1042:
270:
36:
880:
643:, is similar to MFM, but suppresses additional clock bits, producing a longer maximum run length (a (1,4) RLL code). In particular, a clock pulse is only inserted between a pair of adjacent 0-bits if the first bit of the pair did not have a clock pulse inserted before it. In the example below, clock bits that would have been present in MFM are noted in bold:
517:
In FM encoding, the number of 0-bits that may appear between consecutive 1-bits is either 0 or 1. In MFM encoding there is a minimum of 1 zero bit between adjacent ones (there are never two adjacent one bits), and the maximum number of zeros in a row is 3. Thus, FM is a (0,1) RLL code, while MFM is a
233:
Disk drives are subject to a variety of mechanical and materials effects that cause the original pattern of data to "jitter" in time. If a long string of "0" are sent to disk, there is nothing to indicate which bit a following "1" might belong to - due to the effects of jitter it may become misplaced
229:
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,
260:
The upside to the FM approach is that it is extremely easy to implement the writing circuity and the clock recovery on reading is also relatively simple. The downside is that it uses up half of the disk surface for the clock signal, thus halving the total amount of data the disk can store. This led
256:
includes an accurate clock running at half the selected data rate of the disk media. When data is written to the disk, the clock signal is interleaved with the data. On reading, the clock signals are used as short-term triggers to time the presence or lack of a following signal that represents the
572:
In the IBM formats, the start of a sector's header information and the start of the data itself are indicated with special "sync mark", a pattern of 0s and 1s that cannot appear in the data itself. This is accomplished by not encoding this data using the FM or MFM encoding, making it easy for the
568:
FM and MFM are used to indicate the position of individual bits in the original data, but the data does not in itself have any higher level of organization like "files". This is the purpose of the format of the disk. Disks are typically formatted into fixed-sized sectors which contain additional
278:
Modified frequency modulation encodes the clock signal and the data in a single "clock window". Unlike FM, a clock bit is only written when needed to achieve synchronization when both current and preceding data bits are not set. On average, MFM achieves double the information density of FM.
681:
In this system, sync marks are made by inserting additional clock pulses between adjacent zero bits (following the MFM rule) where they would normally be omitted. In particular, the data bit pattern "100001" has a clock pulse inserted in the middle, where it would normally be omitted:
545:
series. The original FD1771 supported FM only, but it was quickly paired with the FD1781 and FD1791 which performed MFM based on an externally provided clock signal. Implementing MFM support with these drivers required an external data separator. Rapid improvement in
174:. MFM allowed devices to double the speed data was written to the media as the code guaranteed only one polarity change per encoded data bit. For this reason, MFM disks are typically known as "double density" while the earlier FM became known as "single density".
209:-inch ordinary and high-density floppy diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of RLL codes. Outside of niche applications, MFM encoding is obsolete in magnetic recording.
225:, 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
273:
A byte encoded using FM (above) and MFM (below). Dashed blue line denotes clock pulse (the signal level change in it is ignored), and dashed red line, data pulse (signal level change encodes 1, and absence thereof,
884:
863:
550:
in the late 1970s and early 1980s led to the first low-cost all-in-one MFM drivers in the early 1980s. The WD2791 was the first to directly support MFM using an internal analog
526:
Because the MFM system requires more accurate timing of the clock signal, it was not possible economically to build the required analog and digital components on a single
554:, but it required a number of simple external components to implement a complete system. The WD1770 was the first to implement a complete MFM solution in a single chip.
53:
1285:
234:
in time. Re-aligning the signals on the disk with individual data bits requires some sort of timing encoded to the disk along with the data.
100:
573:
driver to spot it. The sync mark that is commonly used in MFM encoding is known as the "A1 sync" since the data bits form the start of the
72:
440:
Note that the surrounding clock bits are sometimes known, but sometimes require knowledge of the adjacent data bits. A longer example:
79:
950:
992:
86:
889:
798:
68:
915:
1186:
854:
241:, have been developed for this purpose. Their suitability depends on the media or transmission mechanism being used.
119:
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17:
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value A1 (10100001), but the fifth clock bit is different from the normal encoding of the A1 byte.
1232:
46:
1237:
530:
using late 1970s technology. Instead, MFM drivers required the drive vendor to design their own
719:
542:
970:
744:
A detailed description is furnished of the limiting properties of runlength limited sequences.
1248:
978:
698:
1254:
1227:
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298:. The number of magnetic transitions per one bit of encoded data is on average 0.75 to 1.
