(For my house)

I have a HDTV reciever w/ digitial sound out. I have a soundblaster live! 5.1 card w/ a digital input add-on card. I have my output from the reciver going to the input on the sound card add-on. When i set the reciever to PCM sound, it plays thru the pc speakers, but when i set it to dolby digial, it doesn't. I only have regular spearkers hooked up right now, but i am going to add on 5.1 speakers soon. What's the deal here? Is PCM sound 6 channel, digital?