Editor's Note: We are posting this feature from our February 2007 issue because the transition to DTV has been back in the news.

February 17, 2009, is D-day—when the term “digital divide” will take on a whole new meaning unrelated to computer access. That is when the nation’s 1,700 analog television stations will shut down in the long-promised changeover to all-digital broadcasting. Cable and satellite viewers or those whose TV has a digital tuner will be able to watch CSI and American Idol unaware that anything has changed. But the 21 million households using a conventional set with rabbit ears or a rusty roof antenna—typically people who are poor, elderly or living in rural America—will turn on their TVs and see ... nothing.

Not since color made black-and-white sets passé has a new technology so compelled consumers to replace their primary entertainment appliance. In contrast to the variable radio waves of analog broadcasting, digital TV (or DTV) uses electrical pulses to transmit information precisely and efficiently. Thus, DTV offers startlingly sharp pictures, capable of revealing individual blades of grass on a field or the writing on a ransom note held in a fictional detective’s hand. DTV also will enable new interactive features, thanks to its ability to pack enormous amounts of information in scarce bandwidth. For instance, it could let viewers viewers call up stats on a ballplayer during the game or view a recipe from a cooking show. Broadcasters will be able to offer even more channels in the available bandwidth, too. The freed-up space will make room for better communications for public safety agencies during emergencies such as hurricanes and for next-generation cellular services.

Several thorny technology issues remain, however. Even after the end of TV broadcasting as we have known it, analog TV’s legacy will be with us for years to come. While analog-retentive households contemplate the grim reality of no TV at all, others who seem to be digitally outfitted will still experience stumbling blocks. Cable systems, which are already set up for DTV, may not actually be able to deliver purely digital signals, because many of their customers’ older “cable ready” sets still have analog-only tuners. And even in areas where digital is already available, it has experienced service hiccups: picture problems and even clogged bandwidth. Perhaps it is more accurate to say that long after 2009, viewers will encounter a series of digital divides, between analog and digital and among different kinds of digital.

The idea of switching the country to digital TV first gained momentum in Congress during the 1980s, when the legislature became aware that Japan already had high-definition TV, albeit produced by an analog system. (There has been much confusion over the difference between DTV and high-definition television, or HDTV. HDTV is simply the sexier subset of DTV, and it has a few special requirements: more bandwidth to deliver it, extra capacity to store it, a higher-resolution screen to show it and a larger display to appreciate it.) The U.S. government proceeded to run a series of tests with competing manufacturers designed to determine a next-generation broadcast system. The initial results showed that high-definition broadcasting was not practical without compression—and the most important thing to know about digital is that, unlike analog, it compresses video extremely efficiently. So the idea of retrofitting analog broadcasting for greater resolution was abandoned.

The Advanced Television System Committee, working with an alliance of companies that initially had incompatible systems, ultimately succeeded in unifying a standard. The consortium authorized 18 DTV formats, two of which—1080i and 720p—are high-definition and meant for viewing on wide screens. The 1080i format puts 1,080 lines on the screen by transmitting the odd lines, then the even lines, and interlacing them. Hence the “i.” The second puts 720 lines on the screen one after the other—that is, progressively. Hence the “p.” (In contrast, analog broadcasts offer up to 480 horizontal lines.) Debates over the merits of each are confined to broadcast engineers and videophiles, who cite the spatial advantage of displaying more lines (1080i) versus the temporal advantage of showing fast motion (720p). Most viewers do not care.

The first publicly available high-definition broadcasts in the U.S. began in 1998, as HDTV sets started going on sale. In the late 1990s stations were expected to give up their analog frequencies by 2006. But that plan fell through because too few people bought the expensive digital TVs or the set-top boxes for receiving the signals and because high-definition programming rolled out slowly as well. NBC, for example, offered only one series in high definition, The Tonight Show, beginning in 1999. It took another five years before the network showed all its prime-time dramas and sitcoms in 1080i. Broadcasters had little incentive to give up their analog frequencies. Compared with DVD, a home video format introduced just a year earlier that became the most successful new product launch in consumer electronics history, HDTV barely registered in sales or public awareness.

In March 2005, when manufacturers met in Washington, D.C., at an HDTV summit, there was still no definitive cutoff for analog broadcasting. But in early 2006 President George W. Bush signed a law compelling a transition in just three years. What changed? The name of the legislation was a clue. It was called the Deficit Reduction Act of 2005. The government expects to raise billions of dollars when it auctions off much of the 108 megahertz (MHz) of “returned” analog frequencies. Other factors that make analog’s demise more likely this time are that DTV tuners are now mandated in all new TV sets with a screen size of 25 inches or larger (and will be required in all sets, regardless of screen size, by March 1 of this year), most prime-time network series and all major sports events are broadcast in high-def, and most cable systems carry from four to a dozen high-def channels (although that number still represents a mere fraction of their overall channel lineups).

