Both these responses seem to assume specific receivers. Ie for some receivers, like the
Mercury, you only get PBS audio via an optical port, while other receivers like Lifetime and Ultra you only get PBS audio via a
coaxial spdif port, while other receivers, like the
Twinhan PCI receivers, the computer does all the AC3 processing so you don't need any special AC3 decoder.
I assume that "boz" is referring to a computer card like the Twinhan, otherwise I don't know how he's getting any audio. And for the STBs like "elgemcdlf" is referring to, there are other options besides a surround sound receiver, ie as mentioned in other threads there are converters available on ebay in the $5-$20 range that work well.
Anyway, since the first poster never specified a receiver, I think it might be useful for him to know that his choice of receiver affects what he'd need to get AC3 audio from the PBS channels.