Yannes incorporated data registers into the 6581 SID so a developer could define an envelope and then leave it to the chip to control the intensity, rather than adjusting the intensity by programming the CPU to send volume-control commands as notes played (something few developers bothered to attempt).

Photo of four children looking at a Commodore computer screen. That '80s Vibe: Millions of Commodore 64 home computers were sold. The distinctive sound of the computer's SID chip is prized by modern “chiptune" composers. Photo: Karl Staedele/Picture Alliance/Getty Images

The SID chip has three sound channels that can play simultaneously using three basic waveforms, plus a fourth “noise" waveform that produces rumbling to hissing static sounds, depending on the frequency. The chip has the ability to filter and modulate the channels to produce an even wider range of sounds. Some programmers discovered they could tease the chip into doing things it was never designed to do, such as speech synthesis. This was perhaps most famously used in Ghostbusters, a 1984 game based on the movie of the same name in which the C64 would utter low-fidelity catchphrases from the movie, such as “He slimed me!"

But stunts like speech synthesis aside, the SID chip's design meant that home computer games could have truly musical soundtracks. Developers started hiring composers to create original works for C64 games—indeed, some titles today are solely remembered because of a catchy soundtrack.

Unlike in modern game development, in which soundtrack creation is technically similar to conventional music recording (up to, and including, using orchestras and choirs), these early composers had to be familiar with how the SID chip was programmed at the hardware level, as well as its behavioral quirks. (Because the chip got to market so quickly, MOS's documentation of the 6581 SID was notoriously lousy, with Yannes acknowledging to Spectrum in 1985 that “the spec sheet got distributed and copied and rewritten by various people until it made practically no sense anymore.")

At the time, these composers were generally unknown outside the games industry. Many of them moved on to other things after the home computer boom faded and their peculiar hybrid combination of musical and programming expertise was less in demand. In more recent years however, some of them have been celebrated, such as the prolific Ben Daglish, who composed the music for dozens of popular games.

Daglish (who created my favorite C64 soundtrack, for 1987's Re-Bounder) was initially bemused that people in the 21st century were still interested in music created for, and by, the SID chip, but he became a popular guest at retrocomputing and so-called chiptunes events before his untimely death in late 2018.

Chiptunes (also known as bitpop) is a genre of original music that leans into the distinctive sound of 1980s computer sound chips. Some composers use modern synthesizers programmed to replicate that sound, but others like to use the original hardware, especially the SID chips (with or without the surrounding C64 system). Because the 6581 SID hasn't been in production for many years, this has resulted in a brisk aftermarket for old chips—and one that's big enough that crooks have made fake chips, or reconditioned dead chips, to sell to enthusiasts. Other people have created modern drop-in replacements for the SID chip, such as the SwinSID.

There are several options if you'd like to listen to a classic C64 game soundtrack or a modern chiptune without investing in hardware. You can find many on YouTube, and projects like SOASC= are dedicated to playing tunes on original SID chips and recording the output using modern audio formats. But for a good balance between modern convenience and hard-core authenticity, I'd recommend using a player like Sidplay, which emulates the SID chip and can play music data extracted from original software code. Even after the last SID chip finally burns out, its sound will live on.

An abridged version of this article appears in the July 2019 print issue as “Chip Hall of Fame: SID 6581."

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The HX-63 cipher machine

The HX-63 cipher machine is an electromechanical, rotor-based system designed and built by Crypto AG.
The machine uses nine rotors [center right] to encrypt messages. A dual paper-tape printer is at the upper left.


Growing up in New York City, I always wanted to be a spy. But when I graduated from college in January 1968, the Cold War and Vietnam War were raging, and spying seemed like a risky career choice. So I became an electrical engineer, working on real-time spectrum analyzers for a U.S. defense contractor.

In 1976, during a visit to the Polish Army Museum in Warsaw, I saw an Enigma, the famous German World War II cipher machine. I was fascinated. Some years later, I had the good fortune of visiting the huge headquarters of the cipher machine company Crypto AG (CAG), in Steinhausen, Switzerland, and befriending a high-level cryptographer there. My friend gave me an internal history of the company written by its founder, Boris Hagelin. It mentioned a 1963 cipher machine, the HX-63.

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