I don't have Comcast, but I have Dishnetwork and this seems to be the case for them as well. I have two boxes: my HD box (in my living room), and a regular one (in the bedroom). The HD box is the only one that gets Dolby 5.1 if it's broadcast. In my bedroom it may say it's broadcast as that, but it only come in DD 2 channel. My neighbor and I were just complaining about this same thing the other day!