Bitstream or PCM? Which does AppleTV output?

Hi all,
I'm still learning the fine points of digital audio, and now that I have an AppleTV, I'd like to find out a few items:
1. Does AppleTV output Bitstream or Linear PCM?
2. Is there a difference in what gets output in the HDMI and Optical ports?
3. Is Dolby Digital 5.1 available in both HDMI and Optical ports, or just the optical?
4. If AppleTV supports both Bitstream and PCM, does it output Dolby Digital in only one of the formats, or both?

Thanks much for all your help!

Carlos