Hubbry Logo
Modified frequency modulationModified frequency modulationMain
Open search
Modified frequency modulation
Community hub
Modified frequency modulation
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Contribute something
Modified frequency modulation
Modified frequency modulation
Modified frequency modulation
View on Wikipedia
from Wikipedia

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.

MFM is a modification to the original frequency modulation encoding (FM) code specifically for use with magnetic storage. 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".

MFM is used with a data rate of 250–500 kbit/s (500–1000 kbit/s encoded) on industry-standard 5+1⁄4-inch and 3+1⁄2-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 Outside of niche applications, MFM encoding is obsolete in magnetic recording.

Magnetic storage

[edit]

Magnetic storage devices, like hard drives 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 codes, have been developed for this purpose. Their suitability depends on the media or transmission mechanism being used.

Frequency modulation

[edit]

Frequency modulation encoding (FM) was the first widely used system to perform this operation on disk drives. The drive controller 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 data bits.[2]

The upside to the FM approach is that it is extremely easy to implement the writing circuitry, 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 to the development of new forms of encoding that were more efficient.

MFM coding

[edit]
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, 0).

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.[3]

The basic encoding rule for MFM is that (x, y, z, ...) encodes to (x, x NOR y, y, y NOR z, z, z NOR...). A zero bit is encoded as 10 if preceded by a zero bit in the input stream, and 00 if preceded by a one; a one bit is always encoded as 01. The number of magnetic transitions per one bit of encoded data is on average 0.75 to 1.[4]

Data ... 0 0 ... ... 0 1 ... ... 1 0 ... ... 1 1 ...
MFM clock bits ...? 1 ?... ...? 0 0... ...0 0 ?... ...0 0 0...
MFM encoding ...?010?... ...?0010... ...0100?... ...01010...

Note that the surrounding clock bits are sometimes known, but sometimes require knowledge of the adjacent data bits.[citation needed] A longer example: 

Data:          0 0 0 1 1 0 1 1 ...
FM encoded:   10101011111011111...
MFM clock:    ? 1 1 0 0 0 0 0 0...
MFM encoded:  ?0101001010001010...

(The bold bits are the data bits, the others are the clock bits.)

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.[citation needed] Thus, FM is a (0,1) RLL code, while MFM is a (1,3) code.

Data separator

[edit]

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 integrated circuit using late 1970s technology. Instead, MFM drivers required the drive vendor to design their own clock recovery circuitry, a system known as the data separator. Data separator design was an art form of its own.[5]

Among the most widely used controllers of the era was the Western Digital FD1771 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 IC manufacturing 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 phase-locked loop, 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.

Overall format

[edit]
Main article: Disk formatting

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 header information to link them back to files.

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 (an "illegal" encoding). This is accomplished by not encoding this data using the FM or MFM encoding, making it easy for the 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 hexadecimal value A1 (10100001), but the fifth clock bit is different from the normal encoding of the A1 byte. 

Data:      1 0 1 0 0 0 0 1
Clock:      0 0 0 1 1 1 0
Encoded:   100010010101001
Sync clock: 0 0 0 1 0 1 0
Sync Mark: 100010010001001
                    ^ Missing clock bit

In some older disk formats this same problem was solved by the index hole, of which there was one for each sector, all positioned at the same radius (on one "track") beyond the innermost data track. A signal could be generated via a phototransistor when the start of each sector passed below the read head. No illegal encoding was thus necessary to distinguish data from the sync word. Later diskettes were manufactured with one index hole sufficient to indicate one particular sector, but they were mostly ignored with the sector number instead being part of the sector header.

