Cable systems can handle 750 MHz of bandwidth, with hundreds of channels, video on demand, telephone services, broadband Internet access and interactive programming guides vying for carriage. But even the most advanced cable systems offer only about a dozen high-definition channels to their customers today. Why is that the case? The biggest hog is analog TV channels, which cable operators must continue to carry well past 2009, according to CableLabs, the research and development arm of the cable TV industry. Those linear (continually transmitting, one-way-only) analog channels occupy as much as 550 MHz of the coaxial cable entering homes. Cable companies cannot deep-six all their analog channels and move everything into the bandwidth-efficient digital realm because of the legacy of cable-ready TVs. Millions of viewers plug the cable directly into the analog tuners on their old tubes. Digital cable boxes leased by subscribers do bypass the tuners in their TVs.
So cable operators are faced with the costs of offering a box to everyone or multiple boxes to households that have several TVs. Or another option is that the companies may simply wait well into the next decade, when sufficient numbers of viewers will have finally replaced their long-lasting analog sets with ones containing DTV tuners as well as other so-called conditional-access systems, such as credit-card-size CableCARDs or their software-only counterparts.
One way to manage the pipeline problem is to adjust the transmission techniques. Cable operators have begun employing a modified system architecture called switched broadcasting (also known as switched digital video). A hybrid fiber coaxial cable system brings an optical fiber trunk with seemingly limitless bandwidth to a neighborhood, but then branches to coaxial cable, with its 750-MHz limitation, for the last leg, serving 300 to 500 homes on a node that loops from the fiber. Conventional signal distribution carries every channel in parallel into each home, so that a parent might be tuning into ESPN-HD in one room while a child enjoys MHD (the high-def sibling of MTV) in her room. At any particular time, though, nobody is watching most of the channels on the node.
The new way is to stream only the channels that tuners on that node have requested at that moment. The beauty of the system is that node bandwidth, which was once allocated equally to popular and hardly watched channels alike, can now be freed up to make room for more high-definition channels as needed -- because most people are likely to be choosing from among the same popular channels rather than from obscure options. Part of the reason the cable industry is now talking about a hybrid fiber-coaxial system is increasing competition from the telephone companies Verizon and AT&T, which have begun rolling out fiber to homes (or groups of homes) in select communities. In a couple of years many more households are expected to be able to get TV service from their "phone" company.
Unlike cable's fiefdoms, Direct Broadcast Satellite (DBS) companies (DirecTV and DISH Network) operate nationally and have been digitally efficient all along. Every subscriber uses an external tuner/decoder to receive channels, bypassing the TV tuner. But DBS faces its own bandwidth constraints as channels overall have proliferated, the number of network HDTV affiliates has swelled, and subscribers have increasingly had their local channels beamed to them by satellite.
DBS has been coping in three ways. First, it has added satellites. Subscribers using older equipment who want to get more recently added channels may need to buy another dish or replace a round dish with an elliptical one so they can receive signals from more than one position in the sky. Second, it has increased the use of spot beams as a way to make the most of the available frequencies. Instead of raining down identical bits on the entire continent, a spot beam narrowly focuses channels meant for a particular metropolitan area. Other spot beams are pointed at other cities. Although there is redundancy transmitting Grey's Anatomy on each spot beam, the technology helps to preserve a network like ABC's affiliate system, because local commercials and programs are contained in the signal just as they are if the viewer received the program over the air or by cable.
Last, satellite systems are changing over from the MPEG-2 compression scheme to MPEG-4, which accommodates about twice as many channels; viewers may need to replace their set-top box with one that can decompress MPEG-4. MPEG-4 coding is more efficient because various objects in the same scene can be scaled for different levels of spatial detail. For example, the full resolution for an important foreground object, such as a football, can be maintained, whereas a less important object, such as a group of fans in the background, can be updated at a lower rate. The concept is similar to the perceptual coding technique used for MP3 audio compression, in which, at a particular moment, one instrument masks another; the encoding process discards what is imperceptible anyway to save bits and consume less bandwidth.
The Last Leap No matter how the signal arrives, consumers who want to record their shows may have one more digital hurdle after the transition. A VCR's analog-only tuner will no longer be able to record over-the-air TV channels, although it may be compatible with analog channels still being carried on cable. Cable and satellite subscribers are more likely to lease or buy a high-definition-capable set-top box with a built-in hard drive to record programs at the highest resolution. Alternatives include CableCARD hard-drive recorders such as a new high-def model from TiVo, HDTV sets with built-in hard drives, and -- if viewers want to play their old VHS cassettes in the same device -- DVHS machines that can also record over-the-air HDTV programs.
Of course, occurrences of digital-picture losses and the inconvenience of getting new set-top boxes are worth putting up with, HDTV advocates say, because conventional TV cannot hold a candle to the amazing clarity of high definition or the services it will provide. When February 2009 rolls around, one thing is certain: television will never be the same again.
Digital TV Overview
After many years of anticipation, the end of conventional analog broadcasting now appears to be set for February 17, 2009; after that date, all broadcasts will be digital.
Digital broadcasts offer consumers advantages such as crystal-clear pictures and new information services. Frequencies no longer used for analog broadcasting will be available for communication by emergency response teams or auctioned for other uses, such as advanced cellular systems.
Analog's legacy, however, will continue to dog the new digital era for several years past the cutoff.