Editing Modified frequency modulation (section)

==Magnetic storage==
Magnetic storage devices, like [[hard drive]]s and [[magnetic tape]], store data not as absolute values, but in the changes in polarity. This is because a changing magnetic field will induce an electrical current in a nearby wire, and vice versa. By sending a series of changing currents to the [[read/write head]] while the media moves past it, the result will be a pattern of magnetic polarities on the media that change where the data was a "1". The exact nature of the media determines how many of these changes can occur within a given surface area, and when this is combined with the nominal speed of movement, it produces the maximum data rate for that system.

Disk drives are subject to a variety of mechanical and materials effects that cause the original pattern of data to "jitter" in time. MFM as a run-length limited code limits the distance between recorded transitions so the jitter does not cause a transition to be misaligned in time, thereby causing a data error. Other limitations defined by the media place additional constraints on the way the data is recorded. A diverse range of suitable encodings, known generally as [[line code]]s, have been developed for this purpose. Their suitability depends on the media or transmission mechanism being used.