Eight-to-fourteen modulation (EFM) is a data encoding technique – formally, a ''

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained ...

'' – used by

compact disc The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in O ...

s (CD),

laserdisc The LaserDisc (LD) is a home video format and the first commercial optical disc storage medium, initially licensed, sold and marketed as MCA DiscoVision (also known simply as "DiscoVision") in the United States in 1978. Its diameter typical ...

s (LD) and pre- Hi-MD

MiniDisc MiniDisc (MD) is an erasable magneto-optical disc-based data storage format offering a capacity of 60, 74, and later, 80 minutes of digitized audio. Sony announced the MiniDisc in September 1992 and released it in November of that year fo ...

s. EFMPlus is a related code, used in DVDs and

Super Audio CD Super Audio CD (SACD) is an optical disc format for audio storage introduced in 1999. It was developed jointly by Sony and Philips Electronics and intended to be the successor to the Compact Disc (CD) format. The SACD format allows multiple a ...

s (SACDs). EFM and EFMPlus were both invented by Kees A. Schouhamer Immink. According to

European Patent Office The European Patent Office (EPO) is one of the two organs of the European Patent Organisation (EPOrg), the other being the Administrative Council. The EPO acts as executive body for the organisation

former President Benoît Battistelli, "Immink's invention of EFM made a decisive contribution to the digital revolution."

Technological classification

EFM belongs to the class of DC-free

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. RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m'' ...

(RLL) codes; these have the following two properties: * the spectrum ( power density function) 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 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 eight-bit blocks (bytes). Each eight-bit block is translated into a corresponding fourteen-bit codeword using a

lookup table In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v w ...

. 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 In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the '' modulus'' of the operation). Given two positive numbers and , modulo (often abbreviated as ) is ...

-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 two consecutive zeroes between two ones, then the written sequence will have three 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 two zeroes between every two 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 serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...

in the player. EFM requires three merging bits between adjacent fourteen-bit codewords. Although they are not needed for decoding, they ensure that consecutive codewords can be concatenated without violating the specified minimum and maximum runlength constraint. They are also selected to maintain

DC balance In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...

of the encoded sequence. Thus, in the final analysis, seventeen bits of disc space are needed to encode eight bits of data.

EFMPlus

EFMPlus, EFMPlus Patent, applied in DVD, DVD±RW,

SACD Super Audio CD (SACD) is an optical disc format for audio storage introduced in 1999. It was developed jointly by Sony and Philips Electronics and intended to be the successor to the Compact Disc (CD) format. The SACD format allows multiple au ...

is the channel code used in DVDs and

s. The EFMPlus encoder is based on a

deterministic finite automaton In the theory of computation, a branch of theoretical computer science, a deterministic finite automaton (DFA)—also known as deterministic finite acceptor (DFA), deterministic finite-state machine (DFSM), or deterministic finite-state autom ...

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, 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.

Technological classification

How it works

EFMPlus

References

External links