17 - Cable and QAM
Cable TV network operators were able to make their own rules for transmitting HDTV over the cable. Some of the early ones experimented with the same 8VSB as the OTA broadcasters. The thought being that HDTVs would have 8VSB tuers built in, so could be ‘HD-cable’ ready. However the cable operators settled on a 
different modulation method called Quadrature Amplitude Modulation, or QAM. QAM combines several levels of phase modulation and amplitutde modulation. If plotted on a graph, all the combinations are called a constellation. A system that has 16 dots in it’s constellation is called QAM-16. Most cable operators are using QAM-256, however there is also QAM-1024.
Cable operators typically use MPEG-2 for compression since most of their set top boxes they rent out only support that.
The important thing to know about installing a digital cable box is that they need bi-directional access just like a cable modem. Most boxes will get an IP address assigned just like a cable modem, and need to communicate with the head end to be provisioned.
Another deceiving failure is signal level. In some cases with older cable system, you might see perfectly clear picture on the analog channels, but have some missing digital channels. This might be caused by excessive loss on the upper frequencies used by the digital channels.
on May 2nd, 2007 at 8:21 pm
Uhhm. that’s a 64-QAM constellation. kthxbye.
on June 21st, 2007 at 8:52 am
I stand corrected! My diagram doesn’t have enough dots to be QAM 256. I guess I should change the caption on the picture now.. Thanks for the feedback