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THE BASICS OF DIGITAL TELEVISION |
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Digital television is here, and it's changing the way we watch and interact with TV. Datacasting, HDTV, Dolby surround soundtracks, and multiple sub-channels of programs are all part of the DTV revolution. You've no doubt heard that your old analog TV "will be obsolete as of 2006." You're probably wondering just how many hundreds - or thousands - of dollars you'll need to spend to enjoy DTV. Maybe you aren't sure if the end result will be worth all the time, money, and aggravation. As one of those "early adopters" fortunate enough to observe and participate in the evolution of DTV, I frequently find myself trying to explain to people just what DTV and HDTV are all about. To help you get started, I've prepared this short fact sheet to answer your questions about digital television and HDTV, and cut through all the "hype" and "clutter" you may be hearing. What exactly is digital television? DTV is a new all-digital system for transmitting, receiving, and viewing higher-quality television images and stereo (surround) audio. What are the goals of digital television? The primary goals are to offer better picture and sound quality through the use of digital signal processing, and to allow the introduction of new digital services such as multicasting (transmitting multiple streams of video on a single channel) and datacasting. The FCC has mandated that DTV signals be freely available over the air in every locality nationwide that has analog television. How do current TV broadcasts and DTV compare to each other? There are some similarities. Both use VHF and UHF broadcast frequencies, although some stations may wind up switching frequencies after the full implementation of DTV. Once the transition to digital TV is complete, TV channels 52 through 69 will be re-assigned to other services. While analog and digital television broadcasts have a modulated carrier wave, the way that signal is modulated is quite different. Analog TV uses an amplitude-modulated (AM) signal for pictures and frequency modulation (FM) for audio, while DTV signals use digital "packets", or bursts of data, to transmit pictures and audio. Three modulation systems are currently being used for DTV:
Combination tuners are already appearing that can support one or more DTV modulation standards. An example would be an 8VSB / QAM tuner for off-air and cable reception, or a DirecTV / Dish tuner that supports QPSK and 8VSB. I've heard that all television broadcasts must be digital by 2006. True? The FCC's plan for DTV implementation commenced in November 1998 with 26 "pioneer" DTV stations. 40 network DTV stations were supposed to be on air by May 1999, with all remaining commercial and non-commercial stations by May 2003. At this writing (03/04) there are about 1200 DTV stations broadcasting, depending on whether you read the NAB or FCC lists. The scheduled date for analog TV to “go dark” is January 1, 2007 – but 85% of TV homes must have access to digital TV before that can happen. Digital cable and satellite TV services already reach a large part of the population, so it remains to be seen when and how the FCC considers the 85% threshold to have been reached. Some industry experts think the digital TV deadline might be pushed as far back as 2015! However, the FCC indicated in March 2004 that they might count digital cable TV signal penetration to speed up the transition and stay on the original 1/1/07 schedule. What will happen to my old analog TV set? Good news! You'll still be able to use it, if it has AV inputs. All current DTV set-top boxes provide at least one down-converted video signal. This is usually composite video or S-video, which connects into the appropriate AV inputs on your TV set. (Don't have AV inputs? Get with the program!) All DTV set-top converter boxes manufactured to date will support older analog TV sets. How many DTV standards are there? Can you explain them? There are four basic DTV picture resolution standards. To start with; our analog TV system makes pictures with about 480 scan lines from the top to bottom of the TV screen that are refreshed 30 times per second. These are actually interlaced (alternating) scan lines - half the picture is traced in 1/60 of a second, and the other half in 1/60th of a second. The DTV transmission standards do include a digital version of this 480-line, 30 Hz signal known as 480i. The new DTV standards also allow us to draw those 480 lines progressively (480p), or all at once; just like a computer monitor. This 480p system results in a picture with fewer motion artifacts and no visible flicker. Both 480i and 480p digital signals are considered to be Standard Definition Television (SDTV), for the images they present are not transmitted with any more vertical lines of picture information than the current NTSC system, and will often have the same aspect ratio (4:3). However, 480p images will appear to have improved vertical picture detail over 480I images, thanks to the use of progressive scanning. Two DTV picture scan rates -- 1080 interlaced (1080i) and 720 progressive (720p) -- are considered High Definition Television (HDTV). In the 1080i system, 1080 picture scan lines are traced from top to bottom as interlaced fields (540 lines in the first field, and 540 lines in the second). There are 1920 pixels (picture-forming elements) on each line. Therefore, the total image resolution is 1920x1080, or just over 2 million pixels. The 720p system scans 720 picture lines from top to bottom in 1/60th of a second, to eliminate flicker. There are 1280 pixels on each line, resulting in a total image resolution of 1280x720 or 921,600 pixels. In the same time interval that one-half of a 1080i image is shown -- or about 1 million pixels -- all of the 720p image will be scanned, or just under 1 million pixels. For this reason, proponents of the 720p system claim that it has the same perceived image resolution as the 1080i system. Who is using each system? At present, CBS delivers its filmed and live HD programs in 1080i, as does NBC, PBS, The WB, and UPN stations owned by CBS. Cable and satellite premium networks such as HBO, Showtime, Discovery HD, HDNet, HDNet Movies, INHD, and NBA TV use the 1080i format exclusively to broadcast movies and special programs. ABC and ESPN HD networks broadcast prime-time programs,
live events, and sports exclusively in 720p. Do I need a special TV set to watch DTV? Depending on the TV set you have now, you may be able to watch DTV simply by adding an antenna or cable hookup, external DTV receiver, or set-top box (STB).
