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Eight-to-fourteen modulation
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== Technological classification == EFM<ref>{{US patent|4,501,000}}, EFM Patent, applied in [[Compact Disc]], [[CD-R]], [[MiniDisc]].</ref> belongs to the class of [[DC bias|DC]]-free [[run-length limited]] (RLL) codes; these have the following two properties: * the spectrum ([[Spectral density|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.<ref>{{Cite journal |journal=Proceedings of the IEEE |volume=78 |issue=11 |date=December 1990 |title=Runlength-Limited Sequences |author=Kees Schouhamer Immink |author-link=Kees Schouhamer Immink |url=https://www.researchgate.net/publication/2984369 |pages=1745β1759 |quote=A detailed description is furnished of the limiting properties of runlength limited sequences.|doi=10.1109/5.63306 }}</ref><ref>{{cite book |url=https://archive.org/details/codesformassdata0000scho |title=Codes for Mass Data Storage Systems |edition=Second fully revised |publisher=Shannon Foundation Publishers |location=Eindhoven, The Netherlands |date=November 2004 |isbn=90-74249-27-2 |author=Kees A. Schouhamer Immink |author-link=Kees Schouhamer Immink |access-date=2015-08-23 |url-access=registration }}</ref> In optical recording systems, [[Servomechanism|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.
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