253

DIGITAL TV (DTV): THE STUMBLING REVOLUTION

Which came first, the chicken or the egg? And what daring soul opened a gas station when automobiles were first invented? And who took the first long drive when there were no gas stations? All of this leads us to the catch-22 of digital television (DTV): Who's going to fork over a couple kilobucks for a snazzy new DTV receiver if there are only a few hours of shows being broadcast? And what advertiser is going to pay to broadcast DTV to an audience of a few hundred techno-yuppie viewers? DTV is definitely coming, but adapting to it when it gets here promises to be a thrill ride.

It was easier to start television than it will be to change it. In 1929, experimental television was in the hands of ham radio operators and electronic tinkerers (counterpart to today's computer nerds) and not much was at stake. By 1935, the Radio Manufacturer's Association developed standards so that TV sets could be mass produced and they would all work the same. By 1940, the National Television System Committee (NTSC) under the sponsorship of the RMA presented the Federal Communications Commission (FCC) the television standards that, except for a few minor changes, have been in use ever since. That old system has carried us a long way. Even the addition of color and stereo sound hardly changed the basic way television signals were produced and transmitted. But our venerable old friend, analog NTSC television has stretched about as far as it can go. New technologies, especially the digital ones, promise to offer wider, sharper pictures with better sound. The only problem is, we have a multi-billion dollar investment in the old system. Unlike television's early days, many powerful entities have a financial stake in how this revolution turns out. There are advertisers who demand a substantial viewership before they'll fund digital broadcasting. Broadcasters will have to spend millions of dollars converting to a new system which may not earn them any additional income for a long time. About one hundred million working TV sets will get tossed into the garbage, replaced by expensive new DTV sets. Yes, it's harder to change something than it is to start it.

What Is DTV Anyway?

First, a few things DTV is not. It's not DV (Digital video as found on a digital camcorder), DVD (Digital Versitile Disc), digital satellite , or digital cable TV. Yes, these technologies are all digital and involve video, but they're not the specific type of digital TV described by the "DTV Standard". DTV is more than just digitized video, it is a digital television signal manufactured a particular way. Now that we've cleared up that recurrent source of confusion, let's go on to what DTV is.

But first (another first?), a little history. Man, a creature ever striving to improve things, has long diddled with the idea of making TV pictures sharper and shaping the screen wider than its boxy 4:3 shape. In 1973 high definition television (HDTV) was first demonstrated. The HDTV screen was 1/3 wider, giving it a pleasing, movie-like 16:9 aspect ratio.

The HDTV picture was created using 1,125 horizontal scanning lines (as opposed to NTSC's 525 lines) and the HDTV picture had more detail in each line, 1920 pixels (picture elements or dots) as opposed to today's 720 pixels. I saw one of those demonstrations and it blew my boxers off. The picture was utterly smooth and the extra sharpness made it delightfully realistic. It wasn't television anymore, it was a medium alltogether different. After watching HDTV for a while, when I looked back at normal NTSC screens, it felt as though I was going back to a manual typewriter after using a word processor. Once you've seen HDTV, you love it. It's addictive.

One problem with HDTV: There is so much detail in the picture that it requires very high frequencies to transmit the information. In the analog world, it would require the equivalent of five TV channels to broadcast just one HDTV image. This is where digital television (DTV) has come to the rescue with its mighty compression schemes that can fit six digitally compressed TV channels where one analog channel used to be. Or, that can fit one digital HDTV channel where one analog channel used to be.

