The loading routines from that era just count the number of DC zero-crossings and convert X crossings into a 0 and Y crossings into a 1, they don't care about f

The loading routines from that era just count the number of DC zero-crossings and convert X crossings into a 0 and Y crossings into a 1, they don't care about frequency or amplitude (the raw signal is usually fed into a Schmitt trigger to implement hysteresis for a stable edge). That way, the polarity of the tape signal and the variation of the motor is compensated for.