MMFM

[edit]

MMFM (modified modified frequency modulation), also abbreviated M²FM or M2FM, 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.[6] In the example below, clock bits that would have been present in MFM are noted in bold: 

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 0 0 0 1 0 1 0 0 1 0 1 0 0
Encoded: 01010001001001001000010010001001001000100001

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: 

Data:  1 0 0 0 0 1
Normal: 0 1 0 1 0
Sync:   0 1 1 1 0

See also

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Modified frequency modulation

View on Grokipedia
from Grokipedia
Modified frequency modulation (MFM) is a run-length limited (RLL) , specifically a (1,3) RLL scheme, designed to encode digital data on media by representing bits through controlled transitions on the recording surface. It serves as a modification of the earlier (FM) encoding method, allowing for higher data density—typically doubling the capacity of FM—while maintaining compatibility with existing drive hardware. In MFM, each bit cell has potential flux transitions: a clock transition at the cell boundary (omitted if the previous bit was 1), and an additional transition in the middle of the cell for a data bit of 1. This enforces the (1,3) RLL constraints, allowing no adjacent 1s and no more than three consecutive 0s. This self-clocking nature embeds synchronization information within the , though it requires more sophisticated read circuitry, such as phase-locked loops, to detect the narrower timing windows (typically 2 μs per bit cell at 500 kbit/s). Address marks and special patterns, like the "A1" sync byte, violate these rules intentionally (e.g., by omitting a clock bit) to mark sector boundaries uniquely. Introduced in the late 1970s as an industry standard for double-density floppy disks, MFM enabled capacities up to 820 kB on 8-inch disks at data rates of 250–500 kbit/s. It was widely adopted in early hard disk drives via the ST-506 interface, supporting up to 5 Mbit/s transfer rates and capacities of several megabytes per drive, such as Seagate's original 5 MB ST-506 model. By the 1980s, MFM became the de facto standard for personal computer storage until supplanted by more efficient run-length limited codes like (2,7) RLL in later SCSI and IDE drives.

Background and Context

Origins in Magnetic Storage

Magnetic storage technologies rely on the principle of encoding digital data through localized magnetic flux reversals on a rotating medium, such as coated disks in floppy drives or rigid platters in hard disk drives (HDDs). These reversals, created by a write head that alternates the polarity of magnetic domains on the media, represent binary states: a transition indicates a '1' bit, while the absence or positioning of transitions denotes '0' bits, with the read head detecting variations in the magnetic field to recover the data. This flux-based approach enabled non-volatile, removable storage but faced density limitations in early systems due to the need for frequent clock synchronization pulses that consumed media space. Modified frequency modulation (MFM) emerged in the late and early as an advancement over basic (FM) encoding, which served as its precursor by using dedicated clock bits alongside data bits. Developed to address the density constraints of emerging digital storage needs, MFM was first implemented by in the 3330 series HDD, introduced in 1970, marking a shift toward more efficient run-length limited coding schemes. By the mid-, as adoption grew for and applications, MFM was adapted for flexible media; IBM's 53FD 8-inch double-density floppy drive, released in 1976, utilized MFM to achieve capacities up to 1.2 MB per disk, doubling the effective storage over single-density FM formats. MFM became the dominant encoding method for subsequent generations of floppy disks, particularly in the 5.25-inch and 3.5-inch formats that standardized personal computing storage. It powered the PC's double-density floppy configurations starting in 1981, supporting up to 360 KB on 5.25-inch disks and 1.44 MB on 3.5-inch high-density variants, which were ubiquitous in systems from , Apple, and others through the 1980s and 1990s. In rigid media, MFM underpinned early HDD interfaces, including Seagate's ST-506 standard introduced in 1980, which delivered 5 MB capacities and set benchmarks for PC-compatible drives with transfer rates of 5 Mbit/s. Compared to non-encoded or FM methods, MFM's key advantage lies in halving the number of required flux transitions by embedding clock information within patterns, thereby doubling areal density without altering media or head . This efficiency reduced overhead from unnecessary clock pulses, enabling higher bit rates and storage capacities in an era when magnetic media constraints limited system performance.

