Search results
Results from the WOW.Com Content Network
Modified frequency modulation (MFM) is a run-length limited (RLL) line code [1] used to encode data on most floppy disks and some hard disk drives.It was first introduced on hard disks in 1970 with the IBM 3330 and then in floppy disk drives beginning with the IBM 53FD in 1976.
Frequency modulation encoding, or simply FM, is a method of storing data that saw widespread use in early floppy disk drives and hard disk drives. The data is modified using differential Manchester encoding when written to allow clock recovery to address timing effects known as "jitter" seen on disk media.
A modified frequency modulation, or (1,3) RLL encoding, stores each data bit as two bits on tape, but since there is guaranteed to be one 0 (no flux reversal) bit between any 1 (flux reversal) bits, then it is possible to store 6400 encoded bits per inch on the tape, or 3200 data bits per inch.
Modified frequency modulation (MFM) controllers were the most common type in small computers, used for both floppy disk and hard disk drives. Run length limited (RLL) controllers used data compression to increase storage capacity by about 50%. Priam created a proprietary storage algorithm that could double the disk storage.
English: Graph illustrating encoding of byte 01011000 (in binary) using Frequency modulation (above), Modified frequency modulation (middle), and Run-length limited code (bottom). Date 25 November 2021
English: Graph illustrating encoding of byte 0xA1 (10100001 in binary) using Frequency modulation (above) and Modified frequency modulation (below). Individual bits being encoded are displayed at the top; blue dashed line (short dashes) indicates synchronization pulse; red one (long dashes), the pulse carrying the actual data (change of phase corresponds to "1" and lack thereof to "0").
Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, followed by RLL (1,3) MFM code which were widely used in hard disk drives until the mid-1980s and are still used in digital optical discs such as CD, DVD, MD, Hi-MD and Blu-ray using EFM and EFMPLus codes. [11]
MFM—Modified Frequency Modulation; MF—Medium Frequency; MGCP—Media Gateway Control Protocol; MHz—Megahertz; MIB—Management Information Base; MICR—Magnetic Ink Character Recognition or Magnetic Ink Character Reader; MIDI—Musical Instrument Digital Interface; MIMD—Multiple Instruction, Multiple Data; MIME—Multipurpose Internet ...