See this site for a good explanation of Manchester and
Differential Manchester coding.

http://ppdbooks.pok.ibm.com/cgi-bin/bookmgr/bookmgr.cmd/BOOKS/EZ306400/REVIS
IONS

Diff. Manchester has 2 big advantages:
  - less clock jitter
  - not polarity sensitive

I'm still hunting for a source of good info on MFM and GCR.

Eric Engler