Pat Cook said:
Not true in this case. NFL Network CAN be seen on both channels AND is listed in the lineup as being on both channels. No virtual addressing involved (If there is, it likely involves a 3rd channel)
*sigh...*
Once more, with feeling: there's no such thing, really, as "channel 168" or "channel 417" on a digital cable system, any more than there's really any such thing as "boards.radio-info.com."
Those are virtual concepts - addresses you punch in, if you will, that direct the device you're using (be it a cable box or a web browser) to look up in an address table where to actually connect you to.
Here's what's actually happening under the hood:
You know all those analog channels that went away on your system at some point, or perhaps the uppermost analog channels that were never occupied?
Each of those analog channels was 6 MHz wide and carried a single channel of SD video. Horribly inefficient, because it meant that a typical 750 MHz cable system could pass at most 125 channels of video, though the real-world limit was usually about 75 or 80 for reasons that are off-topic here.
But each of those 6 MHz slices, if used digitally, can carry much more: 3 or 4 HD channels or as many as a dozen SD signals. Much more efficient use of limited spectrum, of course...but then the question arose of how to identify all those channels to the subscriber.
Let's say, hypothetically, that the NFL Network is one of ten services occupying the space that used to be...oh, say, channel 75. That's 528-534 MHz. (You can find the whole chart here -
http://www.eaglecomtronics.com/Cable_Television_Channel_-_Frequency_Chart.pdf - but note that it goes up much higher in frequency than just about any real-world cable system can pass.)
The cable company could have labelled the digital feed of NFL Network as "75.3" or "75.7" or something like that...but cable companies have very good reasons to not want to tie their channel numbering into physical RF channels anymore. They move services around all the time behind the scenes, whether for technical reasons (analog cable "75" overlaps frequencies with UHF broadcast channels 23 and 24, so perhaps you might need to move digital services off that channel if there's strong ingress from a local DTV on 23 or 24) or other internal needs. And they don't want to have to constantly notify customers of new channel lineups if they don't have to.
Enter the virtual channel! With a modern digital cable box, the cable company can keep its labeling consistent and logical, and can even make a single physical RF stream appear as multiple virtual "channels" if there's a reason to do so. On my Time Warner Cable system, for instance, all the news channels appear grouped together in the 100s on digital - CNN on 101, HLN on 101, CNNI on 102, MSNBC on 103 and so on. But for those viewers accustomed to the old analog channels, they also appear on their old analog channel numbers on a digital box - CNN on 20, HLN on 21, MSNBC on 48, etc. It doesn't matter whether I select "48" or "103" on my digital box: all that does is send the box to a lookup table (which can change on a regular basis, without my needing to know about it) that tells it that the stream it's really looking for can be found on QAM 78.2 or 83.24 or wherever the engineers at TWC might need to put it.
You don't need to know
any of that to watch TV. The cable company doesn't want you to know: those QAM channels are all scrambled, anyway, so even if you had a device (a QAM-capable digital TV, for instance) that could tune directly to "78.2" or "83.24," you probably wouldn't be able to decode it. (Most cable companies do pass the local broadcast channels as unencrypted "clear QAM," at least; on the cable-connected digital set in my kitchen, I can see the local access channels and the local broadcasters and sometimes one or two other random signals in the clear.)
Here's an actual example of what one Comcast customer in Aurora found on his QAM lineup:
http://www.silicondust.com/hdhomerun/lineup_web/US:80015#lineup_8259642
You can see from this, for instance, that what you might punch up as "channel 654" for KCNC-DT is actually running on the cable (at least on the day this scan was taken) as one of several services occupying the old cable channel 64.
If Comcast wanted to "duplicate" KCNC-DT as "1004" or "224" or even to have HD boxes automatically point to the HD stream when you punch in "4," all it has to do is update its internal channel map to also point those virtual channels to "64-2." That's all it's doing when it "duplicates" NFL Network on "417" and "168," or NBA TV on "416" and "439."
And in some cases, when a cable company wants to offer more HD channels than it has the physical RF bandwidth to carry, it turns to something called "Switched Digital Video," or "SDV." With SDV, there is
never a fixed channel on which the program you want to watch is being constantly carried. My system does this, for instance, with MSNBC HD. There's a block of RF space set aside for SDV channels, and when I punch in "1074" on my cable box, the box tells the cable node in my neighborhood to use one of those RF channels for MSNBC HD. It could be a different physical RF channel every time - maybe it's 82.9 one day and 86.14 the next...but
it doesn't matter, because the box knows where to find it, and all I know is that I'm watching "channel 1074." (Except when lots of people are watching SDV channels at night and there might not be an open channel available, in which case I'll occasionally get an error message. Hate when that happens.)
Even more advanced boxes might just be pulling a video-over-IP stream from a server at the cable headend, without modulating it as an RF video signal at all.
Is this making sense now? The "channel numbers" you see on your digital box are just marketing. They have no fixed relationship to any physical RF channels, the way the old analog cable systems did. If Comcast wanted to use "channels" 2 through 12 and then leave a gap all the way to "channel" 934, they could do that - and it wouldn't tell you anything at all about how much of the system's actual capacity is being used, or for what.
If they don't carry Tennis Channel, it's not a question of whether there's "space" available for it at "407," it's purely a question of whether Tennis Channel's owners have reached an amenable business deal with Comcast to carry the service. If they carry Versus at "674" instead of "409", that, too, is purely a business decision, not a marketing one.
Same deal with "sharing" Goal Line and Altitude 2 and Red Zone on a single "channel" - it's entirely possible that the bandwidth that's used to carry Red Zone on game days is being used for something else entirely during the week. And if Comcast did want to use that bandwidth to carry Goal Line or Altitude 2, it could use that very same bandwidth but label it "419" or "420" or "1657" if it wants to...and only your box, and the cable company engineers, will ever know.
It's a brave new world we're in, isn't it?
73 de sf
