Eight bit to Fourteen bit Modulation code
EFM is the modulation code used in all CD recording. This low-level and very critical channel coding technique maximizes pit sizes on the disc by reducing frequent transitions from 0 to 1 or 1 to 0. CD employs pulse width modulation, representing 1's as Land-pit transitions along the track. The 8/14 code maps 8 user data bits into 14 channel bits. In the 1982 compact disc standard (IEC 908 standard), 3 merge bits are added to the 14 bit block to further eliminate 1-0 or 0-1 transitions between adjacent 8/14 blocks.