8:
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Data: 1 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0 1 Clock: 0 0 0 0 0 1 0 0 1
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The basic encoding rule for MFM is that (x, y, z, ...) encodes to (x, x
269:
218:
1051:
252:(FM) was the first widely used system to perform this operation. The
238:
144:
856:
SBC 202 Double
Density Diskette Controller Hardware Reference Manual
739:
35:
1217:
1207:
156:
514:(The bold bits are the data bits, the others are the clock bits.)
479:
1... MFM clock: ? 1 1 0 0 0 0 0 0... MFM encoded: ?
1170:
1000:
635:, (Modified Modified Frequency Modulation), also abbreviated
1212:
1115:
1110:
1105:
541:
Among the most widely used controllers of the era was the
237:
A diverse range of suitable encodings, known generally as
155:. It was first introduced on hard disks in 1970 with the
836:
Floppy Disk Data
Separator Design Guide for the DP8473
793:. Springer Science & Business Media. p. 109.
718:
286:
y, y, y NOR z, z, z NOR...). A zero bit is encoded as
294:
if preceded by a one; a one bit is always encoded as
787:
Sammes, Anthony; Jenkinson, Brian (18 August 2007).
538:. Data separator design was an art form of its own.
181:(500–1000 kbit/s encoded) on industry-standard
290:if preceded by a zero bit in the input stream, and
261:to new forms of encoding that were more efficient.
230:it produces the maximum data rate for that system.
60:. Unsourced material may be challenged and removed.
833:Lutz, Bob; Melloni, Paolo; Wakeman, Larry (1982).
832:
159:and then in floppy disk drives beginning with the
849:
685:Data: 1 0 0 0 0 1 Normal: 0 1 0 1 0 Sync: 0 1
1272:
843:
786:
569:header information to link them back to files.
624:01001 ^ Missing clock bit
986:
177:MFM is used with a data rate of 250–500
27:Line code used in early magnetic data storage
839:(Technical report). National Semiconductor.
993:
979:
755:
120:Learn how and when to remove this message
268:
944:
813:
244:
14:
1273:
1085:Differential Manchester/biphase (Bi-φ)
913:
166:MFM is a modification to the original
1065:Non-return-to-zero, level (NRZ/NRZ-L)
974:
584:Clock: 0 0 0 1 1 1 0 Encoded:
1286:Rotating disc computer storage media
1070:Non-return-to-zero, inverted (NRZ-I)
170:(FM) code specifically for use with
58:adding citations to reliable sources
29:
212:
24:
945:Johnson, Herbert R. (2016-07-06).
907:
756:Hecht, M.; Guida, A. (July 1969).
25:
1302:
1187:Carrier-suppressed return-to-zero
1075:Non-return-to-zero, space (NRZ-S)
938:
916:"Digital Magnetic Tape Recording"
814:Prosise, Jeff (30 October 1990).
557:
534:circuitry, a system known as the
521:
1040:
883: This article incorporates
878:
869:from the original on 2017-06-18.
34:
1004:(digital baseband transmission)
953:from the original on 2017-06-18
896:General Services Administration
217:Magnetic storage devices, like
69:"Modified frequency modulation"
45:needs additional citations for
1192:Alternate-phase return-to-zero
947:"M2FM or MMFM diskette format"
826:
807:
780:
749:
712:
13:
1:
724:"Runlength-Limited Sequences"
705:
662:0 Encoded: 010100010010010010
264:
250:Frequency modulation encoding
168:frequency modulation encoding
133:Modified frequency modulation
1161:Eight-to-fourteen modulation
914:Savard, John J. G. (2018) .
147:used to encode data on most
7:
692:
10:
1307:
1243:Pulse-amplitude modulation
561:
1200:
1179:
1093:
1049:
1038:
1009:
1238:Pulse modulation methods
1121:Alternate mark inversion
620:1 0 Sync Mark: 100010010
1233:Ethernet physical layer
762:Proceedings of the IEEE
728:Proceedings of the IEEE
627:
891:Federal Standard 1037C
885:public domain material
816:"Choosing a Hard Disk"
774:10.1109/PROC.1969.7249
768:(7). IEEE: 1314–1316.
