What Is Digital TV?

Snapshot:

• DIGITAL TV is the new U.S. TV broadcasting system, already in use.

• TV stations use digital channels to offer wider, high quality pictures and up to 6 channels of SURROUND SOUND.

• DIGITAL TV can include crisp, detailed high definition TV (HDTV) programs.

• TV stations can show several programs at the same time on one digital channel.

On this website, "DIGITAL TV" refers to the new digital broadcasting system that U.S. TV stations are using to send their TV signals from their transmitters OVER-THE-AIR to viewers who receive the signals with ANTENNAS, for free.  This system is called "digital TV broadcasting" or "ATSC broadcasting" (after the Advanced Television Systems Committee, which developed the digital broadcasting standards). 

Using digital technology to transport TV channels is not new.  Because digital technology allows many TV channels to be sent in the same electrical space as one non-digital TV channel while still maintaining great picture quality, it has been used for many years to send TV channels over CABLE TV systems and SATELLITE TV systems.  Now it is being applied to local TV broadcasting.  This website focuses on the new features and the differences that viewers will find when watching DIGITAL TV channels from local TV stations. 

Today, over 95% of the "FULL POWER" TV stations (stations with powerful signals that cover a large viewing area) have turned on a new, digital channel.  Most of the channels can be watched today on TVs with DIGITAL TUNERS or DIGITAL CONVERTERS.  We will tell you how you can watch those digital channels. 

What does DIGITAL TV mean to me?  These are the features of digital TV that you will notice, especially when you watch digital TV channels over-the-air, with an antenna.  For the really curious, there are more details on the digital TV system later on this page.

High quality OVER-THE-AIR reception.  Program pictures and sound are sent as digital files.  The files don't change as they are transmitted over-the-air and received.  Thus digital TV viewers using an ANTENNA see programs in the same quality that that they had going into the transmitter.

WIDESCREEN pictures.  Digital TV includes a picture shape that is one-third wider than traditional TV, giving a cinema-like, panoramic image.  Digital TVs are built with wider screens to display this shape. 

HIGH DEFINITION TV (HDTV).  Digital TV includes the ability to send HIGH DEFINITION (HDTV) programs.  These programs use five times more picture information to capture and display the pictures, resulting in larger, more detailed, exciting picture images.  HDTV is the most obvious and popular feature of digital TV.  Most broadcasters do transmit some HDTV programming, but  broadcasters can also send STANDARD DEFINITION  programs on their digital TV channels that are not in HD.

Realistic surround sound.  Digital TV includes several audio formats that broadcasters can use for each program.  The highest quality is called "5.1 SURROUND SOUND," and it features 6 channels of sound (five surround channels and a low frequency channel).  Audio can also be in two-channel stereo or four-channel "Dolby Pro-Logic," or it can be in mono (one channel).  Because the sound is sent in a digital format, it can have terrific dynamic range (difference between loudest and softest sounds), matching the sound quality of CDs and DVDs.  

Multiple programs on one channel.  Broadcasters can send several programs over a digital channel at the same time.  This is called "MULTICASTING," and it represents an exciting opportunity for new programming.  Many TV stations are already broadcasting two or three programs full time on their digital channels.

Different languages for programs.  Broadcasters can send several soundtracks for a single program in different languages, and it's easy for viewers to choose the different languages. 

Convenience features.  The digital technology also supports these convenient viewing features:
channel guides on digital TVs that are automatically created, for free, from TV station program data
many choices for setting up displays of closed captions
the ability to set up parental viewing limits based on U.S. TV program ratings

Future features.  Digital broadcasting will allow future enhancements such as programs for mobile devices and interactive TV. 

Stop here if that's enough technical talk!   Go on to "How To Watch Digital TV" if you're ready to start watching.  If you're curious about the magic behind digital broadcasting, read on!

