Modern cable systems have 750MHz to 860MHz bandwidth. Every analog channel consumes 6MHz of bandwidth, so those systems support 125-143 channels, assuming you take away VOD and Internet. A cable system with 60 analog channels is using almost 50% of its capacity on just those channels.
With digital delivery (256QAM modulation), every SD channel takes the equivalent of 0.6MHz bandwidth -- one-tenth as much. A cable system with 60 digital channels is using less than 5% of its capacity.
With digital delivery (256QAM modulation), a 1920x1080i HD channel supplied with full ATSC bandwidth (i.e. maximum quality) will consume the equivalent of 3MHz, while a 720p60 HD channel will consume the equivalent of 2MHz. Full 1080p60 channels -- which don't exist yet outside of the lab -- consume up to 6MHz. At DirecTV's "HDTV Lite" quality levels, you can reduce that by 30-50%.
That's right, the 860MHz cable system in Chicago could offer 430+ 720p60 HDTV channels, 285+ 1080i HDTV channels, 143 1080p60 HDTV channels, or some mix of the three, at full quality...if they were to get rid of all other services. I believe Comcast Chicago is keeping ~20 local channels in analog on their system, so those numbers are reduced by about 15%.
In addition to providing superior efficiency, digital delivery also eliminates picture defects associated with analog transmission, such as excess picture noise, ghosting, and snow. Alll that extra bandwidth can be used for virtually an unlimited number of new SD channels, dozens of new full HDTV channels (none of that "HDTV Lite" crap), as well as HDTV VOD and faster Internet. Said a different way, digital delivery provides the consumer with improved service and more choice.
All this comes at the expense of a set-top box, which many cable companies now include (no extra cost) with a cable subscription. The rub is that some cable providers want you to pay for extra boxes for other TVs. I don't know if that is the case with Comcast Chicago. Ideally, a cable provider would include STBs to support a reasonable numbers of TVs in a household at no extra cost, and if they can't do that, at least decrypt the cable channels in the "digital basic" tier so they are receivable with the built-in digital cable (QAM) tuners found on virtually all new TVs.
In a year or two, this will be a moot point with new TVs. CableCard provides displays with the means to decrypt all cable channels, but most manufacturers don't incorporate it because of cost and usability concerns. Current CableCard implementations -- which use unidirectional OpenCable receivers -- cannot access guide information, interactive services, VOD, or channels delivered using Switched Digital Video (SDV) technology. That will all change next year.
Beginning in mid-2008, you will see televisions start to implement bidirectional digital tuners with OCAP in their TVs. With these TVs, you will simply plug the access card (CableCard) into the back of your display, and it will provide you full access to every channel and service you get with a set-top box. You will get the program guide with the same interface you get on the set-top box. By 2010, they probably won't even sell >$1000 TVs without this functionality.
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Modern cable systems have 750MHz to 860MHz bandwidth. Every analog channel consumes 6MHz of bandwidth, so those systems support 125-143 channels, assuming you take away VOD and Internet. A cable system with 60 analog channels is using almost 50% of its capacity on just those channels.
With digital delivery (256QAM modulation), every SD channel takes the equivalent of 0.6MHz bandwidth -- one-tenth as much. A cable system with 60 digital channels is using less than 5% of its capacity.
With digital delivery (256QAM modulation), a 1920x1080i HD channel supplied with full ATSC bandwidth (i.e. maximum quality) will consume the equivalent of 3MHz, while a 720p60 HD channel will consume the equivalent of 2MHz. Full 1080p60 channels -- which don't exist yet outside of the lab -- consume up to 6MHz. At DirecTV's "HDTV Lite" quality levels, you can reduce that by 30-50%.
That's right, the 860MHz cable system in Chicago could offer 430+ 720p60 HDTV channels, 285+ 1080i HDTV channels, 143 1080p60 HDTV channels, or some mix of the three, at full quality...if they were to get rid of all other services. I believe Comcast Chicago is keeping ~20 local channels in analog on their system, so those numbers are reduced by about 15%.
In addition to providing superior efficiency, digital delivery also eliminates picture defects associated with analog transmission, such as excess picture noise, ghosting, and snow. Alll that extra bandwidth can be used for virtually an unlimited number of new SD channels, dozens of new full HDTV channels (none of that "HDTV Lite" crap), as well as HDTV VOD and faster Internet. Said a different way, digital delivery provides the consumer with improved service and more choice.
All this comes at the expense of a set-top box, which many cable companies now include (no extra cost) with a cable subscription. The rub is that some cable providers want you to pay for extra boxes for other TVs. I don't know if that is the case with Comcast Chicago. Ideally, a cable provider would include STBs to support a reasonable numbers of TVs in a household at no extra cost, and if they can't do that, at least decrypt the cable channels in the "digital basic" tier so they are receivable with the built-in digital cable (QAM) tuners found on virtually all new TVs.
In a year or two, this will be a moot point with new TVs. CableCard provides displays with the means to decrypt all cable channels, but most manufacturers don't incorporate it because of cost and usability concerns. Current CableCard implementations -- which use unidirectional OpenCable receivers -- cannot access guide information, interactive services, VOD, or channels delivered using Switched Digital Video (SDV) technology. That will all change next year.
Beginning in mid-2008, you will see televisions start to implement bidirectional digital tuners with OCAP in their TVs. With these TVs, you will simply plug the access card (CableCard) into the back of your display, and it will provide you full access to every channel and service you get with a set-top box. You will get the program guide with the same interface you get on the set-top box. By 2010, they probably won't even sell >$1000 TVs without this functionality.