Why does my HD or Premiere box need a CableCARD to display Clear QAM channels?
Some cable providers include unencrypted channels in their cable lineups. These channels usually are rebroadcasts of ATSC (antenna) channels. If you perform a channel scan, these unencrypted channels (which are also known as "clear QAM channels") appear in your channel list, and your HD TiVo DVR can tune to them, but you will not receive Guide data for them unless you order CableCARDs from your cable provider. Here is why CableCARDs are necessary:
Unlike analog channels, which are mapped to fixed base frequencies to comply with industry-wide broadcast standards, digital channels can be broadcast on any frequency your cable provider chooses, and the frequency can be changed at any time.
Tribune Media Services, which provides program guide data to TiVo, does not collect tuning frequency data from cable providers.
As soon as a CableCARD is installed and activated, the card downloads a channel map from your cable provider that tell the DVR which frequency to tune to in order to display each channel in your lineup. With this information, the DVR can match the program guide data with the tuning frequency.
Without the CableCARD's channel map, the DVR cannot automatically tune to the correct channel to begin a scheduled recording. This means that without a CableCARD, you can only record a program on a digital channel by pressing the RECORD button while the show is playing in Live TV.