With the latest SA/Cisco S26 Guide software, with the STB turned off, you press the "HD Zoom" button on the remote to bring up the HD Setup Wizard. See this Comcast guide about it:
Preparing Your HDTV for HD»
info.xfinity.com/Comcast ··· etup.pdfAlso see this previous post of mine: »
Re: [STB] Component Versus Composite Cables With RNG110