Hello,
I am guessing that you want to input USB signal out of your PC to a DAC that only has SPDIF (rca) input. This is my limited understanding of the two formats. Someone correct me if I am wrong. Universal Serial Bus uses multiple dedicated signal lines (serial) each carrying different informations whereas SPDIF uses one line to carry all (clock, word and what not) signals in synchronized batches. Because of that a USB to SPDIF conversion requires circuitry that will receive the serial information and regenerate the same information in SPDIF protocol. You will need converters like M-Audio Transit (only optical out) or it's suped up audiophilized versions like Empirical Audio Off Ramp/Freeway or Redwine Audio USB Select to do the conversion. Also, the converter is not a stand alone device but need a PC to process the conversion.
