It has been a long time since we stored software and computer data on audiotape. But it used to be the de facto standard for hobby computers and [Noel] has a great video about the Amstrad’s system (embedded below) which was pretty typical and how the process could be sped up since today, you have perfect audio reproduction, especially compared to consumer-grade audiotape.
The cassette tapes suffered from several problems. The tape had an inherently low bandwidth, there was quite a bit of noise present from the analog circuitry and heads, and the transport speed wasn’t necessarily constant. However, you can easily digitally synthesize relatively noise-free sound at high fidelity and rock-solid frequency. So basically a microcontroller, like an Arduino, can look like an extremely high-quality tape drive.
The idea is similar to a modem except instead of a computer talking to another computer over a phone line, it records it and listens to it again, later. The normal tape system could only get about 2 kbps. How fast can you get if you are generating a nearly perfect digital signal? Watch the video, and find out.