Magnetic disk drive and method of setting a notch filter of the drive

A magnetic disk drive in which a sampling period of positional information recorded on a magnetic disk changes according to a rotation speed of the magnetic disk, comprising:

• a notch filter provided in a controller which controls positioning of the magnetic head to suppress a specific angular frequency by setting a set of parameters related to at least a suppression angular frequency, a damping ratio and a depth in consideration of the sampling period;
• a parameter storage unit which stores a first set of parameters of the notch filter and a first sampling period assumed when the first set of parameters is designed;
• and
• a computation unit that computes a discrete-time system transfer function from the sampling period and the set of parameters using a continuous-time system transfer function and a bilinear transform,
• wherein:
• when the sampling period changes from the first sampling period to a second sampling period, the computation unit computes a damping ratio parameter related to the damping ratio in a second set of parameters at the second sampling period, based on a first absolute value at a first angular frequency of a first transfer function computed from the first set of parameters at the first sampling period and a second absolute value at the first angular frequency of a second transfer function computed from the first set of parameters at the second sampling period;
• and
• the notch filter is set with the second set of parameters at the second sampling period.