The channel coding system Eight-to-Fourteen Modulation (EFM) is a coding technique used in DVD, CD and Minidiscs. EFM belongs to the type of limited-length free codes of continuous component, since they ensure, among other things that:
In optical recording systems, servo-controlled systems accurately track the track in three dimensions, the radial dimension, the focus, and the rotation speed. Everyday damages, such as those caused by dust, fingerprints, small scratches and others, not only affect the recovered data, but also harm the servo functions.
In the worst case, the servos can skip the track or get blocked. Some specific sequences of ridges and valleys are especially susceptible to disc defects, and the possibility of reproducing a disc correctly can be improved if these sequences are excluded when recording. EFM, which is very resistant to everyday handling, meets the design requirements very efficiently.
Following the rules of EFM, the data to be stored is cut first in blocks of 8 bits. Each block of 8 bits is translated into its corresponding 14-bit code word, chosen so that the binary data is separated with a minimum of two and a maximum of ten binary zeros. This is because a binary one is stored on the disk as a change from a crest to a valley, while a binary zero is recorded without making changes. Since EFM ensures that there are at least two zeros between every two ones, it also ensures that each peak and each valley contains at least three clock cycles. This reduces the requirements of the optical pickup system used in the reproduction mechanism. The maximum of ten consecutive zeros ensures the recovery of the clock signal in the worst case.
EFMPlus is the channel code used on DVDs and SACDs. The EFMPlus encoder is based on a deterministic finite state machine that has four states. This machine translates the 8-bit code words into 16-bit code words. The binary sequence generated by the finite-state machine has at least two and at most ten zeros between consecutive ones, just like the classical EFM. In this way, we effectively reduce the storage requirements of one channel bit per user byte, increasing the storage capacity by 17/16 = 7%. The decoding of the EFMPlus sequences is achieved by means of a sliding block decoder of length 2, that is, two consecutive codewords are necessary to reconstruct the sequence of input words.
EFM and EFMPlus have been invented by Kees A. Schouhamer Immink.
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