Digital Television

Basics of Television
Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. When color television was introduced, the hue and saturation information was added to the monochrome signals in a way that black and white televisions ignore. In this way backwards compatibility was achieved. That concept is true for all analog television standards.

There are traditionally three coexisting television scanning standards in the world today. These standards are based on technology available in the 1930s taking in to account cost vs performance. The first was the American NTSC (National Television Systems Committee) color television system, The European/Australian PAL (Phase Alternation Line rate) and the French-former Soviet Union SECAM (Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.

In principle, all three color encoding systems can be combined with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as a capital letter. For example, the United States, Canada, Mexico and South Korea used NTSC-M, Japan used NTSC-J, the UK used PAL-I, France used SECAM-L, much of Western Europe and Australia used PAL-B/G, most of Eastern Europe used SECAM-D/K or PAL-D/K.

However, not all of these possible combinations actually exist. NTSC is only used with system M, even though there were experiments with NTSC-A (405 line) in the UK and NTSC-N (625 line) in part of South America. PAL is used with a variety of 625-line standards (B, G, D, K, I, N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.

For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals; for example, on DVD-Video, which does not contain any analog color encoding, and thus no PAL or NTSC signals at all.

Video Scanning
Video Scanning refer to the manner in which a television scene defines its luminance and chrominance values. They specify the number of lines per frame and the number of frames per second. Technical and economic considerations in various countries around the world have led to the development on numerous different compromises in transmission. These considerations are restrained by the fact that only one bit of information can be transmitted at a time. To work around this, the transmission has to be broken down into small elements transmitted sequentially and reassembled locally at the receiving end. The reconstructed images then has to be displayed in rapid succession to imitate movement.

A cathode-ray tube (CRT) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line the beam returns to the start of the next line; the end of the last line is a link that returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the luminance of that point. A color television system is identical except that an additional signal known as chrominance controls the color of the spot.

Raster scanning is shown in a slightly simplified form below.

When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at the same time at which it is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization with the scanning in the television.

The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this.

Resolution
In television, Resolution is used to refer to the number of lines per picture height (LPH). Television systems have been developed to have square pixels, or equal ratio of horizontal and vertical resolution. Vertical resolution is the ability of the a broadcast format to resolve horizontal lines. It is usually displayed as the number of horizontal lines that can be distinctly resolved on a television screen.

Frame rate
The human eye has a characteristic called Phi phenomenon. The phi phenomenon has been referred to as "first-order" motion perception. It's modeled in terms of relatively simple "motion sensors" in the visual system, that have evolved to detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighbouring point on the retina after a short delay, and therefore quickly displayed successive scan images will allow the apparent illusion of smooth motion. Flickering of the image can be partially solved using a long persistence phosphor coating on the CRT, so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is a large amount of rapid on-screen motion occurring.

The maximum frame rate depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame is used to build the image. This process doubles the apparent number of video frames per second and further reduces flicker and other defects in transmission.

Broadcast Standards
Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards:

ATSC
Advanced Television Systems Committee (ATSC) standards are a set of standards for digital television transmission over terrestrial, cable, and satellite networks. ATSC standards are marked A/x (x is the standard number). The digital television broadcast standard designated A/53 describes the system characteristics of the U.S. digital television system. The standard addresses a wide variety of subsystems required for originating, encoding, transporting, transmitting, and receiving of video, audio, and data by over-the-air broadcast and cable systems. The ATSC standard specifies a system designed to transmit high-quality digital video, digital audio, and data over existing 6-MHz channels. The system is designed to deliver digital information at a rate of 19.29 megabits per second (Mb/s) and uses eight-level vestigial sideband (8VSB) for terrestrial broadcasting.

