Turn a Commodore 64 Into a Bangin' Synth
Gavin Lyons took advantage of the Commodore 64's SID chip to create a synthesizer called the Cyanodore 64.
The Commodore 64 is probably the most iconic computer in American history and it is definitely the best-selling single model. If you're older than 40 or so, there is a very good chance that you used a Commodore 64 in your childhood. Like many kids of the era, you may have played games from the extensive C64 library. Many of those games included some great sound effects thanks to the SID (Sound Interface Device) chip. Today, you can take advantage of that chip to turn a Commodore 64 into a bangin' synth called the Cyanodore 64.
The Commodore 64's SID was way ahead of its time and worked using subtractive synthesis. It could generate four different kinds of waves (sawtooth, triangle, rectangle pulse-width modulation, and "noise") through three different tone generators between 0-4kHz.
Like every computer of the era, controlling sound was a very low-level endeavor that involved poking specific memory addresses. It would be difficult to produce music that way, so Gavin Lyons provides recommendations for different synthesizer, sequencer, and drum machine software options. Lyons also provides thorough instructions on how to get that software running on a C64 and how to get audio output that you can pass along to the rest of your system for amplification or recording.
First, you'll want to connect a display so you can see the software. The C64 has composite video output, so you'll want either a vintage TV/monitor or something like a modern in-car LCD that accepts composite video. Next, you need a way to load the software onto your C64 and Lyons recommends a SD2IEC converter, which uses an SD card and acts like a disk drive. That is much easier than most other era-authentic methods.
Finally, you'll want a way to get clean audio output. That will require some modification to the C64 mainboard. If you want to keep your machine authentic, you can use the lower-quality audio from the A/V port.
Optionally, you can also add a MIDI cartridge. That will let you connect external MIDI controllers so you don't have to do everything through the C64 keyboard. That isn't necessary to use your C64 as a synthesizer, but it can make the process a lot easier.
With everything connected, it is as simple as using the software to produce your music. The software will be far more rudimentary than anything modern, but it will be perfectly usable and will give you a very unique sound.