Since the OP is not using a Mac, this is more general info but according to the
Soundtrack Pro specs, you can "Export mixes as AIFF, WAV, NeXT, SDII, MP3, AAC/Podcast, Dolby Digital Professional (AC-3), or using Compressor." I don't have STP installed on the machine I'm on at the moment so I can't say for sure how it works, but it does come bundled with Logic Studio. I don't think Compressor does, though.
Can someone else verify this? I wonder if these specs are outdated and based on the fact that previously STP came bundled with Final Cut Studio which also comes bundled with Compressor. I wonder if you need Compressor to do the AC-3 export from STP?