Simple unstable baseline detection methods for perpendicular magnetic recording

Current hard disk drives employ a magneto-resistive head to read and write data. During the reading process, if the magneto-resistive head is unstable or has some defects, the readback signal will experience some missing data zones known as an unstable baseline (UB) event, which causes the signal amplitude to drop drastically and vary around the baseline at 0 volt. Thus, such defective heads must be detected and taken care of during the testing process before assembling a hard disk drive. This paper proposes three simple methods to detect the UB event that can be employed in the testing process when the data have a specific pattern, i.e. 4T-pattern, where T is the bit period. The proposed methods can also be utilised to detect the UB event during normal operation when the data are random. Simulation results indicate that the methods can detect the short UB event (4-bit duration) with 4T-pattern data and the 25-bit UB event with random data at 100% of detection.