Frequency Modulation Fundamentals

Frequency modulation (FM), also known as single-density encoding, is a line code developed for magnetic storage media, where each data bit is represented by clock and data pulses manifested as flux transitions on the recording surface. In this scheme, the clock information is embedded directly into the data stream, eliminating the need for a separate clock track and enabling self-synchronization during read operations. Flux transitions occur to encode both timing and data content, with a logical 1 represented by transitions at both the clock edge and the bit cell center, while a logical 0 is encoded by a transition only at the clock edge. The encoding process operates on bit cells of fixed duration, typically 4 μs in standard implementations, where the clock transition marks the start of each cell, and the optional data transition appears 2 μs later. This results in binary patterns such as "10" for a 0 (clock transition followed by no data transition) and "11" for a 1 (clock plus data transitions), ensuring a transition occurs at least every bit cell to maintain . The embedded clock simplifies circuitry but doubles the number of transitions compared to data-only encoding, as every bit cell includes at least one flux change for timing. FM's structure imposes a maximum of two flux transitions per bit cell, yielding a transition density of up to 2/T, where T is the bit period. This leads to a Nyquist-limited frequency of 1/T for the encoded signal, constraining the overall bandwidth and storage efficiency. In practice, this limits areal density, with typical data rates around 250 kbit/s on early media, as the frequent transitions increase susceptibility to media noise and require precise head positioning to avoid bit-shift errors. The high overhead of dedicated clock pulses in FM, combined with its vulnerability to clock drift without robust synchronization circuits, caps achievable densities at approximately 250 kbit/s, beyond which signal integrity degrades due to intersymbol interference. This encoding was foundational in early magnetic storage applications, such as 8-inch floppy disks.

Core Encoding Principles

MFM Coding Rules

Modified frequency modulation (MFM) encoding follows three core rules to map bits into transitions within fixed-duration bit cells on storage media. First, there is no transition at the start (clock half) of a bit cell unless required for clocking. Second, a transition is placed in the middle (data half) of the bit cell if the data bit is 1. Third, if the data bit is 0 and the immediately preceding data bit was also 0, a transition is inserted at the start of the current bit cell to provide ; otherwise, no such clock transition occurs for an isolated 0 (following a 1). These rules embed clock information directly into the data stream, eliminating the need for separate clock pulses in FM encoding. The encoding process depends on the current data bit and the previous data bit, producing a sequence of channel bits where 1 indicates a flux transition and 0 indicates none. The clock bit for a given cell is 1 only if both the previous and current data bits are 0; the data bit matches the input data bit directly. This dependency ensures self-clocking while adhering to run-length limited (RLL(1,3)) constraints: transitions must be separated by at least 2 half-cells (minimum 1 intervening 0 in the channel stream) and at most 4 half-cells (maximum 3 consecutive 0s), limiting gaps to prevent clock drift. By suppressing clock transitions for isolated 0s—unlike FM, which requires a clock pulse for every 0—MFM effectively doubles the data density, allowing one data bit per full bit cell instead of one per two cells. The following table illustrates the output channel bits for each combination of previous and current data bits:
Previous Data BitCurrent Data BitClock BitData Bit (Channel Output)
0010
0101
1000
1101
For the binary sequence 1010 (assuming an initial previous bit of 1 to start the stream), the first 1 produces a clock bit of 0 and data bit of 1, yielding a transition in the middle of the first cell. The following 0 (previous 1) produces clock 0 and data 0, with no transition in the second cell. The next 1 (previous 0) produces clock 0 and data 1, yielding a transition in the middle of the third cell. The final 0 (previous 1) produces clock 0 and data 0, with no transition in the fourth cell. This results in transitions at the middles of cells 1 and 3, creating a 4-half-cell gap (3 channel 0s) that respects the maximum run length. In the case of 00 (assuming previous 1 for the first 0), the first 0 yields clock 0 and 0 (no transition in the first cell), while the second 0 (previous 0) yields clock 1 and 0, placing a transition at the start of the second cell—effectively after one full cell from the prior transition. For 01 (previous 1 for the 0), the 0 yields no transitions, and the following 1 (previous 0) yields clock 0 and 1, placing a transition in the middle of the second cell. These patterns demonstrate how MFM avoids transitions in isolated 0 cells while inserting them for consecutive 0s to maintain the RLL constraints.

