ATSC vs QAM question

[mkenyon2]

So, I have one of the $40 ATSC digital adapters. It will not get any signal off my cable. It specifies in the directions, only ATSC. So, I'm assuming ATSC and QAM are different frequency sets?

If that's the case, can they be combined into one cable? For instance, can I get an adapter that puts my ATSC antennae into the same wire that has my cable's QAM? If so, how does a digital tuner handle that?

[sgilani]

QAM and over-the-air are both ATSC..

ATSC is the digital specification (MPEG-2 video with AC3 audio, etc). QAM and 8VSB are modulation types.

over-the-air ATSC from an antenna is modulated using 8VSB, and cable operators use QAM since they can operate under a higher signal-to-noise ratio and achieve higher bitrates with the same frequency bandwidth.

You can't combine them into one cable because the frequencies will clash with one another.

[mkenyon2]

So, when I plug the cable only into my DTV converter box, why don't I get any channels?

[sub]

Its looking for 8VSB modulated signals but not finding any. The digital cable channels are QAM modulated, so not seen when its looking for 8VSB modulated channels.