How DTV Works

Analog TV signals consist of vibrating electronic waves. Every detail in a picture is represented by a vibration. The more details you have, the more vibrations the wave must make. There is a limit to how many vibrations you can pack into a 6 MHz TV channel, which explains why HDTV doesn't fit in normal analog TV channel allotments. Analog TV has other problems too. Little errors creep into the signal causing colors to shift. Noise and other interference mixes with the signal causing the picture to become grainy. When the signal becomes weak, it gets snowy. When the TV signal bounces off buildings and mountains, the reflected signal interferes with the original signal, causing ghosts. When the signal is recorded and played back, imperfections in the machinery degrade the picture even more. DTV solves many of these problems. Video's vibrating wave is electronically sliced into millions of tiny slivers, each sliver is converted into a number, and from then on, only numbers are transmitted, recorded, and manipulated by the equipment. Interference, noise, and machine errors could make those numbers fuzzy, but still they are the same numbers, and as long as they are discernible, they can be accurately converted back into the original picture with no loss of quality.

Now for the big bonus: Digital data can be compressed. In the analog world, if you had a picture of a bison in front of a blue sky, that entire image would have to be electronically redrawn every sixtieth of a second; millions of video vibrations would be repeated even if nothing in the picture moved. In the digital world, the circuits would look at the picture and start calculating shortcuts. They'd start with the first pixel in the upper left hand corner which happened to be a dot of blue sky. The machine would assign a number such as 0 units of red, 2 units of green, and 255 units of blue. These three numbers would represent one dot of deep blue sky. What if the next dot is the same color and brightness as the first one? You could repeat all the numbers, or using compression, you would just say, "ditto." If the whole scan line across the TV screen were the same color blue, the machine could say "ditto for the whole line" rather than listing the exact numbers 700 or more times. If nothing moved in the picture, then the digital data one sixtieth of a second later would be a duplicate of the previous data. Again the machine could say, "ditto for the next picture," saving millions of numbers from being encoded, recorded, or transmitted. Although I've oversimplified my explanation, the idea of digital compression is to have electronic circuits notice similarities in pictures and parts of pictures and to use shortcuts to represent them. If the bison is walking across the plain, the compression system can simply keep track of the parts of the picture that change as the bison moves, saving oodles of data. The compression system also takes into consideration that our eyes do not see sharpness very well when things are moving. Thus, the still parts of the picture are given more detail (more data) while the moving parts of the picture are fudged --- but we don't notice it. The DTV compression method is called MPEG-2. MPEG stands for Motion Picture Experts Group.

The same trick applies for TV sound. If you've ever listened carefully to your stereo, you've noticed that what comes out the left speaker is very similar to what comes out the right. It would be possible to digitize the left channel and instead of digitizing the right channel in its entirety, only digitize the differences between the left and right channels. Thus you'd have a whole lot of data for one channel but just a little data for the second channel. Using this technique, called Dolby AC-3, DTV squeezes 5.1 channels of audio (5 channels of surround sound plus one subwoofer channel) into a measly 382 kilobits per second (256kbps for two-channel SDTV).

Thus through the magic of digitization and compression, we can make HDTV fit in the same channel space or bandwidth used by analog TV. In fact, there is some space left over for transmitting additional data such as computer text, internet data, or extra sound tracks.

Money Drives the System

Slip your eyeglasses down your nose and pretend you are an accountant for a moment. Your TV station has just adopted DTV, and let's assume the viewing public has purchased DTV receivers. Your choice, Mr. Beancounter, is: Using the magic of compression, should I transmit one beautiful-looking HDTV image to my viewers or should I transmit six standard definition TV signals (SDTV) over the same bandwidth? One HDTV signal will look better, but six average looking SDTV signals offer the option of presenting six shows at once, collecting income from six sponsors. Which technique will derive the most income: One HDTV program or six SDTV programs? If six times as many people will watch the HDTV program as will watch the SDTV programs, you'll break even; the sponsors will pay six times as much to reach six times the viewership. The sixty-four billion dollar question is, will six times as many viewers watch? Current studies have shown that the American viewers are not clamoring for sharper, wider TV pictures.The American public complains bitterly about TV, but never about the quality of the picture.

Maybe if the public could see HDTV, they would get more excited about it, but would they get six times more excited? Add to this equation one of the realities of technology; a true high quality HDTV receiver will cost a lot more than a simple SDTV receiver. HDTV sets currently cost about $8000 and are expected to come down to $2000 in later years, but that's still a lot of money. Will Charlie Couch Commander blow $2000 for an HDTV set when he can buy an SDTV set for maybe $500? The tough question for Beancounters everywhere in the television industry is: Does the public want HDTV badly enough to pay for it?

