Digital HDTV over the air is ATSC.

Digital HDTV over cables is QAM

QAM is the technology used to overcome bandwidth issues and "squeeze in" larger number of digital channels on one carrier by managing each channel's bandwidth and video/audio frequencies. To view Digital TV in High Definition (720p, 1080i, 1080p) and correct color depth the QAM processor require large memory cash. This digital HDTV CABLE transmission standard is the 256QAM. The other known 64QAM is used for STANDARD (480p) digital transmissions only and is not high definition.

The FREE HDTV channels, non-premium and unencrypted transmission over cable, are called "Clear QAM". Some people misunderstand Clear QAM and complain they can not get HBO with a certain 256QAM tuner.

The problem is not the tuner but the way the cable operator is transmitting the channel.

Paying for the Premium service is not enough. The channel is still encrypted. Only the specific cable operator box can remove the encryption.

For instance: Pioneer Cable Box that will remove the encryption from Time Warner HBO channel will fail when used with Comcast HBO channel. Each cable operator has its own encryption method and keys.

Free QAM channels line-up may be different between regions of the same operator, in accordance with the cable operator's marketing plans.