Btw, in regards to some of the other discussions: sure there are many good ac3 encoder that can turn 2 channel stereo to ac3 2.0, but the point of programs like ac3filter (maybe) and ffdshow is to upmix to 5.1 at the same time. They do this by real-time encoding SPDIF output and passing it to a file (with graphedit). I DID manage in my attempts to get ac3filter to pass 5.1 PCM audio to a file, but the ac3 encoder just wouldn't work right for me. (You should be able to do the same with ffdshow). From there you could take it to whatever high quality ac3 encoder you'd like and you don't have to settle for lower quality from a faster encoding. It increases the steps but gives you the best of both worlds.
Anyway you are expanding the sound fields and providing discrete channels rather than just compressing a matrixed surround. And by using something like DPL2 to provide the upmix you are improving the quality of the surround for people with non-DPL2 surround systems. You can even tweak the matrix yourself.
Sure I sound like a convert, but the fact is still that I used 2-channel PCM on my releases. Mind you 5.1 PCM is available on HD systems.
BD & HD DVD OT in AVC with PCM 5.1! Can you feel it? (And can you understand my abbreviations?)