Both twl and cwlondon make valid comments. But in both cases they cover only a portion of the field. Again, for a more complete and comprehensive history of the subject, as well as a detailed technical discussion, I'd like to refer you all to the DD website at:
http://de.geocities.com/bc1a69/index_eng.html
It's thoroughly researched and nicely presented.
http://de.geocities.com/bc1a69/index_eng.html
It's thoroughly researched and nicely presented.