Enter the FCC, Stage Left

Nobody is going to start manufacturing TV sets without some guarantee that they will actually work with the signals the broadcasters will be using. The FCC's job is to lay out a set of standards that everyone will follow so that the DTV signals are the same for everybody and won't change after people have made an expensive investment. The FCC has failed miserably at selecting a single system for DTV. You can hardly blame them, though. Unlike the 1930's, today's world of television is beset by powerful forces with scillions of sheckles at stake. Each of these forces has a good reason for having DTV take a certain path.

Purists would like to see HDTV in all its glory. This means 1125 vertical lines, each line with 1920 pixels, shown on a TV screen with a 16:9 aspect ratio, and the picture would repeat 30 times per second in its entirety. I must digress for a moment and explain last part. Presently, television pictures are interlaced. If you think of the TV picture as a bunch of scan lines, sort of like the slats in a venetian blind, the TV picture is painted on that venetian blind and each slat (each scan line) holds a piece of the picture. Presently, in one sixtieth of a second, the odd numbered scan lines (the odd numbered slats in the venetian blind) are drawn on your TV screen. In the next one sixtieth of a second, the even numbered scan lines are drawn on the screen, inbetween the odd lines. Your eye doesn't notice this but it is seeing only half of the picture (the odd or the even scan lines) at any moment. Thus, every thirtieth of second, you are presented the entire picture. The interlace system was invented to reduce the amount of data that had to be recorded and transmitted over the air. One problem with the interlace system is that the inaccuracies of placing those odd and even lines make the picture a little fuzzy and less bright.

Another way of creating the picture is called progressive scan. All of the lines are drawn on the screen at once. This is the way computer monitors make their pictures. Progressive scan creates a very sharp image, without those interlace errors, but it doubles the data flow because, instead of creating one half of a picture every sixtieth of a second, the system has to create the entire picture every sixtieth of a second. Twice as much data has to be recorded, transmitted, and processed by the TV set.

This is why the purists are having a hard time. Even with the help of compression, the number of lines, the number of pixels, and the doubling of data for progressive scanning simply adds up to too much data. Nobody in the industry doubts that we will someday find a way to handle this much data and tranmit it within the allowed bandwith, but we can't do it right now. The FCC, not wanting to lock us into an inferior system that we will have to live with for many years, has decided that progressive and interlace should both be embraced by the standard, and further, different numbers of scan lines and pixels per line should be allowed. This flexibility has opened the door for a confusing array of DTV signal formats. Some broadcasters plan to transmit 1080: (1080 scan lines interlaced every 1/60 second) while others will use 720p (720 scan lines repeated progressively every 1/30 second). On the one side, 1080 lines is sharper than 720, but uses more data but is interlaced, halving the data but interlace is relatively fuzzy. On the other side, 720 lines is less sharp than 1080 making it data-efficient but progressive scan transmits more data. Actually, both methods end up making pictures that look fairly equal.

The broadcasters, seeing a market for plain vanilla SDTV broadcasts, have pushed the FCC for a system using 480 scan lines with only 704 pixels. This picture is so data stingy that the broadcasters can make six channels of SDTV from their allotted bandwidth.

The computer people (primarily Microsoft) have pushed for 480 lines by 640 pixels which is the present VGA standard. They also maintain that a 4:3 aspect ratio is perfectly adequate. The TV broadcasters realizing that all of their old shows exist in the 4:3 aspect ratio are also happy to transmit in the 4:3 aspect ratio.

