A digital audio transmitter system capable of transmitting high quality, wideband speech over a transmission channel with a limited bandwidth such as a traditional telephone line. Each of the heat recovery unit and the reheater has a heat exchanger tube, and a circulation line is disposed to connect the heat exchanger tubes. Face image data is extracted and stored in a face image library along with an unique identifier. A current sense circuit monitors the loop current and controls the operation of the shunting circuit in accordance with the magnitude of the loop current. The first optical connector has a connector body, an optical interface disposed within the connector body, a shutter, and a shutter controller that attaches the shutter to the connector body and that permits the shutter to move between a first position that covers the optical interface and a second position that exposes the optical interface. One preferable way to use the bookmark information is to vary the presentation of the web page depending on whether the page is bookmarked or not.