Is the difference in image quality that much better in a digital broadcast? Absolutely! Switching from an analog TV system that is
subject to impulse noise, ghosts, color errors, and "snow"
to an all-digital system will make a tremendous difference. And it gets
even better as image resolution is increased AND the picture aspect
ratio becomes wider. What will it cost me to get DTV and HDTV? Right now, a DTV set-top box for of-air reception will set you back about $350 to $900, depending on the model. (Some more-costly models have hard drive recorders built into them.) DTV-compatible direct-view TVs run start as low as $800, while rear-projection TV with HDTV inputs range from $1,500 to $5,000. You can also watch DTV on a front projector. LCD, LCoS, and DLP models start as low as $1,000 and go as high as $30,000. Plasma TVs can also show DTV signals and the cheapest models start at just under $2,800. 50” plasma monitors can be bought for well under $10,000 – one model is now listing at $5,000. Larger models run to $20,000. (All prices given as examples are suggested retail.) LCD TVs are also popular. 30” and 32” models can be bought for well under $4,000, and there are new models coming out with true 1920x1080 resolution in screen sizes as large as 45”. However, at present plasma TVs are cheaper than LCD in comparable screen sizes. There are other ways to watch DTV. Several companies manufacture DTV tuners that plug into a desktop PC, using its monitor (or an external monitor or projector) to show DTV and HDTV signals. These tuners retail for about $300. If I buy an HDTV-ready set today, will it be obsolete tomorrow? That is a tough question to answer. The modulation standards for off-air, cable, and satellite are established and will not change in the near future. So your integrated DTV set should be just fine for many years. If your set-top box is fairly new, you may be okay there – for a while. The question now is one of copy protection. Hollywood and the TV networks are asking the FCC to mandate copy protection on set-top boxes and integrated digital TVs. This is intended to keep their content off the Internet and minimize piracy. One particular digital interface – DVI – is part of this plan. DVI stands for Digital Visual Interface, and is the preferred connection from any set-top box to a digital TV. That’s because the signal stays 100% digital from the content provider to your set. Another secure digital interface, HDMI (High Definition Multimedia Interface), is coming down the pike and may eventually replace DVI as a secure digital connection. Unlike DVI, HDMI is supposed to carry video, audio, and control signals as a “one connector fits all” solution between components in your system. Yet another system, the so-called Broadcast Flag (officially known as the Re-Transmission Control Descriptor) is being added to broadcast HDTV signals to prevent them from being copied and distributed on the Internet. The FCC ruled in November 2003 that all set-top boxes and integrated digital TVs sold after July, 2005 must be equipped to process the broadcast flag digital bits. Hollywood is even asking the FCC to eliminate the “analog hole” – the ability for someone to take an HDTV signal from a set-top receiver’s analog component outputs and digitize it for copying and distribution over the Internet. There is considerable opposition to this motion, as it would affect thousands of early adopters and older set-top boxes. As you might expect, there is considerable opposition to the broadcast flag mandate and proposals to close the “analog hole”. Advocacy groups like the Electronic Frontier Foundation (EFF), backed by five library associations, Public Knowledge, the Consumer Federation of America, and the Consumers Union, have already sued the FCC to block implementation of the broadcast flag mandate. Will I be able to receive DTV in my area? The FCC has mandated that DTV broadcasts should cover existing analog TV service areas. In many cases, a small antenna (and possibly an amplifier) will be all you need to receive the signals. Field experience has shown that digital TV signals are usually receivable farther out than analog signals. DTV signals also hold up better at much lower levels than analog. If you are on cable, your service provider will have different “tiers” of digital TV channels available. Cable system operators are trying to provide at least five HD channels to comply with the FCC’s wishes to move the digital transition along. These could be a mix of local of-air and premium channels, or the five major networks within a given service area. If you have satellite TV (such as DirecTV and Dish), you'll be able to watch selected DTV and HDTV broadcasts by selecting a satellite-ready STB and subscribing to the appropriate service package – DIRECTV or Dish Network. A new DBS start-up, Voom, promises to carry 39 HDTV channels but is having trouble signing up subscribers at this date (March 2004). One note: CBS/Viacom has made deals with both Dish and DIRECTV to carry the CBS HDTV feeds on satellite. If you live near a CBS station that is owned by the network, you automatically have a waiver to watch these programs off satellite. If you live near a CBS station that is just an affiliate, you must get a waiver from them to watch CBS HD via satellite. Are DVD and HDTV products compatible with each other? As stated earlier, DTV sets with 480p inputs for DVD players can show 480p DTV broadcasts, and may also be able to show 720p