Disk drives typically include one or more disks that define a multiplicity of concentric data tracks. Head position control systems are typically used to move a transducer (head) from a departure track to a destination track location during track seeking operations, to settle the head at the vicinity of the destination track during track settling operations, and to follow the read or write centerline of the destination track during track following operations when data information is written on or read from the disk.
Servo head position information is typically embedded within servo wedges on a disk, which are usually recorded in evenly spaced apart areas or sectors of a track. The embedded servo wedges includes servo head position and track/data identification fields, and typically include a recognizable servo address mark (SAM) pattern which is provided to resynchronize timers for recovering the servo head position and the track/data identification field information, and which mark in time an expected arrival of the next embedded servo wedge. SAM patterns (often simply referred to hereafter as SAMs), in the past, were intended to be unique from patterns that may appear in data or in other portions of a servo wedge. However, that is no longer the case, and patterns equivalent to a SAM may appear in data or in other parts of a servo wedge. Further, a demodulated signal may include a pattern that resembles a SAM pattern because of noise or flaws on the disk media.
Conventionally, a servo demodulator determines when or where to start searching for a SAM pattern by timing from the most recent SAM that was detected. Typically, the servo demodulator searches for the SAM during a timing window, that is centered a pre-determined (SAM-to-SAM) time after the most recently detected SAM, with a width equal to a specified timing-variation tolerance. If the SAM is not detected within the window, then the timing of the search for the next SAM is determined by “free-wheeling,” based upon the last SAM that was actually demodulated. When the next SAM is detected (i.e., the SAM following a missing SAM), the timing circuitry is re-set to begin looking for the following SAM based upon the timing of the SAM just detected. This conventional scheme can typically get though at least one missing SAM, and detect the next SAM (which is hopefully good, and can be detected). However, the servo demodulator may inadvertently detect a SAM pattern in the wrong place. This may occur, for example, because another portion of the servo wedge is substantially identical to the SAM (or due to noise, or media or signal corruption, appears substantially identical to the SAM). If this occurs, the demodulator will begin to search for the next SAM at the wrong time or place. In this manner, a single bad SAM detection could cause the servo demodulator to completely lose lock, adversely affecting the performance of the disk drive. There is a need to decrease, and hopefully prevent, the servo demodulator from losing lock.