The DICT Development Group

  Manchester encoding
       
           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)