Modified Frequency Modulation

What Does Modified Frequency Modulation Mean?

Modified frequency modulation (MFM) is a method of encoding digital data on magnetic media. MFM was used with early hardware, including Control Program for Microcomputers (CP/M), IBM compatible PCs and Amiga PCs.


MFM was used on 3.5-inch and 5.25-inch disks, or floppys, with data transfer rates (DTR) of 250 to 500 kbps, as well as MFM ST-506 hard disks up to five Mbps. MFM is now obsolete, with the exception of 1.44 MB floppy disks.

Because MFM had two times the capacity of previous frequency modulation (FM) encoding, it was also known as “double density.”

Techopedia Explains Modified Frequency Modulation

As an enhanced frequency modulation (FM) encoding scheme, MFM reduces the number of flux reversals incorporated for clock pulses, allowing for greater data density. When compared to FM, MFM doubles linear bit density and decreases lineal flux reversal density without increasing recorded magnetic density. Additionally, the recording code uses solely synchronized clock pulses if data bits are not available.

MFM encoding yields a non-return-to-zero (NRZ) bit stream that is encoded when written to magnetic media. The 1 bit is characterized by a magnetic transition, which is usually a positive voltage. A 0 bit does not have a magnetic transition and is generally a negative voltage. On average, each data bit is encoded as two bits with some delimiters or boundaries at a sequence’s beginning and end.

MFM has five basic encoding rules, as follows:

  • Flux transitions are never at a 0 bit’s midpoint.
  • Flux transitions are always at a 1 bit’s midpoint.
  • Flux transitions are never at a 1 bit’s starting point or at a 1 bit’s endpoint.
  • The run length limit is cells of two bits, which facilitates continuous flux reversals between two neighboring 0 bits.
  • The space following the last data bit and the lead-in just before the first data bit are recorded with clocking bits (0s).

Newer standard interfaces, like Small Computer System Interface (SCSI) and Enhanced Integrated Drive Electronics (EIDE), support faster DTRs.


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Margaret Rouse

Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…