For the Truly Curious

What were the goals in creating the Digital TV broadcasting system?  The main focus of the creators of the digital broadcasting system was to develop a system that could transmit high quality, high resolution pictures and more authentic sound, which we call high definition TV.  But the system also takes advantage of the ability of digital technology to maintain the picture quality during transmission, use channel space efficiently, and be flexible for different types of programming and features. 

A key reason that digital TV could be planned was that the computing power required to decode and present digital TV pictures could be mass-produced inexpensively in digital TVs.  A digital TV is in fact a computer that has been programmed to tune and decode a variety of digital signals and create the proper picture and sound outputs. 

Many companies, organizations, and individuals collaborated on the development of the "ATSC" digital TV broadcasting system.  See "What is the Digital TV Transition?" or click here. 

What are the major parts of the digital TV system?  The digital TV system describes standards for the following elements of digital broadcasting:

Picture and sound formats.  These list the ways that TV pictures are sound are captured, including the picture shape, the number of lines used in a picture, and how many pictures are created each second.  For sound, it includes the number and kinds of sound channels that can be combined with a picture. 

COMPRESSION standard.  This is the "magic" of digital TV.  Digital TV programs are encoded into computer files, or "COMPRESSED," for storage and transmission.  The "MPEG-2" COMPRESSION standard is used, which is also used for DVDs, and cable TV and satellite TV transmission.  The compression process very cleverly keeps only the data needed to create changes in the pictures, and it tosses out the data that is repeated from one picture to the next.  Thus, a compressed program has less than 3% of the data of the the original, uncompressed program!  Because of compression, the immense amount of information in a high definition (HDTV) picture can easily be inserted into a digital TV channel.
Transport standard.  More magic.  This part of the standard defines how the picture files, sound files, and data files are packaged into a digital "program stream" for transmission.

Broadcasting standard. This defines how the "program stream" is turned into a radio transmission using a traditional TV broadcast channel that is 6 Megahertz (MHz) wide.  The digital TV transmission system provides 19.4 megabits per second (Mbps) of data transmission on one digital TV channel. That is the size of the "digital" highway the broadcaster gets to work with.   

Receiver standards. This defines what a digital TV needs to do to tune the digital channels, decode the pictures and sound, and present them on the screen and create audio outputs. 

Picture Formats for Digital TV

The most noticeable change in digital TV is the range of picture formats that can be used.  There are several key factors that affect both the quality of a TV picture and the amount of electrical space or bandwidth used to create and store a picture.  To provide flexibility, the digital broadcasting system includes a variety of standards, as follows.  Fasten your seat belt:

Picture shape.  Also called "ASPECT RATIO."  Traditional TV is based on a picture shape that is 4 units wide and 3 units high.  This is called a "4:3" shape or aspect ratio.  When the digital TV committee developed the digital standard, they created a new picture size that is one-third wider than the traditional picture.  This shape is 16 units wide by 9 units high, or a "16:9" shape.  This picture shape shows a wider image, and is more "movie-like."  The digital TV standard actually includes both of the 4:3 and the 16:9 picture shapes. 

Lines of resolution.   To create a TV picture, the circuit of a TV camera looks at just one line in the picture, from left to right.  As it scans the line, it captures information (the brightness and color) on each point on the line and saves that information.  After it finishes scanning a line, it scans another line below it, until it has reached the bottom of the picture.  Then it starts scanning lines again at the top of the picture. The number of scanning lines that the camera uses affects the quality of the picture output; more scanning lines provide a clearer picture.  Traditional ANALOG TV uses 480 visible scanning lines to make a picture.  The digital TV standard includes 480 line pictures, but for higher quality it also includes formats of 720 scanning lines and 1080 scanning lines. (The 720 and 1080  line formats also look at more "points" along each line, which also adds to the picture quality.)   

