How It Works
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 bits are encoded with NRZI encoding, or modulo-2 integration, so that 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. A sequence 0011 would be changed into 1101 or its inverse 0010 depending on the previous pit written. If there are 2 zeroes between 2 consecutive ones, then the written sequence will have 3 consecutive zeros (or ones), for example, 010010 will translate into 100011 (or 011100). The EFM sequence 000100010010000100 will translate into 111000011100000111 (or its inverse).
Because EFM ensures there are at least 2 zeroes between every 2 ones, it is guaranteed that every pit and land is at least three bit clock cycles long. This property is very useful since it 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 three merging bits between adjacent 14-bit codewords to ensure that consecutive codewords can be cascaded without violating the specified minimum and maximum runlength constraint. The 3 merging bits are also used to shape the spectrum of the encoded sequence. Thus, in the final analysis, 17 bits of disc space are needed to encode 8 bits of data.
Read more about this topic: Eight-to-fourteen Modulation
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