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Topic: Modified Frequency Modulation

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	What is MFM? Modified Frequency Modulation Encoding Digital (Site not responding. Last check: 2007-09-11)
		The name "frequency modulation" can be seen in the patterns that are created if you look at the encoding pattern of a stream of ones or zeros.
		The replacement for FM is modified frequency modulation, or MFM.
		MFM encoding was used on the earliest hard disks, and also on floppy disks.
	bugclub.org /beginners/hardware/mfm.html (393 words)

	Frequency modulation - Encyclopedia, History, Geography and Biography
		Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave.
		Problematically, however, frequency drift or lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel.
		Note that frequency modulation can be regarded as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal.
	www.arikah.com /encyclopedia/FM (1137 words)

	Phase-locked loops - Patent 4069462
		It is desirable to generate a clock signal having a frequency which is an integral multiple of the frequencies assumed by the input signal, and such that the input signal is also always maintained in phase with the input signal no matter which one of the plurality of frequencies such input signal assumes.
		Such frequency lock-in operation is obtained by supplying the servo clock signal as the reference input signal to frequency detector 18, the other input to which is in effect an output signal derived from the voltage controlled oscillator.
		Accordingly, the VCO frequency is adjusted until the frequency of the input signals to frequency detector 19 are locked in synchronism.
	www.freepatentsonline.com /4069462.html (3195 words)

	ANSDIT - The letter "F" (Site not responding. Last check: 2007-09-11)
		A parameter, defined in the declaration of certain modules, that is associated with an actual parameter in a call or generic instantiation.
		The form of angle modulation in which the instantaneous frequency of a sine wave carrier is caused to depart from the carrier frequency by an amount proportional to the instantaneous value of the modulating signal.
		Modulation in which a modulating digital signal varies the frequency of the output signal among a fixed number of predetermined values.
	www.ncits.org /tc_home/k5htm/f2.htm (2519 words)

	ISDL - Resources - Modulation Toolbox
		The ISDL modulation toolbox is a set of Matlab files that enables you to perform modulation spectral analysis of speech and other sounds.
		The low modulation frequency content of the lowest frequency is displayed in the lower left corner of the axes.
		Most of the energy of this signal is in a single coefficient in the modulation spectrogram, at acoustic frequency 800 Hz and modulation frequency 3 Hz.
	www.ee.washington.edu /research/isdl/projects/modulationtoolbox (1542 words)

	Inside Computers: An In Depth Guide - By TQ Team C006208
		Frequency modulation was accomplished by using a flux followed by another data bit.
		MFM improved on the frequency modulation in that it added an extra flux only between 2 consecutive zeros, where frequency modulation would add one before ever piece of data.
		MFM almost cut the average usage down by half, but there were still problems in that the amount of data that a disk could hold would change depending on the binary patterns.
	library.thinkquest.org /C006208/data/device-harddrives4.php (730 words)

	Space Weather (Site not responding. Last check: 2007-09-11)
		If a modulation is made of the radio wave that exactly reproduces the amplitude and frequency characteristics of the original sound wave, then sound can be transmitted rapidly over long distances.
		In FM (frequency modulation) radio transmissions, the frequency of the combined waves changes to reproduce the audio signal.
		For example, higher frequency is associated with the peak amplitude in the audio wave.
	www.windows.ucar.edu /spaceweather/wave_modulation.html (326 words)

	Modified Frequency Modulation (MFM)
		A refinement of the FM encoding method is modified frequency modulation, or MFM.
		Since the average number of reversals per bit is half that of FM, the clock frequency of the encoding pattern can be doubled, allowing for approximately double the storage capacity of FM for the same areal density.
		Since the MFM method about doubles the capacity of floppy disks compared to earlier FM ones, these disks were called "double density".
	www.storagereview.com /guide2000/ref/hdd/geom/dataMFM.html (391 words)

	Micro House PC Hardware Library Volume I: Hard Disks: Hard Disk Drives
		Modified Frequency Modulation (MFM) encoding was devised to reduce the number of flux reversals used in the original FM encoding scheme and, therefore, to pack more data onto the disk.
		MFM is used in virtually all PC floppy drives today and was used in nearly all PC hard disks for a number of years.
		Because MFM encoding places twice as many data bits in the same number of flux reversals as FM, the clock speed of the data is doubled so that the drive actually sees the same number of total flux reversals as with FM.
	www.alasir.com /books/hards/001-002.html (778 words)

	Recording digital data on magnetic disks
		These pulses are generated by passing a frequency modulated (FM) current through the disk drive's magnetic head.
		The MFM method of encoding digital data onto a disk is more popular because it is more efficient and more reliable than straight FM encoding.
		MFM encoding still uses two pulse periods, but uses a lot fewer pulses to store the digital data onto the disk.
	www.tpub.com /neets/book23/103b.htm (1555 words)

