I managed to get 8 octaves of tones out of a ZX-81 in my youth without any extra hardware. Bit banging the cassette port audio out and running that output into

I managed to get 8 octaves of tones out of a ZX-81 in my youth without any extra hardware. Bit banging the cassette port audio out and running that output into line in worked. Wasn't amazing and boring organ tone at best and would use a fair chunk of the CPU. But like most thinks was a case of fellow school friends boasting about their zx-spectrum with audio, one just don't take that sitting down.