The Long Good-bye  Some argue that the cessation of over-the-air analog TV by federal law will be far more disruptive than when color elbowed black-and-white aside in the 1960s. After all, those silvery tubes continue to work even today. That will not be possible with conventional TV sets unless they have a digital converter box. Adding boxes, which are expected to cost about $60 for a no-frills model, to every TV in any given household could be an expensive proposition.

According to the National Association of Broadcasters, the 21 million homes that receive only over-the-air signals own a total of 45 million TV sets. If you add that figure to the 28 million second and third TVs in homes with cable or satellite where only the primary set is hooked up to the pay service, some 73 million TVs nationwide could go dark on D-day. (Because many of those secondary sets are attached to DVD or video game players, they will not necessarily become useless just because they cannot receive broadcasts.)

To take the financial sting out of the changeover, Congress has authorized the U.S. Commerce Department’s National Telecommunications and Information Administration to spend up to $1.5 billion on subsidies. Starting in 2008, those who cannot afford a box will be eligible for two coupons worth $40 each toward their purchase. To record an over-the-air signal, older VCRs and DVRs will also have to be attached to a conversion box, although by July 1, 2007, any new device sold that includes a tuner (DVD player/recorder, DVR, computer TV card) must include a DTV tuner.

Even if your TV can receive over-the-air digital signals, that does not guarantee you can see the pictures. Analog offers what is called “graceful degradation”: people in fringe reception areas can at least see something, even if the picture ghosts or fades in and out. DTV is not as forgiving. You either get it, or you do not.

Analog TV transmits a complete picture 30 times a second, a lavish waste of bandwidth from a digital perspective. Digital compression, on the other hand, is based on the premise that it is only necessary to transmit reference frames and intermittent changes. That is how the same 6 MHz allocated to a single analog TV channel can accommodate up to six standard-definition digital channels or perhaps one HDTV channel. But it also explains why momentary signal losses can be so disruptive to DTV reception. Without the luxury of analog redundancy, a DTV set may freeze up on the last reference frame it received, or the picture may break into a less than gorgeous mosaic or just go black. There is nothing graceful about any of those glitches, especially if they happen as the killer is revealed during a crime show.

And you do not have to be living in a valley 80 miles from the broadcast tower to feel abandoned by a DTV signal. Multipath reflection continues to be a problem within a city; the receiver gets multiple versions of the same broadcast because it bounces off buildings. The U.S. uses a transmission standard called 8-VSB (for 8-level vestigial sideband), which is more susceptible to signal woes than the European standard COFDM (coded orthogonal frequency division multiplexing).

Today’s receivers can better compensate for this problem than ones manufactured at the dawn of DTV, thanks to more powerful processors. Newer receivers are using higher-performance chips capable of calculating a greater quantity of reflected signals and discarding them more quickly than the processors in earlier-generation DTV receivers [see “The Multipath to Clarity,” by Philip Yam; Scientific American, June 2005]. But viewers may still have to reposition an indoor antenna when they switch to a channel coming from a different direction—a frustrating exercise that recalls the days before most homes had rooftop antennas or cable. While multipath is DTV’s Achilles’ heel, the problem is restricted to a small portion of the 15 percent of American households not subscribing to cable or satellite service. Although there is something to be said for commercially supported “free” TV, more HDTV programming (including premium movie, sports and nature channels) is already available by subscription only than from over-the-air DTV, and the gap will likely increase.

Dual Broadcasts  Until d-day, over-the-air stations will continue to simulcast analog and digital signals on their two allotted frequencies of 6 MHz apiece—the federal government has given stations the extra space for digital during the transition period. The DTV frequency lets broadcasters do something they could not do with analog: multicast several programs at the same time. Whereas a station might choose to allot the entire 6 MHz to high definition, it could instead transmit several channels in standard-definition digital. One standard-definition channel could be accommodated around the clock—even when the station is carrying the network’s HDTV programs—without a significant degradation in quality, the National Association of Broadcasters claims.

Those consumers whose signals travel by cable still will not escape analog after 2009. Operators would like to go all-digital because viewers prefer watching the high-definition version of a channel if available. Not only does an HDTV program’s wide-screen format fit their new TVs’ 16:9 aspect ratio, not to mention their old pair of eyes, better than standard definition’s 4:3 shape, but they also can pick out much more detail. If CSI’s mantra is to follow the evidence, then HDTV’s two million pixels (on a 1920 x 1080 screen) point the way. The rising demand for high definition has, however, already become a problem for cable and satellite operators, who have a finite amount of bandwidth to divide among various channels and services.

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.