A roller adapted to be rotatably mounted on a bushing of each link assembly of a chain comprises a split roller composed of two roller segments assembled together. Computed network jitter are averaged to determine the average network jitter. The video effects generator can produce at least one user-initiated visual effect altering an image transmitted by the telephone accessory. Since the sensor cell and the monitor cell have different oxygen reactivities, an output of the monitor cell is smoothed through a band-pass filter or the like so that output responses of the sensor cell and the monitor cell are made to substantially match each other, thereby eliminating any deviation in output.