My housemate and I did this for Tandy Color Computer cassettes a while back. If I recall correctly, a binary 0 was a certain duration of 1200Hz sine waves, and

My housemate and I did this for Tandy Color Computer cassettes a while back. If I recall correctly, a binary 0 was a certain duration of 1200Hz sine waves, and a binary 1 was the same duration of 2400Hz sine waves. The circuitry counted zero crossings over time, which made it fairly resilient to noise. And yes, plugging the cassette aux in cable to the headphone jack of a Macbook and transferring code through it was really fun. :)