EFMPlus

Lore & Background

EFMPlus is based on a deterministic finite automaton having four states, which translates eight-bit input words into sixteen-bit codewords. The binary sequence generated by the finite state machine encoder has at least two and at most ten zeros between consecutive ones, the same constraint as in classic EFM. Unlike classic EFM, there are no packing (merging) bits required between codewords.

EFMPlus effectively reduces storage requirements by one channel bit per user byte, increasing storage capacity by 1/16, or 6.25%. Decoding of EFMPlus-generated sequences is accomplished by a sliding-block decoder of length two, meaning two consecutive codewords are required to uniquely reconstitute the sequence of input words.

The code was invented by Kees A. Schouhamer Immink, whose earlier EFM code was described by European Patent Office former President Benoît Battistelli as having made 'a decisive contribution to the digital revolution.'

Reader's Guide

EFMPlus is significant as the channel code adopted for DVDs and Super Audio CDs, two major optical disc formats. By eliminating the merging bits required in classic EFM, EFMPlus increases storage capacity by 6.25% per user byte, a meaningful improvement for high-density storage. The code maintains the same run-length constraints as EFM—at least two and at most ten zeros between consecutive ones—ensuring compatibility with servo mechanisms that require specific pit and land lengths for reliable tracking and clock recovery. The use of a four-state finite automaton for encoding and a sliding-block decoder for decoding represents a practical application of formal language theory to consumer electronics. EFMPlus contributed to the engineering success of DVDs and SACDs, which became widespread optical storage media. Its invention by Kees A. Schouhamer Immink continued his work on DC-free run-length limited codes, which are essential for optical recording systems that must resist handling damage such as dust, fingerprints, and scratches.

Did You Know?

• EFMPlus is a channel code used in DVDs and Super Audio CDs (SACDs).
• EFMPlus was invented by Kees A. Schouhamer Immink.
• EFMPlus reduces storage requirements by one channel bit per user byte, increasing storage capacity by 6.25%.
• Decoding of EFMPlus sequences requires a sliding-block decoder of length two.