Flux Transition Patterns

In Modified Frequency Modulation (MFM), reversals on storage media define the encoded bit cells, where each cell spans a fixed 2T units of time (with T representing the nominal half-cell interval and minimum transition spacing). A logical '1' bit produces a flux transition at the midpoint of its 2T cell, while a logical '0' bit lacks a midpoint transition but may include a clocking transition at the cell boundary if the preceding bit was also '0', ensuring no more than three consecutive '0's to limit run lengths. This mechanism guarantees a flux transition at least every 4T, enabling self-clocking by providing regular timing references for without dedicated clock tracks. Representative flux transition patterns illustrate these principles through timing diagrams. For a sequence of four '1' bits (1111), dense transitions occur at the midpoint of each 2T cell, resulting in flux reversals every 2T across the entire sequence, maximizing recording density with closely spaced pulses. In contrast, a sequence of four '0' bits (0000) produces clock transitions at the boundary between the first and second cells, between the second and third, and between the third and fourth (with no transition in the first cell), creating 2T intervals between subsequent reversals while adhering to the (1,3) run-length limited constraint to prevent excessive gaps. These patterns can be visualized as follows in a simplified timing diagram (where vertical lines denote cell boundaries and arrows indicate flux transitions):
  • 1111: |→ (mid) |→ (mid) |→ (mid) |→ (mid) | (transitions every 2T)
  • 0000: | → (clock) | → (clock) | → (clock) | (transitions every 2T after the initial cell, no mids)
Such diagrams highlight bit cell boundaries and the absence of overlapping transitions, which supports reliable head readout. The flux patterns in MFM enable an effective recording frequency of up to 2/T—twice that of Frequency Modulation (FM) at 1/T—by eliminating redundant clock transitions, allowing data rates of 1-2 Mbit/s on typical media like double-density floppy disks. This density advantage arises from averaging one flux change per bit cell, compared to FM's higher overhead, while the self-clocking transitions every 2-4T reduce the need for separate synchronization tracks and improve overall storage efficiency. Patterns with long runs of '0's, such as approaching three consecutive zeros, can introduce clock slip errors where timing drifts due to widened intervals between transitions, potentially misaligning bit cell windows during readout. This issue is mitigated by the inherent RLL constraints of MFM, which cap zero runs at three to maintain frequent enough flux reversals for stable operation in recovery circuits.

Signal Processing and Recovery

Data Separator Mechanisms

The data separator in Modified Frequency Modulation (MFM) systems functions as a circuit that processes the analog readback from magnetic media, detecting flux transitions to reconstruct bit cells and separate embedded clock and signals. This involves measuring intervals between transition edges to identify bit cell boundaries and contents, enabling deserialization of the serial stream into parallel for the controller. Operationally, the separator typically employs a (PLL) comprising a , , and (VCO) to synchronize with the incoming signal. In MFM, which features variable transition densities due to omitted clock pulses for consecutive zeros, the PLL uses timing windows to distinguish clock transitions (expected at cell boundaries, approximately 2T to 3T intervals) from data transitions (centered in cells, 1.5T to 2.5T). A quarter-period delay line and pulse gating ensure phase comparisons account for missing pulses, while digital edge detectors in integrated variants sample the at precise intervals to generate clean clock and data outputs. Historically, early 1980s floppy disk controllers like the WD177x series integrated analog PLL-based separators to handle double-density MFM at rates up to 500 kb/s, providing internal without external adjustments. By the mid-1980s, digital implementations emerged in hard disk drives (HDDs), such as the WD10C22B chip, which used self-adjusting digital PLLs with phase-frequency detection for ST506/412 interfaces, supporting MFM at 5 Mbits/s and reducing susceptibility to analog component variations. Key challenges include managing from motor speed variations (up to ±2% instantaneous) and media defects, which can cause fractional or quadrature locking in the PLL, leading to errors. Bit slip, where timing drifts accumulate, is detected and corrected using sync marks like 0xA1 patterns with omitted clock bits to realign the PLL within 80 bit times. Noise and bit shifts from adjacent pulses are mitigated through window margin designs achieving 60-70% tolerance under worst-case conditions, often via pre-compensation and high-bandwidth filters.

