EFM (eight-to-fourteen modulation)

A CD does not store bits directly. In the late 1970s Kees Schouhamer Immink at Philips designed EFM: each eight-bit byte is looked up in a table and replaced by a fourteen-bit pattern, plus three merging bits. The rules guarantee at least two and at most ten zeros between ones, so pits and lands are never so short the laser misses them nor so long the clock loses sync — the same kind of run-length constraint that RLL provides magnetically.

Without EFM a CD's density and error tolerance would not have coexisted. MiniDisc and later discs built on the idea.