Biphase mark code

Differential Manchester encoding is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. In various specific applications, this line code is also called by various other names, including Biphase Mark Code (BMC), Frequency Modulation (FM), F2F (frequency/double frequency), Aiken Biphase, and Conditioned diphase. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It is not necessary to know the polarity of the sent signal since the information is not represented by the absolute voltage levels but in their changes: in other words it does not matter which of the two voltage levels is received, but only whether it is the same or different from the previous one; this makes synchronization easier.

Differential Manchester encoding has the following advantages over some other line codes:

These positive features are achieved at the expense of doubling the bandwidth—there are two clock ticks per bit period (marked with full and dotted lines in the figure). At every second clock tick, marked with a dotted line, there is a potential level transition conditional on the data. At the other ticks, the line state changes unconditionally to ease clock recovery. One version of the code makes a transition for 0 and no transition for 1; the other makes a transition for 1 and no transition for 0.

Differential Manchester is specified in the IEEE 802.5 standard for token ring LANs, and is used for many other applications, including magnetic and optical storage. As Biphase Mark Code (BMC), it is used in AES3, S/PDIF, and SMPTE time code. Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase, according to the ISO/IEC 7811 standard. Differential Manchester is also the original "frequency modulation" (FM) used on "single-density" floppy disks, followed by "double-density" modified frequency modulation (MFM), which gets its name from its relation to FM, or Differential Manchester, encoding.

...
Wikipedia