Sequence, timing and synchronization technique for servo system

Abstract

A servo system controller for a disk drive device which includes a rotating mass storage disk incorporates a timing error determining circuit for determining rotational speed variations in the disk from information contained in a servo field recorded in a track. A timing error signal is supplied when the speed variation exceeds a threshold, thus indicating an unacceptable level of speed variation. A digital synchronizer responds to asynchronous servo data signals from the disk to establish a cell clock signal occurring about a normal frequency, a cell during which only one servo data signal will occur, and a cell data signal which occurs during the cell. The cell clock signal clocks logical operations on a synchronous basis, and servo data signals are reliably captured. A concurrence determining circuit determines the degree of concurrence between the predetermined series of cell data signals and an actual pattern of signals derived from the servo field, and recognizes the servo control information from less than complete concurrence of the derived pattern and the actual expected pattern. Normal operation is permitted even through slight inconsistences may be detected, improving data throughput while still achieving an adequate level of error tolerance.