Scanning Mode.  You're heard about the "i" and "p" in HDTV, so here's the story.  In traditional analog TV, the TV camera actually scans every other line as it goes from the top to the bottom of the picture.  So it looks at lines 1, 3, 5, etc.  Then it starts over again on lines 2, 4, 6 until it's done.  This is called "interlaced" scanning.  This was developed for early TV technology, which could not easily scan the lines in order, and it uses our natural "persistence of vision" to create a continuous image.  But modern video screens, including computer displays, are fast enough to scan all the lines in order (1, 2, 3, etc.), and this is called "progressive" scanning.  Digital TVs are capable of showing programs with both kinds of scanning.  Since a lot of video today is displayed on computer monitors, and since progressive scanning can show "action" scenes with more detail, the digital broadcasting standard includes both interlaced and progressive scanning modes. 

Finally, Frame Rate.  You can imagine that a key element of TV picture quality is the number of new pictures that are created by a TV camera every second. Correct!  This is called the "frame rate."  The digital standard includes a frame rate of 60 frames or pictures per second, and a frame rate of 30 frames per second, and a frame rate of 24 frames per second.  Why 24 frames?  Most theatrical films are shot in a frame rate of 24 frames per second,  which is a key part of the "cinema" mood conveyed by film.  But: all films are eventually converted to video for distribution on DVDs, downloads, and broadcasting.  Traditionally, film had to be converted to video's 30 frames per second to be shown on TV, but new digital TVs can show movies in their original 24 frames per second rate (which also uses less transmission space), so the 24 frame rate is included in the standard.  

Now, let's put them together. 

The digital TV standard is a combination of the format elements above that TV stations can use to produce and send TV shows.  The formats with a widescreen shape and at least 720 lines of resolution create the highest quality pictures, so they are considered "high definition TV" (HDTV).  The other formats using progressive scanning are called "enhanced definition TV."  The remaining formats using interlaced scanning are called "standard definition" TV, and are similar to traditional TV quality.  

There are 18  picture formats in the ATSC digital TV standard: 

Quality                                                Picture shape                          Horizontal by vertical resolution                 Frame rate and scan

HIGH DEFINITION (HDTV)            16:9 (widescreen)                   1920 pixels x 1080 lines of resolution      24 and 30 progressive; 30 interlaced

                                                            16:9 (widescreen)                   1280 pixels x 720 lines of resolution        24, 30, and 60 progressive

ENHANCED DEFINITION              16:9 (widescreen)                   704 x 480                                                        24, 30, 60 progressive

                                                             4:3                                             704 x 480                                                        24, 30, 60 progressive

                                                             4:3                                              640 x 480                                                        24, 30, 60 progressive

STANDARD DEFINITION                16:9 (widescreen)                  704 x 480                                                         30 interlaced

                                                              4:3                                            704 x 480                                                          30 interlaced

                                                              4:3                                            640 x 480                                                          30 interlaced

From this table, you'll see that digital TV programs can be:

• the same picture shape and quality as current analog TV; or
• in a widescreen shape (16x9) but otherwise in the same quality as current analog TV; or
• in the same picture shape as current TV but with higher quality; or
• in many flavors of higher quality!
  

Are broadcasters using all these DTV standards?    No.  In fact, in the late 1990's when digital channels first began, U.S. broadcasters quickly settled on two of the formats for transmitting high definition TV programs (HDTV).  One of the formats is 1920 x 1080i (30 frames, interlaced scan), used by the CBS and NBC networks. They like 1080i as a good "general purpose" format. 

The other format is 1280 x 720p (60 frames, progressive scan), used by the ABC and FOX Networks. They have said that the progressive scan of this format gives a better presentation of fast action shots, especially for sports.  Most viewers find the difference to be fairly subjective; when the data elements of each format are multiplied out (pixels x frame rate per second), 720p provides just 11% less picture data in every second than 1080, but the progressive scan can provide a smoother image.   

