A method of processing supplementary content in a digital receiving apparatus, includes connecting to an external device being different from a broadcaster; receiving uncompressed audio/video (A/V) content from the external device being different from the broadcaster; extracting audio watermark from the uncompressed A/V content, wherein the audio watermark includes domain type information, time information and event flag; constructing a first uniform resource locator (URL) based on the audio watermark, transmitting a request to a remote server based on the first URL; receiving a second URL for the supplementary content from the remote server, and presenting the supplementary content based on the second URL.