Watch a Vintage SWTPC Become a Synthesizer

Even if you aren't a musician or fan of electronic music, the history of synthesizers is quite interesting if you're into retrocomputing. The earliest synthesizers used dedicated hardware to generate waves at different frequencies. But in the late '70s and early '80s, we started seeing personal computers that could run synthesizer software to generate those frequencies through purpose-built chips. One of the first popular examples was the Yamaha CX5M, which included the SFG-01 frequency modulation synthesizer chip. But Sam Battle's vintage SWTPC didn't include any such chip, which forced him to get creative to give that computer synthesis capability.

Unlike modern computers, vintage computers had few layers of abstraction between the user interface and the "bare metal." That meant that users could do things like "poke" a value into a specific memory address. That was a fundamental technique in computer programming before more abstracted languages came along that could manage memory themselves without direct instruction. Vintage computers also tended to have accessible buses for peripherals. By poking values, the user could share data through the bus with that peripheral. This is how Battle was able to get his SWTPC to act as a synthesizer, even though it didn't come with any such hardware of its own.

Computers work entirely with digital signals (ones and zeros). That means that they best they can do is output a square wave — switching back and forth between on and off at a specific frequency. But a DAC (digital-to-analog converter) can accept a digital signal and produce a corresponding analog signal, resulting in a much smoother and more natural sounding wave. The goal of this project was to connect a modern DAC to the vintage SWTPC.

Battle achieved that by using the aforementioned bus poking technique. The DAC connects to the SWTPC's bus, which would normally be used for memory. When the user pokes a value to that "memory" address, it goes to the DAC. The poked value sets the analog output of the DAC, which has four channels of its own selectable through the bus.

On the software side, Battle used Robert Pafford's SWTPCSequencer. That should look familiar to musicians, though it is quite rudimentary in order to work with the modest processing power. The sequencer software handles all of the hard work of poking different values to the bus, so the user only has to worry about composition.