RLL encoding
Squeezed another half again out of 1980s hard disks by controlling how many zeros could sit between flux transitions.
RLL (run-length limited) generalises the idea behind MFM: instead of a fixed rule per bit, groups of bits are translated into patterns where the number of zeros between two ones is guaranteed to stay within a range. The most common variant, RLL (2,7), allowed at least two and at most seven zeros in a row. Fewer needless transitions meant more bits fit — typically 50 % more than MFM on the same hard disk.
A 20 MB ST-506 drive became 30 MB just by swapping the controller, provided the media and heads were clean enough for the tighter transitions. RLL broke through commercially around 1987 and survives in advanced form (PRML) in every modern hard drive.