AC3: Audio Codec 3. This was the original and more technical name for Dolby Digital. Replaced by marketing mavens when they realized that Dolby's name was not in the title. Some RF modulated, 5.1-encoded laser discs were labeled as AC3. Later versions were labeled as Dolby Digital.
From Googling - "ac3 to" - it mentioned many references to it as a sound element and that is all. It leads me to believe that you can get "ac3" from "avi" - and convert ac3 to wav or many other sound types. There is NO mention of it to another video type.
"How to convert DivX/XviD with AC3 to a DVD with AC3 and subtitles" - this statement also supports the fact that it is a sound element, not a video element.
see AC3 at
http://www.hometheatermag.com/glossary/