Does ATSC QAM mean it has a built in HD tuner ?

Over-the-air and cable HD transmissions use different modulation schemes. ATSC (Advanced Television Systems Committee) is the standard for over-the-air transmissions and uses a 8 VSB (vestigial sideband) modulation scheme. As someone else pointed out, cable systems use QAM (quadrature amplitude modulation).

Bottom line is that the set you saw in the ad can receive both over-the-air HD transmissions and unencrypted cable HD transmissions.

Thank you all for the help!

Comcast, with exception of a very few analogue channels still being broadcast that must for NSTC form, both scrambles and encrypts all its digital channels that are transmitted in QAM format. No way to look at digital transmision with one of their boxes.

One other thought on receiving unencrypted QAM channels. Most cable companies don't provide mapping information for their digital cable channel numbers.

I use Cox in New Orleans. For example, CBS HD is mapped to Cox channel 704. If I want to watch it as an unencrypted QAM I have to search for it and remember the native channel number. Last time I tried this it was 53.110. The cable companies frequently move the QAM channels without warning. It can become a pain to periodically rescan and hunt down your favorite channels. The cable's set top box or Cablecards provide the mapping to the stable cable-assigned channel numbers (like 704).

The FCC requires that all cable carriers transmit network HD signals as clear QAMs, but they don't have to make it easy.