The problem is the cable box manufacturers, and the content providers (Comcast in your case) don't want you trying to "place shift" your viewing. They do this by turning on "High-bandwidth Digital Content Protection" (HDCP) which is a form of digital copy protection to prevent copying of digital audio and video content as it travels across connections. So when your TV is off the cable box thinks there is no reason you should be trying to stream your content. To get around this when using a Slingbox, you can use the component video cables (RGB) and separate audio cables. The issue is some of the newer Comcast cable boxes don't have component cable outputs. You can also try an HDMI to component converter. I also have read some people had success just changing the video output of the Comcast box from 1080p to 1080i.