Clock and Data Synchronization

Modified frequency modulation (MFM) employs a self-clocking mechanism by embedding clock information directly into the through periodic flux transitions on the magnetic medium. This design ensures that clock bits are inserted to prevent runs of more than three consecutive zeros, limiting the maximum interval without a transition to four bit times (4T), which allows the receiver to maintain without a separate . The (1,3) run-length limited nature of MFM guarantees transitions at least every 2T and at most every 4T, enabling reliable during read operations. Synchronization begins with the detection of specialized sync bytes, such as 0xA1, which are encoded with a deliberate violation of standard MFM rules by omitting a clock bit between the fourth and fifth data bits. This creates a unique flux transition pattern—typically 0100010010001001 for the data bits 10100001—allowing the data separator to align the phase of the local clock to the incoming signal. In practice, sectors are preceded by 12 bytes of 0x00 followed by three 0xA1 bytes, providing a for initial lock acquisition before the address mark (e.g., 0xFE or 0xFB). During write operations, precompensation adjusts the timing of adjacent transitions to counteract peak shifting caused by magnetic interactions, typically shifting isolated transitions by ±125 ns on higher-density tracks. This technique, implemented via controller signals like EARLY and LATE, ensures that closely spaced transitions (e.g., 2T apart) do not merge or spread excessively, preserving read accuracy. On the read side, a (PLL) with a (VFO) locks onto the signal by dividing bit cells into data and clock windows, using transitions to adjust phase and frequency. The PLL operates in a high-gain sync mode during preambles for rapid acquisition, then switches to low-gain data mode to track variations while minimizing accumulation. Error handling in MFM systems detects loss of when no transition occurs for more than 4T, triggering a resynchronization sequence via address marks like the 0xA1 . This violation-based resync allows recovery without halting the read process, as the unique encoding (e.g., 10101000 in data bits with missing clock) stands out from valid MFM streams. Performance characteristics include jitter tolerance of up to 10% of the bit cell time (typically 2 µs at 500 kHz ), enabling robust operation despite media imperfections. typically achieves lock within fewer than 10 bit cells during preamble detection, supporting efficient in floppy and early hard disk systems.

Data Organization and Format

Track and Sector Structure

In MFM-encoded media, data is organized on the disk surface in concentric tracks, which are narrow circular paths accessed by the read/write head. Each track is subdivided into sectors, representing fixed angular portions of the circle for storing discrete blocks of information. In formats, tracks typically number 40 to 80 per side, with sectors per track varying from 8 to 36 based on disk size and density to optimize storage efficiency. Sectors are delineated by inter-sector gaps consisting of unencoded flux periods that allow for head movement settling, clock synchronization, and error margins during read/write operations. On floppy disks, the commencement of each track is indicated by an index hole—a physical perforation near the disk's hub—detected by the drive to initiate timing and positioning alignment; hard disk drives generate an equivalent index pulse electronically once per revolution. Early MFM implementations often employed variable sector sizes, such as 128 bytes in initial double-density setups, whereas later formats standardized on 512 bytes per sector to simplify data management and enhance compatibility across systems. The write process begins with positioning the head to the desired track via increments from a track zero, calibrated by the drive's loading mechanism and index detection; in more precise systems, dedicated servo patterns on separate surfaces or tracks ensure accurate radial alignment to prevent off-track errors. Once positioned, MFM encoding is applied continuously to the sector stream, generating transitions that embed clock and signals across the entire track without interruption at sector boundaries. Representative capacities illustrate this structure's evolution: an 8-inch single-sided double-density floppy featured 77 tracks and 26 sectors per track with 128 bytes per sector, providing about 256 KB total storage. By contrast, the 3.5-inch high-density variant advanced to 80 tracks per side (160 total), 18 sectors per track, and 512 bytes per sector, resulting in 1.44 MB capacity while maintaining MFM for reliable data density.

