A differential data transmission system that transmits encoded data symbols as differential signals. When print data is obtained, the setting device displays a preview of a print page image represented by the print data. The accessory comprises first and second signal conveyers for conveying electrical audio signal driving audio transducers and electrical optic signal driving optical radiation sources, respectively. The third and fourth sealing lips are each configured as a pressure relief valve, and sealing lips are arranged in series. The occurrence of \u201cpopping\u201d sounds caused by differences in DC voltage due to switching between an AGC circuit and an electronic variable resistor circuit can be suppressed.