The supported formats by S/PDIF are
- Stereo LPCM
- Dolby Digital 5.1
There is no notion of "content protection" for the low-quality S/PDIF audio. And look at the block diagram I posted (or download the datasheet of ALC889). The decryption module is connected to only DAC (digital-to-analog conveter), not to the S/PDIF transmitter.
BTW Realtek ALC889 and NVIDIA HDMI audio have nothing to do with each other. The motherboard (and any other GeForce 9300/9400/8200/8300 motherboard) has two independent HD audio devices:
- HD audio controller in the chipset with ALC889 chip (called "Realtek Digital Output/Speakers")
- HD audio controller in the chipset with HDMI transmitter in the chipset (called "NVIDIA HDMI Output")
I am not sure if these HD audio controllers are shared or not.