	4,509,073b (Site not responding. Last check: 2007-09-11)
		Individual bits of each packet are conveyed as amplitude modulation levels of respective cycles of a carrier signal which unit 28 develops on cable 56 by dividing in half the frequency of the 8.056 Megahertz carrier signal developed by control processor 18.
		From the MFM encoded signal, the phase-locked loop develops the 8.056 Megahertz carrier signal, four signals including a 32.2 Megahertz signal, a 16.1 Megahertz signal, a 10.7 Megahertz signal and a 8.05 Megahertz signal, one of which is employed as the carrier signal to transmit ackets to head-end subsystem 12, and a 32.2 Megahertz signal.
		Each of the quadra-port memory modules is connected so as to be available to each of the processors on respective ones of the two (cable-interface module) local buses and to be available to each of a pair of processors on a respective one of a pair of buses local to system-control module 162.
	www.ictv.com /who_we_are/patents/4509073.html (4817 words)

	Modified Frequency Modulation - Wikipedia, the free encyclopedia
		Modified Frequency Modulation, commonly MFM, is a line code used by most floppy disk formats, notably by most CP/M machines, as well as PCs running DOS.
		MFM was also used in early hard disk designs, before the advent of more efficient types of Run Length Limited (RLL) coding.
		This is referred to as a "sync A1" because it decodes as hexadecimal A1, although it cannot appear in normal data.
	en.wikipedia.org /wiki/Modified_Frequency_Modulation (403 words)

	ITS, Hard Disk Drive Glossary of Terms
		The interface was later modified for hard drives and this modified version became the ST-506 interface.
		MFM ENCODING (Modified Frequency Modulation Encoding) - This is an encoding scheme was was an enhancement to FM.
		MFM does away with the need for timing signals thus it is more efficient than FM.
	www.i-t-s.com /corporate/terms.html (1113 words)

	Terms and Definitions
		A module or area of execution which is to be kept isolated from other domains; a domain may have special properties that define the nature of the isolation and the limits on communication to and from the domain.
		Modulation is converting signals from a binary-digit pattern [pulse form] to a continuous wave form.
		FM is superseded by MFM (modified frequency modulation) is an encoding method used in floppy disk drives and older hard drives.
	www-ece.engr.ucf.edu /~jza/glossary/terms.html (20644 words)

	glossary of computer terms
		MFM encoding is based on an earlier technique called frequency modulation encoding but improves on its efficiency by reducing the need for synchronizing information and by basing the magnetic coding of each bit on the status of the previously recorded bit.
		MFM encoding stores more information on a disk than does frequency modulation encoding and is used on many hard disks.
		Modules usually consist of two parts: an interface, which lists the constants, data types, variables, and routines that can be accessed by other modules or routines, and an implementation, which is private (accessible only to the module) and which contains the source code that actually implements the routines in the module.
	www.skeeterbytes.com /pc/pcglossary/m.htm (6007 words)

	Digital Magnetic Tape Recording
		More bandwidth-efficient forms of modulation which avoid a DC component that we will see later have been used for magnetic recording, but this form of modulation has found applications in fiber-optic communications.
		MFM is quite a simple code, and polarity transitions are quite frequent in it.
		Two ways of recording data on tape which had this desirable property were ZM (Zero Modulation), invented by Arvind Patel at IBM, and used in the IBM 3850 data storage system, and the code known as Miller Squared, used in the Ampex D2 digital videotape recording format.
	www.quadibloc.com /comp/tapeint.htm (4876 words)

	Glossary
		Software, complete with source code, that is distributed freely to users who are in turn free to use, modify, and distribute it, provided that all alterations are clearly marked and that the name and copyright notice of the original author are not deleted or modified in any way.
		Frequency is usually measured in hertz (Hz), with 1 Hz equaling 1 occurrence (cycle) per second.
		Frequency is also measured in kilohertz (kHz, or 1,000 Hz), megahertz (MHz, or 1,000 kHz), gigahertz (GHz, or 1,000 MHz), or terahertz (THz, or 1,000 GHz).
	www.gis.net /~sotis/galaxy/glossary/f.html (8175 words)

	Wikinfo \| Modulation
		Modulation describes a range of techniques for encoding information on a carrier signal, typically a sine-wave signal.
		When OFDM is used in conjunction with channel coding techniques, it is described as 'Coded orthogonal frequency division modulation' (COFDM).
		Modulation is frequently used in conjunction with various channel access methods.
	www.wikinfo.org /wiki.php?title=modulation (231 words)

