A method for superimposed recording of a first signal and a second signal with frequency which is less than the frequency for the first signal, in a perpendicular magnetic recording medium by means of a ring-shaped magnetic head. The mutually superimposed first and second signals are supplied to the...http://www.google.com/patents/US4719520?utm_source=gb-gplus-sharePatent US4719520 - Method and apparatus for setting recording current in perpendicular magnetic recording apparatus