Header, Gap, and Data Fields

In Modified Frequency Modulation (MFM) encoding, the internal structure of a sector on a floppy disk or early hard drive track is organized into distinct fields for identification, timing, and data storage, enabling reliable access and error detection. The sector begins with sync bytes consisting of 12 bytes of 0x00, followed by three 0xA1 bytes and the ID Address Mark (IDAM) byte 0xFE, which signals the start of the header. This is followed by four bytes containing the track (cylinder) number, head (side) number, sector number, and sector size code (typically 0x02 for 512 bytes), plus a 2-byte Cyclic Redundancy Check (CRC) for integrity verification. The header allows the disk controller to locate and validate the sector before accessing its contents. The address marks use special encoding where the clock field for the three preceding 0xA1 bytes has a specific pattern (clock bits inverted on the position where data and clock differ) to allow the data separator to detect the mark uniquely. Preceding the IDAM are the sync bytes of 12 bytes 0x00, which facilitate (PLL) lock-in for during readout. After the ID field and its CRC, gap fields—filled with 0x4E bytes in MFM to provide regular clock transitions every two bit cells—provide timing buffers for head settling and inter-field spacing. The data portion follows with sync bytes of 12 bytes 0x00, three 0xA1 bytes, and the Data Address Mark (DAM) byte 0xFB, succeeded by 512 bytes of user and another 2-byte CRC. The CRC for both the ID and fields uses the x16+x12+x5+1x^{16} + x^{12} + x^5 + 1, computed over the address mark, field bytes, and appended as two bytes for error detection. Gap fields are categorized by their position and purpose within the track, which typically contains multiple such sectors. In standard PC double-density formats, GAP1, positioned post-index pulse, comprises 80 bytes of 0x4E to allow initial stabilization after track start. GAP2 serves as the inter-sector gap of 22 bytes of 0x4E, separating consecutive sectors for rotational timing. GAP3, immediately post-ID field, is a short 3-byte gap of 0x4E to transition quickly to the data field. GAP4, post-data field, has variable length (often adjusted to fill the remaining track space before the next index) to accommodate the total track capacity. These gaps are essential for maintaining consistent rotational speed and preventing overlap in soft-sectored formats. The read and write sequence relies on the to navigate these fields sequentially. Upon detecting the index pulse, the controller skips GAP1 and searches for the IDAM sync and mark; it then reads and verifies the header fields, including CRC, to confirm the target sector. If valid, it skips GAP3, locks onto the DAM sync, reads the data field and CRC, then skips GAP4 or the next GAP2 to proceed to the subsequent sector. During writes, the controller similarly times insertions into gaps while generating on-the-fly encoding and CRC. This process ensures precise data transfer without interference from adjacent fields.

Variants and Evolutions

Modified Modified Frequency Modulation (MMFM)

