I made this with a tape recorder this morning.

I will not be able to verify that it’s working 100% until I get the cassette read card built, but it sounds a lot like the sound of an original SCELBI tape that Mark Arnold sent me a little while back.

The format is substantially different than the format WOZ used later on for his Apple computers. The SCELBI actually uses the ones and zeros to switch output between two frequencies, instead of directly generating the waveform in software. This is very similar to how RTTY AFSK (audio frequency shift keying) is done. Though I don’t have the audio format entirely figured out, it appears substantially more complex than the Apple format, with each nibble being sent as a sort of independant packet.