In a rotating disk computer mass storage device, data and control information is typically recorded on concentric tracks on the disk. One type of arrangement imbeds the control information in servo fields. The servo fields are spaced periodically along the length of each track. The servo fields include information which describes the track number, the position on the track, synchronization information, and other information. Between each of the servo fields, data is recorded in a data field. Each servo field is utilized to identify the following data field and to position accurately a head or transducer which reads or writes the data field, among other functions. This type of arrangement is referred to as an imbedded servo field and control system because the servo field is imbedded in each of the tracks, rather than other arrangements which record the servo and control information on separate tracks from those which contain data.
Typically, the information contained in the servo fields is recorded by a precise laser/magnetic recording or other suitable device at the time of manufacture of the disk drive. By using the servo field information, the user may then write data into the data fields interspersed between the servo fields, or may access and read data previously recorded between the intervening servo fields.
Proper functionality of a rotating disk computer mass storage device requires that the disk be rotated at a substantially constant speed. Servo system controllers are employed to, among other functions, regulate the rotational speed of the disk. However, due to the practical response limits of a control system, it is sometimes very difficult or impossible to maintain a high degree of precision in controlling the rate of disk rotation. Furthermore, because the frequency at which information is recorded on each track is extremely high compared to the rotational speed of the disk, slight variations in disk rotational speed translate into significant variations in the signalling frequency for writing to or reading from each track. These signalling frequency variations present significant difficulties when attempting to synchronize the logic of the servo system controller to the asynchronous information received from the disk. Thus, despite very effective motor speed regulation control systems, it is virtually impossible to avoid signalling frequency variations.
The problems of controlling the rotational speed and synchronizing the reading of data from or writing data to rotating disk mass storage devices can be particularly difficult in portable computers. Portable computers are frequently used on unstable surfaces, and therefore are subject to being physically disturbed and moved. Each physical movement of the computer applies forces on the rotating disk, and those forces typically cause minor perturbations or variances in the speed of the disk.
Even in circumstances where physical movement of the computer does not translate into variations in disk rotational speed, other factors may intervene. For example, despite the normally reliable and consistent manner in which the servo field information is recorded on the tracks, occasionally there are variations in the frequency or periodicity at which the information is recorded. These variations may cause variations in timing and synchronization even though the computer disk is rotating at precisely the desired rate.
The information recorded on the tracks is in the form of a curvilinear series of dibits, or magnetic field reversals. The strength of the magnetic field reversals is very small, thereby presenting a situation where electrical noise may represent a significant component of the signals induced in a transducer or head of the disk drive. The presence of noise raises the risk that the noise may obscure or corrupt the recorded information. The noise may cause substantial difficulty in correctly detecting and responding to the servo information when controlling the operation of the disk drive. In some situations, noise may be the most or a highly significant detriment to the best operation of the servo system controller and the disk drive device.