manchester encoding

Manchester encoding

   <communications, protocol> A method of transmitting bits which
   enables the receiver to easily synchronise with the sender.

   A simple way of signalling bits might be to transmit a high
   voltage for some period for a 1-bit and a low voltage for a 0
   bit:

    Bits Sent:             1     1     0     0

    Signal:      High    ___________
   	      Low                |___________

    Time: ->            .     .     .     .     .

   However, when several identical bits are sent in succession,
   this provides no information to the receiver about when each
   bit starts and stops.

   Manchester encoding splits each bit period into two, and
   ensures that there is always a transition between the signal
   levels in the middle of each bit.  This allows the receiver to
   synchronise with the sender.

   In normal Manchester encoding, a 1-bit is transmitted with a
   high voltage in the first period, and a low voltage in the
   second, and vice verse for the 0 bit:

    Bits Sent:             1     1     0     0

    Signal:      High    __    __       __    __
   	      Low       |__|  |_____|  |__|

    Time: ->            .  '  .  '  .  '  .  '  .

   In Differential Manchester encoding, a 1-bit is indicated by
   making the first half of the signal equal to the last half of
   the previous bit's signal and a 0-bit is indicated by making
   the first half of the signal opposite to the last half of the
   previous bit's signal.  That is, a zero bit is indicated by a
   transition at the beginning of the bit.

   Like normal Manchester encoding, there is always a transition
   in the middle of the transmission of the bit.

          Differential Manchester Encoding

    Bits Sent:            1     1    0     0

    Signal:      High  ____       __    __    __
   	      Low       |_____|  |__|  |__|

    Time: ->            .  '  .  '  .  '  .  '  .

   With each bit period half as long, twice as much {bandwidth}
   is required when using either of the Manchester encoding
   schemes.

   (1995-11-23)