	Tom's Computer Troubleshooting Tips
		When a drive reads data off a disk, the drive head is passed over a disk surface in which a magnetic flux has been previously placed, a current is generated in the head coils and the direction of the current flow depends on the direction of the flux.
		MFM and RLL are the types of encoding schemes used.
		MFM or Modified Frequency Modulation is a scheme in which a set pattern of bits always uses the same amount of space on a disk or it puts clocking information to the data that allows single bit errors to be detected.
	home.earthlink.net /~tomscomputertips/tc6.html (2239 words)

	RF Simulation Demo: Amplitude Modulation- Developer Zone - National Instruments
		Modulation is a process by which characteristics of a high-frequency carrier signal are altered to convey information contained in a lower-frequency message.
		Though it is theoretically possible to transmit baseband signals (or information) without modulating the data, it is far more efficient to send messages by modulating the information onto a carrier wave.
		High frequency waveforms require smaller antennae for reception, efficiently use available bandwidth, and are flexible enough to carry different types of data.
	zone.ni.com /devzone/cda/epd/p/id/5146 (730 words)

	Bit clock reproducing circuit - Patent 4385395
		However, in analog PLL type bit clock reproducing circuits, the free running frequency of the voltage controlled oscillator is astable due to variations of temperature and humidity and hence the phase locked state of the PLL is apt to be released.
		In this example, the frequency of the clock signal C.sub.0 is selected to be 16 times that of a bit clock which is to be provided, so that the counter 14 is of the 4 bits and hexadecimal type.
		As described above, according to the present invention a certain constant numerical value is not unconditionally loaded into the counter at the edge of the data as in the prior art, but the numerical value determined by the state of the output from the counter at that time is loaded into the counter.
	www.freepatentsonline.com /4385395.html (2642 words)

	Modified Frequency Modulation from FOLDOC (Site not responding. Last check: 2007-09-11)
		(MFM, Modified FM, or sometimes "Multiple Frequency Modulation") A modification to the original frequency modulation scheme for encoding data on magnetic disks.
		MFM allows more than 1 symbol per flux transition (up to 3), giving greater density of data.
		It is used with a data rate of between 250-500 kbit/s on industry standard 3.5" and 5.25" low and high density diskettes, and up to 5 Mbit/s on ST-506 hard disks.
	foldoc.org /?Modified+Frequency+Modulation (108 words)

	Personal Computer System Components
		Frequency Modulation (FM) encoding was used on the first floppy drives installed in PCs.
		MFM was designed to reduce the number of flux reversals used in the original FM encoding scheme, thereby allowing more data to be placed on the disk.
		MFM technology is still used for floppy disk drives but not hard drives.
	www.windowsitlibrary.com /Content/175/02/5.html (1599 words)

	MFM - Modified Frequency Modulation
		Abbreviation of modified frequency modulation, an encoding scheme used by PC floppy disk drives and older hard drives.
		Modified Frequency Modulation: the data storage system used by floppies and older, smaller hard drives.
		Every attempt has been made to provide you with the correct acronym for MFM.
	www.auditmypc.com /acronym/MFM.asp (147 words)

	Frequency Modulation (FM)
		This is a simple scheme, where a one is recorded as two consecutive flux reversals, and a zero is recorded as a flux reversal followed by no flux reversal.
		The name "frequency modulation" comes from the fact that the number of reversals is doubled for ones compared to that for zeros.
		FM was actually made obsolete by MFM before the IBM PC was introduced, but it provides the basis for understanding MFM.
	www.storagereview.com /guide2000/ref/hdd/geom/dataFM.html (442 words)

	History Of Frequency Modulation
		modulation had been investigated and rejected, because the mathematics said that...
		Modulation, was invented in the late 1960s by the composer John Chowning...
		modulation had been investigated and rejected, because the mathematics said that hertz...
	www.netactics.co.uk /history_of_frequency_modulation.html (225 words)

	Data Recovery and Hard Risk Drive Glossary of Terms
		Although the pin outs of this interface have changed slightly over the years it is still the standard for 5.25" and 3.5" floppy disk drives.
		This is an encoding scheme that reduces the amount of data-checking information that is stored and thus requires less flux reversals for a given amount of data.
		This is an advanced version of RLL that has a run length of zeros from 3 to 9 which even further increases disk space up to (100% over MFM).
	www.datarecoveryspecialist.com /glossaryofterms.htm (1827 words)

	Champions Technical Support Library's hardware concepts: MNO
		A repeater connects the internal bus to the external bus which is a simple, low-power synchronous DRAM-like interface that may have a frequency between 75 MHz abd 200 MHz; much wider than an SDRAM's.
		Modified Frequency Modulation is the standard for ST506/412-inter-faced drives.
		ST506 drives, using the MFM data-encoding, have the typical 17 sectors per track.
	home.earthlink.net /~champions/h_mno.htm (1576 words)

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