A method and apparatus is disclosed to overcome the effects of intersymbol interference during data transmission. Then, the size in the axial direction of the inner ring 102 is adjusted on the basis of a difference between the first distance \u201ca\u201d and the second distance \u201cb\u201d. An apparatus can comprise ambulatory physiological sensors that can provide a first HF status indicator and a second HF status indicator to a user. A wedge member engages the positioning groove in the barrel and the engaging groove in the switch housing, and has a thickness that gradually increases toward the open end of the barrel so as to prevent movement of the switch housing in the barrel.