Pattern dependent overwrite on thick media

Overwrite measurements on thick media as a function of the recorded data pattern and record current level are presented. Two recording methods are addressed, full wave NRZI recording and double density NRZI (D²NRZI). The overwrite using NRZI recording is poor on thick media as low density signals are recorded deep into the media. Significant improvement is seen when extra higher density transitions are introduced between data ONEs for the D²-NRZI method which restricts the recording depth of low frequency components. However, the residual signal at the fundamental frequency of the data pattern is higher for long strings of ZEROs between ONEs than short strings, which is attributed to the reproduce head response