A servo loop controls an actuator (24) and read head (10), the loop being responsive to training signals (60) to cause the actuator (24) to move the read head (10). An error signal is produced that indicates a less than optimum positioning action by the actuator (24). A compensating filter (46) alters the servo loop's response to a training signal (60) to cause the actuator (24) to produce a more optimum positioning action, the filter (46) exhibiting an initial value filter characteristic. A processor (40) is responsive to a first training signal (60) to alter the filter's initial characteristic value in a first direction to cause the filter (46) to act upon the first training signal in accordance with the altered characteristic value, and is further responsive to a second training signal to alter the filter's initial characteristic in a second direction to cause said filter (46) to act upon the second training signal in accordance with the altered characteristic value. The processor (40) adjusts the filter characteristic in a direction dependent upon which training signal causes a lower value error signal.