I learnt this term CAS (Computer As Source) recently from various local hifi websites and forums. While I enjoy more people are using computer technology for media playback, I read too many mysterious complicated methods. Some of them are even non-related to audio quality. A lot of discussions has shown lacking basic knowledge in digital audio. I decided to give some comments and build FAQ in this topic. This is not a technical paper, but aimed to explain some facts to audiophile in easy to understand perspective.