Modified Modified Frequency Modulation (MMFM), also known as M²FM or M2FM, is a run-length limited encoding variant of MFM that relaxes the maximum run length constraint to allow up to four consecutive 0-bits between 1-bits, forming an RLL(1,4) code compared to MFM's stricter RLL(1,3) limitation. This extension suppresses certain clock bit insertions, specifically omitting flux transitions between two 0-bits when followed by a 1, thereby reducing the number of transitions and enabling higher data density in double-density recording formats. As a result, MMFM achieves greater storage efficiency on magnetic media by packing more data bits per unit length while maintaining self-clocking properties through strategic pulse placement. The encoding modifications in MMFM build directly on MFM principles but introduce an additional rule for clock synchronization: a clock pulse is inserted only after four 0-bits to prevent excessive run lengths, extending the maximum flux transition interval to 5T (where T represents the bit cell period) rather than MFM's 4T limit. In practice, this means patterns like four consecutive 0s (0000) trigger a dedicated clock transition at the end of the run to aid , while shorter runs follow MFM-like rules for 1-bit encoding at cell centers and clock bits at boundaries when needed. These changes require specialized hardware, such as phase-locked oscillators tuned for longer intervals, to decode the signal accurately without losing . MMFM found niche applications in early double-density 8-inch systems during the late , particularly in controllers from (e.g., iSBC 202 Diskette Controller) and formats supported by and (e.g., HP 9895A Flexible Disc Memory). It was employed to boost capacity in industrial and environments, such as 's ISIS operating system setups, where formatted disks could reach approximately 600 KB per side (1.2 MB total) with 77 tracks and 30 sectors. However, its adoption remained limited due to incompatibility with standard MFM decoders and the need for custom data separators. While MMFM offered improved density over MFM—potentially up to 25% more bits per track in optimized setups—it introduced trade-offs in , including heightened complexity for circuits to handle the extended 5T runs, which could degrade performance in noisy environments or with marginal media. This susceptibility to errors, combined with the rise of more advanced run-length codes like RLL(2,7), led to MMFM being largely superseded by the early in favor of broader-compatible schemes.

Transitions to Advanced Schemes

As magnetic storage technologies advanced in the late , MFM encoding encountered significant limitations in achieving higher areal densities, primarily due to its run-length limited (RLL(1,3)) constraints that mandated frequent flux transitions to maintain , thereby restricting the minimum transition spacing and making it susceptible to timing and media defects in drives with increasing track counts. This resulted in MFM-based hard disk drives typically plateauing at capacities of 20 to 100 MB by the end of the decade, as further density improvements required relaxing these constraints to allow longer runs of zeros without synchronization loss. The transition to more advanced schemes began in the mid-1980s with the adoption of Run-Length Limited codes, particularly RLL(2,7), which permitted a higher by allowing up to seven consecutive zeros while ensuring at least two, effectively increasing storage capacity by approximately 33% over MFM on the same hardware, and up to 50% in practice with optimized controllers and higher in SCSI drives during the early 1990s. For instance, RLL(2,7) enabled -based systems to double effective capacities in many cases by leveraging faster bit clocks without altering the physical interface, marking a key shift in enterprise storage. In parallel, Group Code Recording (GCR) emerged as an alternative for applications, notably in Apple systems where it encoded more efficiently than MFM for single- and double-sided 3.5-inch drives, supporting up to 800 KB per disk while maintaining compatibility with variable-speed motors. By the early 1990s, Partial Response Maximum Likelihood (PRML) detection supplanted peak-detection methods used with MFM and early RLL, introducing to equalize and employ Viterbi decoding for reliable recovery of data sequences at densities previously unattainable, thereby enabling error-tolerant capacities exceeding 1 GB in hard drives. IBM's 1990 introduction of PRML in the 0681 model exemplified this evolution, combining it with thin-film heads to achieve linear densities over bits per inch, a substantial leap that facilitated the of hard disk capacities into the era. MFM's legacy persists in niche applications, such as the encoding of Track 3 on magnetic stripe cards per ISO/IEC 7811-7 standards, where its simplicity supports high-coercivity media for financial and identification purposes without requiring complex error correction. In retro computing, software emulators like PCem replicate MFM interfaces for preserving 1980s systems, allowing archival of floppy and hard disk images on modern hardware. However, with the dominance of solid-state storage post-2000, MFM has seen no widespread adoption in new designs, though its principles influenced early embedded controllers in 1980s industrial systems for reliable, low-cost magnetic interfacing.