720:Kees Schouhamer Immink
543:Western Digital FD1771
275:
1249:Pulse-code modulation
1166:Delay/Miller encoding
699:Group coded recording
272:
1255:Serial communication
1228:Digital transmission
1131:Coded mark inversion
967:Frequency Modulation
616:Sync clock: 0 0 0 1
245:Frequency modulation
54:improve this article
1260:Category:Line codes
1141:Hybrid ternary code
1101:Conditioned diphase
1094:Extended line codes
1060:Return to zero (RZ)
447:... FM encoded: 1
1180:Optical line codes
790:Forensic Computing
758:"Delay Modulation"
528:integrated circuit
276:
141:run-length limited
1268:
1267:
1126:Modified AMI code
1017:Unipolar encoding
862:. pp. 4–26.
851:Intel Corporation
800:978-1-84628-732-9
734:(11): 1745–1759.
722:(December 1990).
552:phase locked loop
438:
437:
130:
129:
122:
104:
16:(Redirected from
1298:
1156:64b/66b encoding
1044:
1022:Bipolar encoding
995:
988:
981:
972:
971:
961:
959:
958:
934:
932:
931:
922:. Archived from
904:
903:
898:. Archived from
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548:IC manufacturing
434:
421:
408:
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372:
371:...0 0 ?...
367:
362:
361:...? 1 ?...
352:
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328:
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254:drive controller
213:Magnetic storage
208:
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172:magnetic storage
153:hard disk drives
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1151:8b/10b encoding
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908:Further reading
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582:1 0 1 0 0 0 0 1
566:
564:Disk formatting
560:
524:
512:
445:0 0 0 1 1 0 1 1
443:Data:
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398:
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365:
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357:MFM clock bits
343:
331:
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227:read/write head
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1177:
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1146:6b/8b encoding
1143:
1138:
1136:MLT-3 encoding
1133:
1128:
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1113:
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939:External links
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902:on 2022-01-22.
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822:. p. 427.
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562:Main article:
559:
558:Overall format
556:
536:data separator
532:clock recovery
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522:Data separator
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1027:On-off keying
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1010:Main articles
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926:on 2018-07-02
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382:MFM encoding
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71: –
70:
66:
65:Find sources:
59:
55:
49:
48:
43:This article
41:
37:
32:
31:
19:
1204:
965:The PC Guide
955:. Retrieved
928:. Retrieved
924:the original
919:
900:the original
890:
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855:
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518:(1,3) code.
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149:floppy disks
136:
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64:
52:Please help
47:verification
44:
18:MFM encoding
1002:Line coding
820:PC Magazine
580:Data:
575:hexadecimal
376:...0 0 0...
366:...? 0 0...
257:data bits.
219:hard drives
1281:Line codes
1275:Categories
1080:Manchester
1052:line codes
957:2017-06-19
930:2018-07-16
706:References
265:MFM coding
239:line codes
195:-inch and
163:in 1976.
80:newspapers
1291:Encodings
1205:See also:
920:quadibloc
670:010010010
151:and some
145:line code
110:June 2023
1218:Bit rate
1208:Baseband
951:Archived
864:Archived
853:(1977).
693:See also
654:1 0 0 1
161:IBM 53FD
157:IBM 3330
666:0010010
204:⁄
190:⁄
139:) is a
94:scholar
1171:TC-PAM
1050:Basic
797:
650:0 0 1
179:kbit/s
143:(RLL)
96:
89:
82:
75:
67:
1251:(PCM)
1245:(PAM)
887:from
867:(PDF)
860:(PDF)
701:(GCR)
639:, or
511:0...
304:Data
101:JSTOR
87:books
1213:Baud
1116:2B1Q
1111:4B5B
1106:4B3T
795:ISBN
689:1 0
678:001
641:M2FM
637:M²FM
633:MMFM
628:MMFM
433:0...
425:...0
420:?...
412:...0
407:0...
399:...?
394:?...
386:...?
344:...
332:...
320:...
308:...
221:and
73:news
770:doi
736:doi
674:010
351:...
339:...
327:...
315:...
284:NOR
274:0).
137:MFM
56:by
1277::
949:.
918:.
894:.
818:.
766:57
764:.
760:.
742:.
732:78
730:.
726:.
658:1
296:01
292:00
288:10
994:e
987:t
980:v
960:.
933:.
803:.
776:.
772::
738::
687:1
676:0
672:0
668:0
664:0
660:0
656:0
652:0
648:0
622:0
618:0
614:1
612:0
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604:1
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592:0
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507:0
505:1
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457:0
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325:1
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313:0
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206:2
202:1
199:+
197:3
192:4
188:1
185:+
183:5
135:(
123:)
117:(
112:)
108:(
98:·
91:·
84:·
77:·
50:.
20:)
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