Eight-to-Fourteen Modulation


Eight-to-Fourteen Modulation - Definition

Eight-to-Fourteen Modulation (EFM) is an encoding technique used in the mastering of both CDs and MiniDiscs.

The data to be stored is first broken into 8-bit blocks. Each block is substituted by a corresponding 14-bit code, using a straightforward lookup table (http://www.physics.udel.edu/wwwusers/watson/scen103/efm.html). The 14-bit values are chosen such that binary ones are always separated by a minimum of two and a maximum of ten binary zeroes. This is because a binary one is stored on the disc as a change from a land to a pit or a pit to a land, while a binary zero is indicated by no change. Because EFM ensures there are at least two zeroes between every two ones, it also ensures that every pit and land is at least three clock cycles long. This reduces the demands on the optical pickup used in the playback mechanism. The ten consecutive-zero maximum ensures the tracking mechanism always has changing data to track.

EFM+ is the system used in DVDs. Standard EFM requires 3 packing bits between each 14 bit code to ensure that bits from the end of one code and the beginning of the next cannot be interpreted as a valid code. EFM+ uses reworked codes that are now 16 bits in length to prevent this problem, and no packing bits. This effectively reduces storage requirements by one bit per byte, slightly increasing storage capacity.

See also