I think he's talking about an AC-3 RF input, which is actually quite different than a normal digital coax input. To answer your question, no, the STR-DE845 does not have an AC-3 RF input. You'll need to find a separate RF demodulator (they usually cost somewhere around $100-150US).
Generally, built-in AC-3 demoduation is only found on high-end receivers.
------------------