Sunday, 31 May 2020


The Commodore 64 is generally agreed to have the best sound of any 8-bit computer released in the 1980s, and its Sound Interface Device (SID) chip is one of the most distinctive sound chips made for any computer. This is borne out through games like “Last Ninja 2,” “Bubble Bobble” and Elite’s “Commando,” and through the sampling of speech found in games like the “Ghostbusters” and “Impossible Mission”: “Another visitor. Stay a while... stay forever!” 

The C64 is sought out as a music synthesiser in its own right, and the SID chip is default sound that an 8-bit computer makes – considering Commodore sold seventeen million of them before they went out of business in 1994, that isn’t much of a surprise. What makes that sound so distinctive, and the SID chip so versatile, tends not to be questioned as a result.

In its simplest form, an analogue synthesiser, made by companies like Moog, Yamaha and Korg, creates a sound using a wave generated by an oscillator, which is then filtered and shaped to create different effects. The sound chips found in most 8-bit computers do not contain oscillators, relying instead on having an oscillating signal fed into it from elsewhere. For this reason, while you can still create different sounds by programming the attack, decay, sustain and release of the sound – known as ADSR, or the sound envelope - the sound itself is a uniform pulse wave, known as a “square wave” because of how it looks on an oscilloscope. Square waves are responsible for the stereotypical “beep boop” sound of an old computer.

Most 8-bit computer companies used one of two off-the-shelf sound chips, making them all sound similar: one chip was the Texas Instruments SN76489, as used in their own TI-99/4A, the BBC Micro, Tandy 1000, the Sega Master System, Game Gear and Mega Drive; the other chip is the General Instrument AY-3-8910, as used in some Sinclair ZX Spectrum models, the Amstrad CPC range, the MSX computer standard, and the original 16-bit Atari ST. Both chips have four sound channels: one to generate white noise, which you could use for rhythm, and three square wave tone generators. 

Meanwhile, Commodore designed and made their own processor chips, including for sound. The SID chip’s designer, Robert Yannes, was a musical hobbyist who wanted to create a high-quality instrument within a computer. However, some of the most sought-after characteristics of the chip came as a result of meeting a deadline. 

The SID chip had its own oscillator, but not only could it create a square wave signal, but it also could produce sawtooth and triangle waves as well as white noise, putting the chip among proper analogue synthesisers. Originally, this oscillator would have served as many as thirty-two channels, but time constraints led the work completed on the first channel to be copied, for a total of three channels, each with its own oscillator. Each channel can therefore be programmed to produce their own sound, using different wave forms, or they can be locked together. Each channel also had its own ring modulator, to produce additional effects, and their own separate ADSR controls built into the hardware, instead of attempting to replicate it through programming. Settings could also be switched almost instantaneously through programming, making it sound like you had more than three channels to play with. The SID chip also had its own filter to add in further effects. In other words, this was uncharted territory.

The C64 went on sale in 1982 at a price of $595. At the same time, the electronics shop Radio Shack sold, for $499, the Realistic Concertmate MG-1, an analogue synthesiser keyboard made for them by Moog. The MG-1 can only play one note at a time, with polyphony available as an effects option. The oscillators in the MG-1 were less versatile, assigned to certain effects that could be completed by any of the SID chip’s oscillators. The MG-1 has one overall ADSR envelope, when the SID chip has one for each channel. Radio Shack catalogues in 1983 still sold the MG-1 for $499 – the C64 could be found for $199 by the end of that year.

Adventurous programming of the C64 led to the discovery that the SID chip could be made to reproduce samples, often used for recordings of speech. Exploiting imperfections in the manufacture of the chip, and a bias in the sound output, you could create an effect that could effectively be used as a fourth sound channel, although one that was used sparingly due to the intensive processing power required. This is why the original SID chip, code number 6581, is more highly-prized: when the cost-reduced Commodore 64C was released in 1986, the new SID chip (8580) had been refined, making the effect quieter. Modern recreations of C64 hardware usually have to accommodate for which version of the chip people wish to use.

I had originally looked at all of this when The C64, a full-sized emulator of the original 1982 machine, was released in 2019. When faced with the decision of learning Microsoft Basic 2.0 to program the virtual SID chip, or learning my scales, I opted for a Yamaha synthesiser instead. While you can replicate an 8-bit computer sound using an analogue synthesiser, or a digital recreation of one, there is a certain timbre to the sound of a SID chip that means you need to seek out the real thing – many people do, judging by the £40-50 price commanded on eBay for just the chip, let alone an entire C64.

After leaving Commodore, Robert Yannes set up his own keyboard company, Ensoniq – the digital oscillator chip from its Mirage sampling synthesiser, which contained the thirty-two sound channels Yannes originally wanted in the SID chip, made its way into the Apple IIGS computer, causing The Beatles’ record label Apple Corps to sue Apple Computer for breaking their legal agreement not to enter the music business.

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