Eight-to-fourteen modulation

Lore & Background

EFM belongs to the class of DC-free run-length limited (RLL) codes, which have two properties: the spectrum of the encoded sequence vanishes at low frequencies, and both the minimum and maximum number of consecutive bits of the same kind are within specified bounds. In optical recording systems, servo mechanisms accurately follow the track in three dimensions: radial, focus, and rotational speed. Everyday handling damage such as dust, fingerprints, and tiny scratches can disrupt servo functions, causing skipped tracks or stuck mechanisms. The use of EFM produces a disc highly resilient to handling and solves this engineering challenge efficiently.

Reader's Guide

Under EFM rules, data is first broken into eight-bit blocks (bytes), each translated into a corresponding fourteen-bit codeword using a lookup table. The 14-bit words ensure binary ones are always separated by a minimum of two and a maximum of ten binary zeros. Bits are encoded with NRZI encoding, so a binary one is stored as a change from land to pit or pit to land, while a binary zero indicates no change. This guarantees every pit and land is at least three bit-clock cycles long, reducing demands on the optical pickup, and the ten-zero maximum ensures worst-case clock recovery. Three merging bits are required between adjacent codewords to maintain runlength constraints and DC balance, meaning seventeen bits of disc space encode eight bits of data. EFMPlus, used in DVDs and SACDs, uses a deterministic finite automaton with four states to translate eight-bit words into sixteen-bit codewords, with no merging bits, increasing storage capacity by 6.25%. Decoding EFMPlus requires a sliding-block decoder of length two.

Did You Know?

• EFM ensures at least two zeros between every two ones, making every pit and land at least three bit-clock cycles long.
• EFMPlus reduces storage requirements by one channel bit per user byte compared to classic EFM.
• EFMPlus decoding requires two consecutive codewords to uniquely reconstitute the input sequence.
• EFM and EFMPlus were both invented by Kees A. Schouhamer Immink.