A feedback control system applicable to a magnetic disk unit is capable of improving the accuracy of positioning at a low sampling frequency. When multiple episodes are recorded, the device computes statistics on the apnea durations, such as average apnea duration and total apnea duration for a preselected time period. One or more measurement devices are responsive to magnetic fields in close proximity to the device, allowing compensation by controlling current to the compensation coil. The apparatus may also include a control unit configured to define a first size of a stereoscopic image, based on at least the first spatial position, and generate a signal to display the stereoscopic image at the first stereoscopic image size. The apparatus also includes a plurality of process members provided in contact with the drum.