References

  1. http://bitsavers.trailing-edge.com/magazines/Computer_Design/Computer_Design_V19_N02_198002_Encoding-Decoding_Techniques_Double_Floppy_Disc_Capacity.pdf
  2. https://minuszerodegrees.net/manuals/Seagate/Seagate%20ST506%20ST412%20-%20OEM%20Manual.pdf
  3. https://www.micropolis.com/support/kb/mfm-drive-interface
  4. https://www.sbe.org/handbook/fundamentals/Audio/Audio-Magnetic_Recording.pdf
  5. https://illumin.usc.edu/the-magnetic-stripe-technology/
  6. https://d1yx3ys82bpsa0.cloudfront.net/groups/ibm-3330.pdf
  7. https://ieeexplore.ieee.org/document/729138
  8. http://www.3480-3590-data-conversion.com/article-floppy-disks.html
  9. https://gunkies.org/wiki/ST506_disk_interface
  10. https://sinclair.wiki.zxnet.co.uk/wiki/Magnetic_Disk_Storage_System
  11. https://ub.fnwi.uva.nl/computermuseum/diskettes.html
  12. https://hxc2001.com/download/datasheet/floppy/thirdparty/FDC/Data_Separator/TN6-1_9216_Floppy_Disk_Data_Separator_Jun82.pdf
  13. https://docs.mamedev.org/techspecs/floppy.html
  14. https://floppy.cafe/mfm.html
  15. https://info-coach.fr/atari/hardware/_fd-hard/AN-505.pdf
  16. https://hxc2001.com/download/datasheet/floppy/thirdparty/FDC/Applications_notes/07MAGREC.pdf
  17. https://www.ti.com/lit/an/snoa710/snoa710.pdf
  18. https://patents.google.com/patent/EP0182701A2/en
  19. https://www.ti.com/lit/pdf/snoa707
  20. http://bitsavers.org/components/westernDigital/_dataSheets/WD10C22B_Self-Adjusting_Data_Separator_1988.pdf
  21. http://www.bitsavers.org/pdf/shugart/_appNotes/SA1000_Data_Separator_Guide_and_Recommended_Track_Format.pdf
  22. http://nerdlypleasures.blogspot.com/2021/04/floppy-drives-single-density-and-double.html
  23. https://cmrr-star.ucsd.edu/static/pubs/psrll.pdf
  24. https://map.grauw.nl/articles/low-level-disk/
  25. https://www.5volts.ch/posts/mfmreader/
  26. https://github.com/keirf/greaseweazle/wiki/Write-Precompensation
  27. http://bitsavers.org/components/westernDigital/FD179X_Application_Notes_Jun1980.pdf
  28. https://retrocmp.de/fdd/general/floppy-formats.htm
  29. https://www.dosdays.co.uk/topics/floppy_drives.php
  30. https://bitsavers.org/pdf/ibm/pc/pc/6025008_PC_Technical_Reference_Aug81.pdf
  31. http://www.bitsavers.org/components/ti/_dataBooks/TMS279X_Floppy_Disk_Formatter_Jun84.pdf
  32. https://encyclopedia.pub/entry/35704
  33. https://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/disk-format
  34. https://oldcomputers.dyndns.org/public/pub/manuals/m4853_mitsubishi.pdf
  35. https://www.worldradiohistory.com/Archive-Company-Publications/HP-Journal/70s/HPJ-1979-04.pdf
  36. http://bitsavers.org/pdf/intel/iSBC/9800420A_iSBC_202_Hardware_Reference_Sep77.pdf
  37. https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
  38. https://www.redhill.net.au/d/10.php
  39. https://www.ardent-tool.com/FAQ/FAQ_3.html
  40. https://www.datarecoverytools.co.uk/data-recovery-vocabulary/vocabulary-a-e/data-encoding-and-decoding-of-hard-disk/run-length-limited-rll/
  41. https://mirrors.apple2.org.za/ftp.apple.asimov.net/documentation/hardware/storage/disks/apple_floppy_disk_drives.pdf
  42. https://www.storagenewsletter.com/2018/07/19/history-1990-ibm-681-first-hdd-with-prml-channel/
  43. https://jacobsschool.ucsd.edu/news/release/716
  44. https://www.iso.org/standard/61938.html
  45. https://www.computerhistory.org/collections/catalog/102741157
Add your contribution
Related Hubs
Contribute something
User Avatar
No comments yet.