Using built in QAM tuner

There are no "channels" with a composite (yellow video, white/red audio) connection. Composite is a "line input" not "channels."

If you connect the Comcast cable converter box RF (threaded) output to your TV and run the channel scan you'll find one "analog" channel, 3 or 4, corresponding to the "output channel" of the Comcast converter box.

If you connect the raw Comcast coax cable to your TV and run the channel scan you'll find whatever clear QAM channels are being provided by Comcast. In my area Comcast is currently providing more than one hundred clear QAM sub-channels. These clear QAM sub-channels are generally those found in the "expanded basic" tier of service.

Cable company converter boxes "map" cable "services" to "locations" according to that cable company's numbering scheme. The same services received by a clear QAM tuner-equipped device will appear on sub-channels that may have no similarity to cable company assigned "locations."

Cable company provided converter boxes or Cable Cards are required to receive "scrambled" services. Comcast's basic "digital to analog converter" is the Pace DC50X Digital Transport Adapter (DTA) that provides only an RF output and no "scrambled" services.

To try to keep it simple...

Get a splitter. Take the primary feed and connect to the input of the splitter. Connect one output from the splitter to the TV the other to the "cable box".

On the TV perform a channel search/scan and you should be able to get digital unencrypted channels, typically "locals" which should be HD, but not all channels. (just so you know )

Use the cable box as you have been for all the other channels that your TV's internal tuner doesn't provide.

Hi - I'm faced with the same problem - don't I also need a coax A/B switch to switch between the TV QAM HDTV stations and the Comcast On Demand Digital Box? I have only one coax input on my Samsung HDTV, but have S-video, composite video and component video inputs to select from as TV inputs?

Nope. You really only need the splitter.

You'd run one feed into the RF input on the TV for the clear QAM, and the other to the cable box, which you should connect through a line input (HDMI/Component if the tuner's HD capable, and S-Video if not - only use composite or RF if you have no other choice - in that order).

Then you just switch between inputs to watch either.