Question: is spdif optical or coaxial?
Answer: They are both digital connections. Optical usually refers to the ADAT protocol over a fiber optic cable (TOSLINK), while SPDIF is usually transmitted over a coaxial "RCA" cable. "Usually", optical can carry 8 channels at 44.1/48KHz (or 4 at 88.2/96KHz in certain situations) while SPDIF is stereo (two channels).