Detecting changes in signal/data stream is easier than detecting levels.

So, any protocol that increases the number of changes in a data stream 
improves detection.

RLL - Run Length Limited (Huffman coding)

  Encodes block of data bits on larger block of bits 
    to guarentee changes in signal level with in a limited span.

  Data is encoded so that the number or "run" of consecutive zeros is limited.

  Also limits number of consecutive one's to limit high frequency attenuation.

  Cost - Increases the number of bits needed.

   Generic example 
     
   Data   RLL 
          000 
          001 
   00     010   010
   01     011   011
          100 
   10     101   101
   11     110   110
          111 
   
   When combined as a stream of bits,
     At most 2 zeros or 4 ones next to each other.

   Implementations of real RLL encoding is more complex.

   Once data is encoded as RLL, any of the transmission/storage methods 
     may be used.

  FM - Digital Frequency Modulation or Delay encoding

  

  Delay encoding - (0,2) RLL
    0  10
    1  11

    2x data stream.

    Change at beginning of clock period.

    Change in middle if data = 1

    No change in middle if data = 0

    Delay encoding is also known differential Manchester encoding.

  MFM - Data is encoded so that there is a zero after any one and no more than
    3 zeros in a row.  Sometimes called (1,3)RLL

  

    Used on floppies and early hard drives.  

    Notice there is always a change at the edge of the bit. 
    Like NRZI with a clock signal embedded.

    Modified Frequency Modulation - MFM - form of FM combined with simple
      RLL encoding.  

     Produced higher bit density that FM.


   RLL(2,7) - encodes n bits of data on 2n bits. Most common on current storage.
     But done in data bit groupings of 2, 3, or 4.

     Guarantees that at least 2 consecutive 0s between bits

     Guarantees that at most 7 consecutive 0s

     RLL vs. MFM.
       The number of actual transitions in the RLL signal allow for
       1/3 smaller foot print when storing bit sequences.
   
       Early hard drives 20 MB (MFM), 30 MB (RLL) - same physical drive.

       Standard coding for hard drives for a long time.

     Produced higher bit density that FM.

     Used on floppies and earliest hard drives.  

   8b,10b - used with NRZ on wired Gigabit Ethernet.
      DC free.