and 1080i. Some DVD players are now coming to market with DVI output connectors, which should work fine with current-model DTV sets. Do VCRs, PVRs, and other video equipment work with DTV sets? All DTV sets have at least one composite input (sometimes two or three), and at least one S-video input (possibly more). You can connect your existing VCRs, PVRs, and other accessories to these jacks as you would with a regular TV. DTV sets also have cable-ready analog TV tuners and can be connected to older cable set-top boxes. Combination off-air / satellite set-top boxes from DIRECTV and Dish can tune both analog and digital stations sequentially. A second antenna input may be provided for tuning analog signals off cable, or off-air. (Don’t expect satellite tuners to support digital cable!) What's the difference between the DTV component video standards? There are two different ways to connect an analog component video signal. The first, identified as "YPbPr", is a three-wire connection. It consists of one luminance signal marked "Y" and two color difference signals marked "Pb" and "Pr". The “Y” input is typically colored green, while the “Pb” and “Pr” jacks are colored blue and red, respectively. The second standard is identified as "RGB". This can be a three-wire, four-wire, or five-wire connection, and is similar to that used by your computer monitor. In the four-wire RGBS and five-wire RGBHV formats, there are three separate color signals - red, green, and blue - and either one or two sync signals. Most set-top receivers support both YPbPr and RGB output, although not at the same time. Are DTV signals broadcast on special frequencies? No. DTV broadcasts use exactly the same channels as regular analog television. While many DTV stations are now occupying UHF broadcast channels, broadcasters are allowed to move back to their original VHF or UHF TV channel once the transition to DTV is complete. The only caveat is that TV channels 51 through 69 will be auctioned off for other uses at the end of the transition to digital TV. Stations who originally had analog channels in this band will have to move, no matter what. One potential problem with re-using low VHF (2-6) TV channels for DTV is the possibility of interference from other signals during certain times of the year. "Skip" may bring in distant broadcasts on the same channel and create interference. Impulse noise is also a problem on low VHF channels. What’s more, the physical size of low VHF and high VHF antennas is much larger than that of a UHF antenna. Tests so far seem to indicate that high VHF channels (7-13) are quite well suited for DTV broadcasts, and many broadcasters plan to move back to their high VHF channels at the end of the transition. VHF transmitters also cost much less money to operate than UHF transmitters. Why is it so hard to receive DTV signals in some locations? When it comes to digital television, it's an "all or nothing at all" proposition. Once the signal is acquired, a steady stream of data assures you'll get a perfect picture and great audio. If that bitstream is interrupted, however, there will be nothing - just a blank screen. It’s as if the signal went over a cliff! In areas with lots of buildings and multipath, frequent signal dropout causes this "cliff effect". The fix is to use a higher-gain antenna and perhaps even a preamp - assuming the multipath can be tamed. Fortunately, current model set-top DTV receivers are light years ahead of early models in terms of multipath performance. The key to widespread rollout of digital TV is carriage of local DTV stations on cable TV systems. Today, better than 70% of all US households are getting television via cable or satellite, so you can see how important it is for broadcasters and cable/DSS providers to sign carriage agreements. Is there much to watch on DTV? You bet! CBS, ABC, NBC, and WB carry most of their prime-time shows in HDTV. CBS, ABC, and NBC also carry plenty of live sports programming in HDTV, including college and pro basketball, college and pro football, pro hockey, auto racing, and horse racing. Many of these programs are available with Dolby Digital 5.1 sound. CBS has also broadcast the Grammys in HD with surround sound, while ABC has carried the Oscars in HD for the past two years – also with DD 5.1 sound. PBS is getting ready to launch a 24-hour HD channel. UPN is slowly expanding its schedule of HD programs, and Fox will carry 50% of its prime-time programming in 720p HD starting in the fall of 2004. HBO, Showtime, and other movie channels present a selection of movies and live concerts/specials in 1080i. HDNet offers 24-hour sports and special interest programming including selected Major League Baseball, National Hockey League, and National Lacrosse League games in 1080i. Its companion network HDNet Movies also offers a selection of films in HD. In other words – if you’ve been waiting for “enough HDTV programming to justify buying a new HDTV”, we’re well past that point. It’s time to join in the fun of HDTV! Where can I go on the Internet to get more information about a specific DTV topic and program schedules? There are numerous web-based resources you should check out. Here are a few of them. You’ll probably find others as you surf the web. General DTV information High-bandwidth Digital Content Protection - http://www.digital-cp.com
CBS programming – http://www.cbs.com/info/hdtv/ HDNet programming - http://www.hd.net/
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