i am trying to render as 5.1 ac3 in vegas movie studio 17 but there is no option for this my project is in 5.1 so i don't know what else to do.in vegas movie studio14 i get the 5.1 in ac3.please help!
Short answer - it's not available. It's a while since this has cropped up but search the forum and you should find several threads.
From memory the situation is that Vegas (and some other video editors) no longer licence Dolby AC3. So for AC3 Vegas relies on Windows components and Windows doesn't provide 5 channel AC3.
But there may be a workaround. What are you rendering to? Mp4, Blu-ray ....
You can export as wav 5 channel by customising a template. Then if you are using DVDA to create a Blu-ray disk or image get DVDA to convert to AC3 5 channel. It's still available in DVDA.