I dont get that. I dont have ffdshow as post-processing and i still get ac3, dts, ... passthrough.
What does it add?
I believe that it allows PCM5.1 to be re-encoded as AC3.
This is needed for 5.1 AAC and 5.1 WMV over SPDIF.
Hoborg may have other reasons too, but without post processing you would only get 2 Ch SPDIF for those two formats