"The typical encoding method for computer data was simple FSK which resulted in typical data rates of 500 to 2000 bit/s, although some games used special faster loading routines, up to around 4000-bit/s. A rate of 2000-bit/s equates to a capacity of around 660 kilobytes per side of a 90-minute tape." http://en.wikipedia.org/wiki/Compact_Cassette#Data_recording