SPDIF-out through optical from my PC. However, for some reason my PC isn't seeing that I connected anything to the optical out. I have Realtek HD Audio with R2.73 drivers. Usually in windows if you choose properties under your sound devices it will show the jack information and which one is connected to a device. It just isn't seeing the Bifrost. I might try installing my EMU 0404 PCI card and use that for optical SPDIF.
There's no way for your Realtek card to know what is connected to it by S/PDIF. It's not USB. Just stream the audio to Realtek S/PDIF device and your Bifrost will do the job.