So we end up with four possible picture sharpnesses in the FCC standard, and two picture shapes, 16:9 and 4:3. We also have progressive versus interlaced scan. The FCC, caught inbetween these powerful forces, has diluted the standard to permit all these systems to exist. [See DTV Standards Table to compare the various standards accepted by the FCC]

Ah, but there's more. Enter the film people, stage right. Movies are shot at 24 frames per second. TV pictures are made at 30 frames per second. Since these numbers are different, a complicated conversion must occur to make movies viewable on TV. The conversion process (called 3:2 pull-down) shows one movie picture three times (3 video fields) and the next movie picture two times (2 video fields). Although the math is a bit complicated, it all boils down to repeating some of the movie pictures the right number of times so that they synchronize with the video pictures. This process works, but it's quite wasteful. Every time you repeat a movie frame you are sending needless data that takes up space. It would be better if you sent the data for a movie picture just once and were done with it. Thus the film people have convinced the television industry and the FCC that the DTV standard should allow the transmission of 24 pictures per second as well as TV's 30 frames per second.

And if you thought this was complicated, try on this other twist for size. Today's TV pictures are not really produced at 30 frames or 60 fields per second. We refer to these numbers because they are simple to say, and black-and-white TV systems used to follow these numbers, but when the US switched to NTSC color TV, it adopted slightly different numbers. Actually there are 29.97 frames per second and 59.94 fields per second comprising our NTSC color TV picture. These odd numbers were implemented for technical reasons involving the circuitry in the tubed TVs of yesteryear. Those circuit problems have been resolved and there is no longer a need to use these weird numbers, but they're entrenched as part of today's NTSC standard. All the color video tapes made these past 40 years use those ungainly numbers, so if you want to stay compatible with the old video tapes without going through expensive and complicated conversions, you need to stick with these numbers. On the other hand, the numbers 30 and 60 are so easy to work with, why not use them? The 30/60 numbers are even useful for counting time while editing (dismissing the need for NTSC's "drop-frame" and "non-drop-frame" time code distinctions, but that's another subject).

So the FCC has adopted both sets of numbers plus another set of numbers to represent what happens when 24fps film is transferred not to 30 fps video but to 29.97fps video. As a result of accepting all of these permutations of different numbers, the DTV television standard has become hardly a standard at all. The screen can be 4:3 or 16:9, the signal can be progressive or interlaced, it can have four different sets of vertical lines and four different sets of horizontal pixels per line and the pictures can be made 23.976, 24, 29.97, 30, 59.94, or 60, times per second. This is a standard?

The Effect of DTV on Joe Camcorder

First let's start with Charlie Couch Commander and his TV receiver. In order for his digital TV receiver to operate on all these different standards, the machine will have to electronically complex (translation: expensive) whether it can display high definition television imagery or not. The circuitry will have to assure that whatever signals are received in high or standard definition, and regardless of picture shape, and regardless of how many channels of audio are transmitted, something will appear on the TV screen and something will come out the speaker. DTV receivers will also have NTSC inputs for backward compatibility. There will also be set top converter boxes that will translate DTV signals, in whatever format they are, into something that older TV sets can display (remember how many years you had to use the cable company's converter box with your TV before TVs became "cable ready"?)

Note that most cable and all satellite systems presently send digital signals. The set-top boxes provided by Cablevision, Cox, DirectTV and other companies already make the conversion from digital to analog for your analog TV set. You don't have to buy a new set to view cable and satellite signals. Over-the-air broadcasts will be different after April 7, 2009; you WILL need a digital TV to pick up those broadcasts UNLESS you have a set-top box that will make the conversion for your analog TV. Set top converter boxes will also make it possible for you to record shows using your standard VHS VCR; once the DTV signals are converted to NTSC audio and video, your old TVs and VCRs will work just fine.

The analog camcorder is likely to die a gradual death. You will still be able to use it to record home videos that you play on your home VCR through your old TV. But eventually you'll no longer be able to buy analog NTSC TVs and VCRs forcing you to jump up to digital camcording. Think for a moment how difficult it is to find a super 8 movie projector these days; even if you had a wonderful super 8 movie camera, you wouldn't bother using it because nobody could show your movie.

