A headrest support system comprises a seatback support frame having an upper cross member adapted to support a headrest. The control circuitry comprises clock adjustment circuitry configured to generate a control signal for adjusting a parameter of a clock signal based at least in part on timing information obtained by detecting a timing pattern on a surface of the storage disk. The optical coupling lens includes a first converging lens and a second converging lens. According to this manufacturing method, the formation of the gates having a MOS structure is performed such that the gettering layer getters dissolved oxygen present in the silicon substrate. An off-format condition can be detected by low pass filtering a tracking control signal, detecting a DC level over a threshold level, and indicating the off-format condition if the DC level is above the threshold level for a maximum number of cycles.