Eight-to-Fourteen Modulation
Encyclopedia
Eight-to-fourteen modulation (EFM) is a data encoding
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....

 technique – formally, a channel code
Channel code
In digital communications, a channel code is a broadly used term mostly referring to the forward error correction code and bit interleaving in communication and storage where the communication media or storage media is viewed as a channel...

– used by compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

s (CD) and pre-Hi-MD
Hi-MD
In January 2004, Sony announced the Hi-MD media storage format as a further development of the MiniDisc format. With its release in later 2004 came the ability to use newly-developed, high-capacity 1 gigabyte Hi-MD discs, sporting the same dimensions as regular MiniDiscs.- Main features :* The...

 MiniDisc
MiniDisc
The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5" floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable or premastered. Recordable MiniDiscs use a magneto-optical system to record...

s. EFMPlus is a related code, used in DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

s and SACD
Super Audio CD
Super Audio CD is a high-resolution, read-only optical disc for audio storage. Sony and Philips Electronics jointly developed the technology, and publicized it in 1999. It is designated as the Scarlet Book standard. Sony and Philips previously collaborated to define the Compact Disc standard...

s. EFM and EFMPlus were both invented by Kees A. Schouhamer Immink
Kees A. Schouhamer Immink
Kornelis Antonie Schouhamer Immink is a Dutch scientist, inventor, and entrepreneur, who pioneered and advanced the era of digital audio, video, and data recording including popular digital media such as Compact Disc, DVD and Blu-Ray Disc. He has been a prolific and influential engineer, who...

.

Technological classification

EFM belongs to the class of DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

-free run length limited
Run Length Limited
Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...

 (RLL) codes; these have the following two properties:
  • the spectrum (power density function
    Spectral density
    In statistical signal processing and physics, the spectral density, power spectral density , or energy spectral density , is a positive real function of a frequency variable associated with a stationary stochastic process, or a deterministic function of time, which has dimensions of power per hertz...

    ) of the encoded sequence vanishes at the low-frequency end and
  • both the minimum and maximum number of consecutive bits of the same kind are within specified bounds.


In optical recording systems, servo mechanisms
Servomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...

 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 some cases, the servos may skip tracks or get stuck. 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. The use of EFM produces a disc that is highly resilient to handling and solves the engineering challenge in a very efficient manner.

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
Lookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...

.

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
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...

 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.

EFMPlus

EFMPlus is the channel code used in DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

s and SACD
Super Audio CD
Super Audio CD is a high-resolution, read-only optical disc for audio storage. Sony and Philips Electronics jointly developed the technology, and publicized it in 1999. It is designated as the Scarlet Book standard. Sony and Philips previously collaborated to define the Compact Disc standard...

s.

The EFMPlus encoder is based on a deterministic finite state machine
Deterministic finite state machine
In the theory of computation and automata theory, a deterministic finite state machine—also known as deterministic finite automaton —is a finite state machine accepting finite strings of symbols. For each state, there is a transition arrow leading out to a next state for each symbol...

 having four states, which translates 8-bit input words into 16-bit codewords. The binary sequence generated by the finite state machine encoder has at least two and at most ten zeros between consecutive ones, which is the same as in classic EFM. There are no packing (merging) bits as in classic EFM.

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

The adoption of EFMPlus modulation in DVDs, instead of the earlier modulation used in the prototype Super Density Disc (SD), is why DVDs have 4.7 GB of storage, rather than the projected 5 GB – EFMPlus is 6% less efficient than the modulation originally used in SD discs, though it was chosen due to improved resistance to disc damage.

External links

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