By default, the Rosetta is set for ADAT optical. If you want to change it to S/PDIF optical, hold down the "Sample Rate (AES FMT)" button. On the Source to Digital Outputs, the ADAT and AES LEDs should be lit. Hit the "Source (SRC)" button to change it to "S/P OPT." If you want to go back to ADAT, just do the exact same procedure again. Keep in mind that if you reset the Rosetta 200 by holding down the sample rate button when powering the unit on, it will reset the optical format to ADAT.