Originally Posted by impala454
I don't think this is correct. OTA is broadcast at full bandwidth, and that could be the case with the cable channel, although the signal passes through a lot more equipment before arriving at your home. The QAM cable signal will use the same amount of bandwidth whether you're transmitting 9Mbps or 1305723057023570Mbps. It does not necessarily indicate that the original signal was that high.
OK, let my try to explain a little different way.
The term "bandwidth" when talking about channels means the channel width - in all cases in the US, that is 6Mhz. Analog and digital TV stations OTA have a 6Mhz bandwidth. Cable channels also are 6Mhz. A single SD analog station takes 6Mhz, and a digital QAM occupies 6Mhz.
There are different modulation techniques used for OTA (8VSB) and cable (QAM) channels. ATSC is capable of carrying around 19Mbps data rate. With various "overhead" elements it is claimed that the actual "payload is about 18Mbps. So, if a ATSC station devotes it's entire payload to a single HD program, then that program can be up to 18Mbps. Each SD digital sub-channel typically takes 2-3Mbps. So if there are 3 such SD subchannels, that will leave 9-12Mbps (usually on the lower end) for the HD. That's the case with many PBS stations.
QAM is twice as efficient (doesn't have to worry about ghosts, etc.), so can carry a payload of about 38Mbps. Many cable systems put 2 HD channels on a single QAM (they do here), giving each channel about 18Mbps. Of course, the topic of this thread is that Comcast is putting 3 HDs on a QAM, thus carrying only about 11-12Mbps.
Now if the local PBS station feeds cable via fiber and sends them the full bit-rate, and cable is only putting 2 HD channels per QAM then that PBS hd channel will go out to cable customers at about 18Mbps. Yet, the same station, because of the 3 subchannels is sending out the HD signal OTA at only 9-12Mbps.
That's correct, and happens more than you think. Many stations feed their local cable head end via fiber or microwave so that their cable viewers still get the signal if the stations transmitter has problems.
Now, in regards to "the signal passes through a lot more equipment before arriving at your home". That is true. But when talking about QAM channels we are talking about DIGITAL, and (up to a point) degradation of the signal from all those amplifiers (and there are less than you think in a modern cable system - see my next post) does not effect the quality of the received DIGITAL signal.
I do hope that this clarifies the situation. Just keep "bandwidth" and "bitrate" separate.