DVB-T
The European Telecommunications Standards Institute has adopted a set of standards for digital broadcasting of television, sound, and data services. Standards have been adopted for satellite, cable, and terrestrial signal delivery. The standard for terrestrial transmission, ETS 300 744, is designated Digital Video Broadcast–Terrestrial (DVB-T). This standard describes a baseline transmission system for digital broadcasting of television. It is similar in many respects to the U.S. DTV standard. However, there are also important and significant differences in both channel coding and modulation. The DVB-T standard specifies a system designed to transmit high-quality digital video, digital audio, and data over existing 7- or 8-MHz channels. The system is designed to deliver digital information at rates from 4.98 to 31.67 Mb/s and uses coded orthogonal frequency-division multiplexing (OFDM) modulation for terrestrial broadcasting.

ISDB-T
Japan’s Digital Broadcasting Experts Group (DiBEG) has developed a standard for digital broadcasting of television, sound, and data services, designated integrated services digital broadcasting (ISDB). Standards have been developed for delivery of satellite, cable, and terrestrial signals. These standards include a description of a baseline transmission system that provides for digital broadcasting of television, including channel coding and modulation. The transmission standard for terrestrial digital television is similar in many respects to the DVBT standard. It is entitled Integrated Services Digital Broadcasting–Terrestrial (ISDB-T). The ISDB-T standard specifies a system designed to transmit over existing 6-, 7-, or 8-MHz channels. The system is designed to deliver digital information at data rates from 3.561 to 30.980 Mb/s. ISDB-T uses coded orthogonal frequency-division multiplexing (OFDM) modulation and two-dimensional interleaving. It supports hierarchical transmission of up to three layers and uses MPEG-2 video and Advanced Audio Coding. This standard has been adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H.264/MPEG-4 AVC, which has been adopted in most of South America and Portuguese-speaking African countries.

DTMB
Previously known as DMB-T/H (Digital Multimedia Broadcast-Terrestrial/Handheld), the DTMB is a merger of the standards ADTB-T (developed by the Shanghai Jiao Tong University), DMB-T (developed by Tsinghua University) and TiMi (Terrestrial Interactive Multiservice Infrastructure); this last one is the standard proposed by the Academy of Broadcasting Science in 2002. The DTMB was created in 2004 and finally became an official DTT standard in 2006. The system is designed to deliver digital information at data rates from 4.813 to 32.486 Mb/s. DTMB adopts time-domain synchronous (TDS) OFDM technology and has been adopted in the People's Republic of China, including Hong Kong and Macau.

Serial Signal Transmission & Ancillary Data
Advances in technology have made it cost-effective to transmit television signals using bit-serial-distribution. Digital video data bits, sync information, and ancillary data (Closed Captioning, AES/EBU Audio, etc.)can be distributed through a single coaxial cable.

Bit-serial data rate is given as

$$Serial \ bitrate = Parallel \ bit rate \ (Mwords/s) \ x \ number \ of \ bits \ per \ word$$

The 4:2:2 component digital serial bitrate is equal to

$$27 \ Mwords/s \ x 10 \ bits/word= 270Mbps$$

DTV Protocols and Technologies
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United States
In the United States, the digital switchover is planned to occur on February 17, 2009; after that date, all major broadcasters will stop transmitting analog signals altogether. These broadcasters have already been broadcasting digital versions of their programming alongside the analog signals for several years. These digital signals can be only received via a digital tuner; such tuners are included in any new television set and some DVD recorders.

Older (analog-only) televisions can use a digital converter box to receive digital programming. The US government has established a coupon program offering up to two $40 coupons per household good toward the purchase of such boxes (see Wikipedia:Coupon-Eligible Converter Boxes and the US government's FAQ on the program). Most converter boxes are being offered for about $60, and only one coupon can be applied toward the purchase of one box, so each box will cost about $20, plus taxes and shipping costs, if any. The coupons can be applied for online at https://www.dtv2009.gov/ or by calling 1-888-DTV-2009. The coupon cards must be redeemed within three months, and are processed by retailers exactly like a credit card (although it is nonrefundable), providing an instant $40 credit.

According to a study performed in 2004, approximately 19% of US households receive television through broadcast signals only; to continue receiving television signals after February 19, 2009, these households must purchase a converter box, a new TV with a digital tuner, or subscribe to a for-pay cable or satellite TV service.

