aniwolfe; here's the (I think) relevant quote:
"For communication between separate transport and DAC, S/PDIF transmission via coaxial cable became the standard. The clear advantage of I²S is that the four signals of Master Clock, Bit Clock, Word Clock and music data are sent down a cable in separate paralleled conductors. S/PDIF combines them in a serial stream and the DAC receiver must ‘unzip’ the various signals and process them further. Clearly I²S has many advantages. It’s simply curious that PS Audio opted for the HDMI standard as the physical carrier for its I²S interface".
"For communication between separate transport and DAC, S/PDIF transmission via coaxial cable became the standard. The clear advantage of I²S is that the four signals of Master Clock, Bit Clock, Word Clock and music data are sent down a cable in separate paralleled conductors. S/PDIF combines them in a serial stream and the DAC receiver must ‘unzip’ the various signals and process them further. Clearly I²S has many advantages. It’s simply curious that PS Audio opted for the HDMI standard as the physical carrier for its I²S interface".