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One form of encoding asynchronous binary data is BiPhase encoding (or Bi-Phase encoding). Given that a message consists of '0's and '1's, and that two different types of signal can be transmitted 'L' and 'H', bi-phase encoding consists of transmitting either 'LH' or 'HL' for every bit. The name comes from the fact that every bit period looks like one cycle of a digitized sine wave, either with phase 0 (HL) or phase π (LH).

The advantage of bi-phase encoding is that every pulse is either half a bit long or one bit long, so it is self-clocking; i.e. so long as the clock used to transmit and the clock used to receive are both reasonably constant, the receiver will never lose track of how many bit-periods (or, to be more specific, half-bit-periods) have elapsed since the beginning of the message. NRZ (Non-Return to Zero) encoding is not self-clocking; therefore, without bit stuffing or framing or some other mechanism, the resolution and accuracy of the receiver's clock may accumulate an error of greater than 1 bit period if the signal is unchanging for too long (imagine trying to guess whether a long beep is 100 seconds long or 101 seconds long). The disadvantage of a self-clocking signal is that it requires more bandwidth than NRZ.

The most obvious form of BiPhase encoding is fully-coherent bi-phase encoding, a.k.a. Manchester encoding. Fully-coherent (or fully coherent) means that the value of the underlying message corresponds directly with the (absolute) value of the signal; a bit '0' is represented by 'LH', and a bit 1 is represented by 'HL' (unless you ask someone from the other half, who will tell you that '0' is 'HL' and '1' is 'LH'). For example:

    <--0--> <--0--> <--0--> <--1--> <--1--> <--1--> <--0--> <--1--> <--0--> <--1-->
H       ---     ---     -------     ---     ---         -------         -------    
     L / H \ L / H \ L / H   H \ L / H \ L / H \ L   L / H   H \ L   L / H   H \ L 
L   ---     ---     ---         ---     ---     -------         -------         ---

A less obvious, but sometimes more useful, form of BiPhase is differentially-coherent bi-phase (sometimes called differential bi-phase, sometimes even just called bi-phase by people who don't realise Manchester encoding is also a form of bi-phase). Differentially coherent means that the value of the message corresponds to changes in the value of the signal. In this scheme, a '0' is represented by a cycle with the same phase as the previous cycle, and a '1' is represented by a cycle with the phase opposite from the previous cycle. (Again, if you talk to some others, they will tell you that a '0' corresponds to a phase change and a '1' corresponds to no phase change. Change on '1' has a mathematical nicety in that converting between fully-coherent and differentially-coherent is identical to changing between binary and gray code.) Differential bi-phase is often described in different terms; one may say that there is an edge (or a transition) in the middle of every bit, and an additional edge at the beginning of a '0' bit; or, there's an edge in the middle of every bit and an edge at the end of a '0' bit; or, there's an edge between every pair of bits and an edge in the middle of a '0' bit. All of these descriptions are equivalent, with the exception of the details of the very first and/or very last bits being transmitted; in RFID, commonly a message is transmitted again and again back-to-back; so even this detail is not significant. The same data in the above example looks rather different (this diagram assumes the signal is at H before the bit sequence shown):

    <--0--> <--0--> <--0--> <--1--> <--1--> <--1--> <--0--> <--1--> <--0--> <--1-->
H -     ---     ---     -------         -------     ---         ---     -------    
   \ L / H \ L / H \ L / H   H \ L   L / H   H \ L / H \ L   L / H \ L / H   H \ L 
L   ---     ---     ---         -------         ---     -------     ---         ---

A comparison of the two signals is worthwhile. For the fully-coherent signal, a regular square wave with a period equal to the bit period appears whenever a long string of '0's or a long string of '1's appears. A regular square wave with a period equal to twice the bit period appears whenever the bits are alternating. By contrast, in the differentially-coherent signal, a regular square wave with a period equal to the bit period only appears where there is a string of '0's, and a regular square wave with a period of twice the bit period appears when there is a string of '1's. Note that a message transmitted as fully-coherent bi-phase is physically indistinguishable from a message transmitted as differentially-coherent bi-phase. The '-coherent' part of the encoding is all about the interpretation of signals, not about the shape of the signals.

Other forms of encoding include NRZ and Miller encoding.

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