Technique : Manchester encoding

  
    
    Change in middle of bit 
      if zero, one direction, 
      if one, then the opposite.

    Improper wiring can invert bits.

    Change guarentees timing. Fequency 2x data transmission.

    Manchester encoding is used in wired (10/100) 10Base T Ethernet  802.3

    No DC component - average transfer of current is zero.
  
    (Like NRZ but based on change rather than level)


Technique : Differential Manchester encoding

  
  
    Change at transition if next bit 0.
 
    Change in middle of bit every time. (clock) 
      * look closely at the digital version of FM. They are essentially
        the same except FM always changes at start of bit and
        differential Manchester shifts in middle of bit.    
 
    This is similar in behavior to NRZI combined with a clock signal. 

    Zero average DC voltage - because switching guaranteed at least every
      2 clock cycles, average voltage zero - more efficient, less susceptible
      to noise.

    Bits still correct with improper wiring.

    Used on IBM Token ring.
    Some types of magnetic and optical storage.
    A variation used on magnetic stripe cards (credit cards, etc.)
    Used on the 1st floppy drives.
      Replaced by MFM - higher density.