Q. I just signed up for Comcast’s TV service and the customer service guy said I could start watching my channels before the installer came, using the Xfinity app on my Roku. But I keep getting an error message when I try to watch one of the channels on the app. Can you help me? — Burke, Boston.

Burke, as you note, Comcast has an app on Roku called Xfinity Stream that allows you to watch the same channels that you subscribe to via your set-top. For example, if have the set-top connected to a living room TV, you could watch your Comcast programming lineup on another TV by simply connecting the Roku to the second set.

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This a nice feature for Comcast subscribers who want TV in multiple rooms without paying for additional set-tops. The Xfinity app does not require an additional fee. (Note: The Xfinity Stream app is also available on smart phones and tablets.)

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However, Xfinity Stream requires two things to work to watch live channels:

1. Internet service from Comcast;
2. One Comcast TV set-top installed and connected in the house.

You didn’t mention if you subscribe to an Xfinity Internet plan, but it’s needed to send the signals from the set-top to your Roku box, using your home wireless network.

On point #2, it sounds like the Comcast customer service representative got a bit ahead of himself. You can’t start watching live channels on the Xfinity app as soon as you sign up for TV service. You actually have to have the service (and set-top) installed. After that, you should have no trouble accessing your subscription channels on Roku.

Last point: The Xfinity app is available on the following Roku devices:

Ultra (4660), Ultra (4640), Premiere + 4630, Premiere (4620), Express + (3710 and 3910), Express (3700, 3900), Roku TV (5XXX, 6XXX: Models within the 5000 and 6000 range), Streaming Stick + (3810), Streaming Stick (3600 and 3800), Roku 4 (4400), Roku 3 (4200, 4230), and Roku 2 (4210).

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Have a question about new TV technologies? Send it to The TV Answer Man at swann@tvpredictions.com. Please include your first name and hometown in your message.

— Phillip Swann