FM encoding
... that early floppy disks used FM encoding that took up only half the available storage?
... that early <a href="./Floppy_disk" id="mwBew" rel="mw:WikiLink" title="Floppy disk">floppy disks</a> used <b id="mwBe0"><a href="./Frequency_modulation_encoding" id="mwBe4" rel="mw:WikiLink" title="Frequency modulation encoding">FM encoding</a></b> that took up only half the available storage?