For transmitting standard definition programs with a 4x3 shape on digital channels, most broadcasters use the same resolution as analog TV: 640 x 480i, (30 frames, interlaced).  (Broadcasters: are you dong anything different? Let us know.) 

Digital TV trivia: what about 1080p?  The digital broadcast standard does NOT include a format of "1080p," (1080 lines of resolution, progressive scanning)...even though you can buy digital televisions today that can display "1080p."  So why purchase a "1080p" TV?  Because high definition DVD players (Blu-Ray and HD DVD), as well as video games and computers, can output their pictures in 1080p, and the picture quality is excellent.

Sound Formats for Digital TV

To augment the high quality picture formats of digital TV and provide a lot of programming flexibility, the digital TV audio standard allows very high quality audio while using digital COMPRESSION to minimize the channel space used to transmit the audio.  Each program can have several "audio services," and most of the audio services can have up to 6 channels of sound.  This is to support a "5.1 SURROUND SOUND" output, where the 6 channels are right, center, left, right surround, left surround, and "low frequency enhancement," for bass frequencies (from 3 to 120 cycles).  This channel has less than 1/10th the frequency information of the other channels, so it's called the "dot 1" channel.   

Besides the 5.1 surround sound, the digital system can transmit 2 channel (stereo) sound, Dolby Surround Sound (4 channel output, transmitted over 2 stereo channels), and 1 channel (mono) sound. 

Audio channels are sampled at a 48kHz rate and have a frequency range and a dynamic range similar to audio CDs.  "AC-3 Digital Audio Compression" is used to compress the audio tracks for transmission. 

There is enormous flexibility available for program audio formats.  A digital TV program can have any of the following "audio services" matched to it:

• "complete main" service.  Full dialog, music, and effects.
• "music and effects" service.  Full soundtrack, but without voices (allows a separate language soundtrack to be inserted).
• "visually impaired" service.  Designed to feature a verbal description of the program.
• "hearing impaired" service.  Designed to feature dialog that is mixed louder than music and effects, for those who might have difficulty hearing dialog in the normal mix.
• "dialog" service.  Designed to support one or more dialogs in different languages; these can be mixed with the "music and effects" service.
• "commentary" service.  Can contain optional program commentary, selectable by the viewer.
• "emergency" service.  Can be used to send a priority audio message that overrides all other program audio.
• "voice over" service. Designed to allow a voice track to be added on top of the main service audio during the transmission.

On the receiving end, a digital TV can usually decode the sound into two channels, and some TVs decode the full 6 channels.  For a full "surround sound"  effect, most digital TVs have a wired or optical audio output that can be connected to an Audio-Video (A-V) receiver/amplifier to decode the sound and distribute it to up to 6 loudspeakers.  

What does all this mean for broadcasters with a digital channel?  Digital TV gives broadcasters a lot of flexibility in how they use a digital channel. 

Programs.  Most broadcasters are focused on offering more HDTV, and their primary digital channels often feature HDTV programs.  They still have room to offer several additional channels, usually in "standard definition" so they take up less space on the channel.  Today many broadcasters are offering two or three new channels of programming on their single digital broadcast channel. 

Program audio. Broadcasters have a lot of flexibility for audio. The 5.1 channel surround sound is a terrific complement to HDTV, and even stereo and mono audio sounds great on digital TV.  Meanwhile, digital audio is being used to add second or third languages to programs. 

Mobile TV.  Broadcasters are developing extensions to their digital broadcasting system that can provide quality TV signals to moving and portable devices, like autos, computers, and mobile phones.  This will enable new programming services targeted to out-of-home viewers. 

Data transmission.  Because full power TV stations have very good signal coverage of their communities, their signals provide a way to broadcast data across entire cities, and broadcasters are looking at ways to commercialize this.  Some stations are already providing data transmission for emergency service providers.

More information at:

Federal Communications Commission's DTV site
Advanced Television Systems Committee

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CAPITALIZED WORDS are defined in the Digital TV Glossary.