Tech moves fast! Stay ahead of the curve with Techopedia!
Join nearly 200,000 subscribers who receive actionable tech insights from Techopedia.
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.”
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: