biphase

Biphase, or bi-phase, refers to a family of line coding schemes used to encode binary data into a two-phase signal. In biphase encodings, bit information is conveyed by transitions in the signal within each bit interval, rather than by constant levels alone. This approach provides DC balance and reliable clock recovery because the signal inherently contains transitions.

The main variants include Manchester encoding, Differential Manchester encoding, and Bi-Phase Mark Code (BMC). Manchester encoding represents bits by a transition in the middle of each bit period, with the direction of that middle transition indicating the bit value. Differential Manchester encodes by always producing a transition in the middle of every bit; the presence or absence of a boundary transition determines the bit value, which makes the scheme robust to polarity reversals. BMC is another form where a transition pattern within each bit time encodes the data, typically ensuring a transition at the bit boundary and/or middle according to the data.

Key properties of biphase schemes are that they are self-clocking and DC-free, trading lower spectral efficiency for improved timing recovery and reduced baseline wander. They generally require more bandwidth than non-return-to-zero encodings but are advantageous in systems where clock recovery from the data stream is important.

Applications of biphase encoding have included data communications and storage systems that rely on reliable timing information, such as certain Ethernet implementations and legacy signaling schemes, as well as various magnetic and optical storage and communication protocols. The term is sometimes used interchangeably with bi-phase in older literature, though specific implementations may differ in transition rules and bit encoding.