Optical DD input to Analog 5.1, Possible?

As the badly drawn diagram below shows, I'm attempting to decode either a Dolby Digital 5.1 or DTS input, and then subsequently output (on the same card, a Montego DDL) the 5.1 stream. I've yet to find a way to do this, so I'm coming to you.

It appears, without a dedicated hardware decoder, my only option is two-channel AAC.

I believe your conclusion is correct. The problem is with the real-time translation of the bitstream signal into analog for the 5.1 .

That's not an easy feat and as you've likely learned requires independent hardware to handle the task.