Thus there will be a gradual change to digital standards for camcorders and home VCRs. Naturally, the digital stuff will be more expensive at first and will slowly come down in price. Right now the prices are so high that only the broadcast and industrial guys can afford the cameras, VCRs, and TVs.

And now for some good news. Remember all the "standards" that the FCC accepted? Some of those standards are quite compatible with today's technology; it won't be hard to build a cheap, consumer camcorder that can record/play one of the digital formats. Naturally, more expensive models will record using one or more of the higher definition formats. How it all works will remain transparent to the user. You simply chuck in a tape and pull the trigger like you did before and the system will record picture and sound. You will play your signal back from your camcorder into your TV just as you do now, or you will need a compatible VCR to play the tape. Many of today's digital camcorders are up to the task of recording a DTV signal but aren't doing it because NTSC is still the defacto standard. Manufacturers will have little trouble converting the digital camcorders of today to outputting one of tomorrow's SDTV signals.

The Editing Dilemma

Editing digital TV will have a few extra hurdles of its own. You remember that DTV involved compressing the data in order to make it fit within a smaller bandwidth. The image looks great when the pictures are moving on your TV set, but what happens if you try to manipulate those pictures, edit them, apply special effects, or add new sound? You could try converting the digital imagery back into analog and editing it the old way. The process would work but would defeat the advantages of digital signals: copyability without degradation, reduced noise and graininess. The important thing to remember is that the MPEG-2 signal transmitted from the DTV broadcaster to your home contains maybe 14% "real" pictures and 86% "fake" imagery. The result looks good but it's not really there. If you want to edit, freeze frame, or manipulate video imagery, you need the "real" pictures to work with. Thus the camcordist will have to shoot the scene and store all or almost all of the data on some kind of a digital recorder. This will be a lot of data because it hasn't been compressed yet or has been compressed in such a mild way that it can be decompressed easily without losing anything. There are digital VCRs that can do this, but they cost a bit more than the home style video editors.

Non-linear DTV editing faces the same problems: If you don't compress the data, your video picture will look good, but will fill up your hard drive in a matter of minutes. It also takes an industrial strength (fast, expensive) computer to handle all that data smoothly. On the other hand, if you compress the data, more will fit on your hard drive, the computer can handle it more easily, but your image degrades and may also become uneditable. There is no easy path out of this dilemma. Rest assured that the design wizards are working into the wee hours to find a solution.

Most Likely Scenerio

When DTV arrives, reasonably priced camcorders will be able to produce a DTV signal matching one of the lower standards. Some camcorders may even be switchable so that they also output plain old NTSC. More expensive camcorders will make sharper pictures using one of the higher DTV standards.

Whatever the camcorder does, rest assured that the DTV receiver will be able to lock onto the standard and display the image. Just as today, a lousy, cheap TV receiver won't make a great picture even if you feed a classy signal into it.

On the editing front, I predict that VCRs and computers will improve to the point where they can record uncompressed or slightly compressed data and store a reasonable amount of it for manipulation. When you are done editing, making special effects, and mixing sound, you'll output the whole works through an MPEG-2 encoder that will change the signal into one of the approved DTV formats for your TV to display or for a regular DTV recorder to record. Note this important distinction: The camcordist may have to record a lot of data on an expensive camcorder in order to create a non-compressed DTV or HDTV image ready for editing and manipulation. Once this step is done, however, the signal can be MPEG-2 compressed to a more manageable form in one of the "DTV standards," becoming viewable on a DTV receiver and recordable on a common DTV home recorder.

The DTV revolution is coming and it's likely to be an awful mess for a while, but those of us close to the industry can see a light at the end of the picture tube.

