If your TV cannot tune them they are either scrambled or the TV has a
poor tuner. My Panasonic tunes all the clear Comcast channels, my Sharp
does not. You would need a QAM tuner. Comcast where I am uses different
types of QAM at the same time. Some TVs and devices (HDHomerun tuner)
will tune them all simultaneously. Apparently other tvs will only tune 1
flavor at a time.
A TivoHD will record the unscrambled channels. All of them, provided
they're actual QAM channels. If they require a tuning adapter you can
usually get one for a Tivo. For the scrambled channels you'd need either
two S or one M cablecards, and the appropriate access added to your comcast
account.
The Tivo can be configured to use both cable and an over-the-air antenna.
It can record two shows at once from either cable or OTA.
-Bill