My first computer-synth project in 2000 was for the Radio Shack TRS-80 Model III
computer. This computer actually has no formal sound capability, but it
does have a cassette port to read and write data to a data cassette. If you alternate
writing a binary 1 and binary 0 to the data
cassette quickly, it will generate a tone.
I wrote a program using a Microsoft Z80 assembler
that generates tones and chooses the frequency based on which key is pressed on
the keyboard. Not all the tones fall on a normal scale, but there are enough
for it to barely be a useful instrument.
Sample of direct audio output from cassette port:
Song by my band Science Club from 2001 using the Trsynth: