A calibration method for adjusting an offset voltage of a unit is proposed. The combination provides the stack with high resolution reading capabilities without the necessity for a narrow read gap formed by closely spaced top and bottom shields. This blocking device allows rotation of the crown to be blocked in a first direction, and to be permitted in a second direction, in a predetermined position of a control lever selectively acting on the first or second of these spring blades through a sector. Each vertical framing section can be made from a plurality of second framing members. Again, film mode may be detected by monitoring this second assessment from field to field.