A monitor includes a monitor main body, a base member supporting the monitor main body, a monitor hinge supporting the monitor main body to be tilted against the base member within a predetermined angle, and a shield rotatably combined to the monitor main body to shield a gap between the monitor main body and the base member rotating downwardly when the monitor main body is tilted up. The escutcheon includes a control panel mounting surface comprising a lower edge, and a plurality of formations depending from said lower edge and defining a retaining slot. The time interval that a varying input signal requires to change its amplitude crossing and in between the two threshold voltages can thus be detected by two subsequent rising or falling edges of the comparator output without the adverse influence of the comparator’s meta-stability.