The article mentions modulation techniques but doesn't really explain the one actually used by these classic computers: pretty much they all used FSK- frequency

The article mentions modulation techniques but doesn't really explain the one actually used by these classic computers: pretty much they all used FSK- frequency shift keying. One tone is a 1 and another tone is a 0. The playback usually has a limiter such that the software gets no amplitude information. It really just gets zero-crossings, and has to time successive crossings to determine the frequency. The cassette recorders used for this were basically the cheapest, lowest fidelity ones. Their frequency response was around 200 - 8000 Hz. Even so, this was good enough for 1200 - 2400 Baud FSK. It's interesting to consider what data rate you could get from these tape recorders using the best possible techniques available in the 90s- QAM from modems, or even OFDM used in Telebit Trailblazer modems (now used in LTE cell phones). Of course the problem is that the modem would have more CPU power than the home computer itself..