Eight-to-Fourteen Modulation
Eight-to-Fourteen Modulation (EFM) is an encoding technique used in the mastering and burning of CDs,
CD-ROMs, and MiniDiscs. EFM belongs to the class of dc-free runlength limited (RLL) codes because it ensures
that a) the spectrum (power density function) of the encoded sequence vanishes at the low-frquency end
and b) both the minimum and maximum number of consective bits of the same kind are within specified bounds. In optical recording systems, servo systems accurately follow the track in three dimensions,
radial, focus, and rotational speed. Everyday handling damage, such as dust, fingerprints, and tiny scratches,
not only affects retrieved data, but also disrupts the servo functions. In worst cases, the servos may skip tracks or
get stuck. A product with such devastating weaknesses could not be successfully marketed and would remain a
laboratory toy. Specific sequences of pits and lands are particularly susceptible to disc defects, and
disc playability can be improved if such sequences are barred from recording. EFM, which is highly resilient to disc
handling, solves the engineering requirements in a very efficient manner. Under EFM rules, the data to be stored is first broken into 8-bit blocks (bytes).
Each 8-bit block is translated into a corresponding 14-bit codeword using a
lookup table.
The 14-bit words 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 worst-case clock recovery in the player. EFM requires 3 merging bits between
each 14 bit codewords to ensure that consecutive codewords can be cascaded without violating the specified minimum and
maximum runlength constraint. The three merging bits are also used to shape the spectrum of the encoded sequence. EFMPlus is the channel code used in DVDs.
EFMPlus 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, increasing storage capacity by 7%. EFM and EFPlus were both invented by Kees A. Schouhamer Immink.
See also
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