DTV Glossary

DTV (Digital Television) TV that is broadcast, recorded, and processed
digitally, possibly with extended definition like HDTV, and adheres to certain
FCC specifications.
HDTV (High Definition Television) One DTV method of displaying sharper,
wider TV pictures than the present NTSC system. Pictures ideally would be
shaped into a 16:9 aspect ratio, composed of 1125 scanning lines, each line
having 1920 pixels.
SDTV (Standard Definition Television) Digitally broadcast TV signals with
about the same sharpness and screen shape as today's NTSC television.
Dolby AC-3 Method of compressing 5.1 channels of high quality sound data
into 382 kbps for use in DTV and DVDs.
Bandwidth Signal transmission capacity, or the size of a channel through
which data has to travel. TV broadcast channels, whether for digital or
analog programming, offer 6 MHz of bandwidth.
Compression The process of fitting a large amount of data into a small
file. MPEG-2 is the method used to compress DTV. Audio is also compressed
using the Dolby AC-3 algorithm.
Datacasting DTV can consist of TV programs as well as digital data. The data
might include program guides, sports statistics, stock quotes, retail
ordering information, etc. Although the data is one-way (to you), dial-up
Internet connections allow you to place orders for products or answer viewer polls.
Interlaced scan The present NTSC method of creating a TV image by
"drawing" the even numbered scan lines (rows of pixels) on a TV screen from
left to right, top to bottom, and then drawing the odd numbered scan
lines between them. The even and odd lines of each frame are referred to
as fields. The 1080i high definition standard to be used by NBC and CBS when
it broadcasts DTV will be interlaced as opposed to progressive scan.
Progressive scan The converse of interlaced scan, a method of "drawing" a
TV image across the screen sequentially --- left to right, top to bottom.
No lines are skipped. This method is presently used on computer monitors and
the 720p method will be used by Fox TV and ABC
when DTV arrives.
Multicast, multichannel, multiplex The ability to transmit more than one
channel of programming within the broadcaster's allotted channel spectrum.
Up to now, a 6 MHz channel could hold only one analog TV program. With DTV,
6 channels can be squeezed into the same spectrum space. Some
of these multicast channels can have higher definition while others have
sub-VHS quality.
NTSC (National Television Standards Committee) The acronym refers to both
the Committee and to the standard video format used in the US, Japan, and
some other countries.
Resolution Picture sharpness, which is roughly proportional to the amount of
data used to make up a picture. Digital TVs will describe their resolutions in terms of the
number of pixels or dots that make up the picture along the vertical and horizontal axes.

Where Will DTV Debut?
Present:
All commercial and noncommercial stations in the US have begun digital broadcasts.
April 7, 2009:
Analog TV signals will be completely eliminated in the US. Only DTV signals will exist.
From the present to April 7, 2009:
Broadcasters who expand DTV transmissions, yet will simultaneously broadcast analog TV.

Summary of What DTV Offers
1. Sharper picture or more channels with standard quality pictures.
2. Wider picture with a width to height ratio of 16:9 (similar to motion pictures)
as opposed to today's 4:3.
3. 5.1 channels of Dolby digital surround sound using the AC-3 system.
This provides 5 separate audio tracks plus one subwoofer audio track.
4. Data channels which could carry related information such as text or
computer software downloads.

New Channel Lineup
The FCC gives every broadcaster a new extra TV channel to use
during the transition period. Their old channel in most cases will
stay the same and will broadcast in analog NTSC. The new channel will be
DTV. In 2009, the FCC will reclaim the old NTSC channels and shut down
NTSC broadcasts for good.

NTSC presently uses channels 2-69 (but not 37), for a total of 66 channels.
The new DTV channels will be 2-51, relinquishing UHF channels 52 - 69
for non-TV uses. The DTV channels, because several SD programs
can fit within the bandwidth, may appear as 23.1, 23.2, 23.3, etc.,
so there will be more than 49 channels if you count the subchannels.

Shooting in High Definition
Shooting in high definition is a tad different from normal TV. For
instance:
1. You can hold shots longer --- it will take time for the detail to sink
in.
2. You can use wider camera angles --- the detail survives the long shot.
3. There are no more cardboard sets --- the all-seeing eye of the
hi-definition camera will spot any fakery. Set construction and painting
will have to be more exacting.

About the author

About Today's Video 4th. ed.

Return home