Purchasing a converter box
Several dozen models of converter boxes are available that are eligible for the coupon program; a list of them, including a breakdown of their basic features, is available at Comparison of CECB units. Most major retailers are only stock one or two models (provided they have any units in stock at all). The following table lists what units are listed for sale on each company's website:

These units may or may not be available in your local store, or even online. You might want to call your local retailer before setting out to see what they actually have in stock.

Kmart and Sears do not list any boxes on their respective websites, although supposedly they both have the Magnavox/Philco TB100 available in some stores. A number of other online, telephone, and local retailers are also participants in the government's coupon program and may have different boxes available than those listed above. See https://www.dtv2009.gov/VendorSearch.aspx for a listing of vendors (fill in the "Locate Retailers Near You" form for additional local listings).

Coupon-eligible boxes are limited in what features they can offer (only very basic video and audio connectors, for instance). Units with more features, such as DVD recorders, or units that can be hooked up to PC monitors, HDTVs, or other non-analog equipment are available, but you cannot use a coupon to purchase them.

Advice about specific models

 * Units using LG's sixth-generation DTV chipset (the LGDT1111 family), including the Digital Stream, Insignia, and Zenith models listed above, will supposedly get better reception of signals, especially in areas with a lot of obstructions (buildings, trees, etc.) between the receiver's antenna and the broadcast antennas.


 * The Insignia NS-DXA1 and Zenith DTT900 models are both manufactured by LG Electronics, and are essentially identical products. The Magnavox TB100MW9 and Philco TB100HH9, both manufactured by Funai, are also believed to be identical models.


 * Users have reported audio artifacts (sibilance) in the high treble range on Zenith DTT900 and Insignia NS-DXA1 models. This problem supposedly has been fixed in units marked April 2008 and later.


 * The Digital Stream DTX9900 may have issues with unwanted picture cropping in both "zoom" and letterboxed mode.


 * A small of models offer "analog pass-through": this feature allows you to receive both digital TV signals and analog signals through the same video setup (and antenna). This feature is important if there are stations in your area that you wish to receive that have a good analog signal but a weak or nonexistent digital signal.  Digital Stream DTX9950, Zenith DTT901, and Insignia NS-DXA1-APT models which incorporate this feature are due to be released in mid-2008, or may already be available.

Advantages and disadvantages of broadcast digital television
Digital television incorporates new functionality not previously available in analog broadcasts. The new features include:


 * High-resolution video (HDTV), including widescreen formats
 * Surround sound audio
 * Multiple programs per channel (through the use of subchannels)
 * Built-in electronic programming guide (program description and broadcast schedule)
 * Near-perfect picture quality (provided the signal is strong and steady)

SDTV and HDTV
Standard-definition digital video (SDTV) is equivalent to the resolution available on a good analog TV set. HDTV is broadcast in two forms: 1080i and 720p. Both are widescreen formats, with an aspect ratio of 16:9 -- this means the picture is 78% wider than it is tall. (Contrast this with the 4:3 aspect ratio of older TV sets and SDTV broadcasts, where the picture is 33% wider than it is tall.) 1080i has five to six times the resolution of standard-definition pictures and 720p has more than twice the resolution. However, 720p is also broadcast at twice the frame rate: the picture is refreshed 50 or 60 times a second instead of 25 or 30 times a second. As a result, both 1080i and 720p convey about the same amount of visual information, with 720p being better-suited to high-motion programming such as sports.

Although digital cable providers may provide local stations (including local affiliates of national networks), to save bandwidth, many provide a reduced-quality HDTV signal compared to the one that is available over the air.

Electronic programming guide
The digital format also allows broadcasters to provide program information and schedules along with their programming; most provide several hours' or days' worth of listings, usually including program titles, planned broadcast times, and often a short description of the content. This information is accessible through your receiver, in the form of a menu overlaid on the screen; how much information is displayed is dependent on what the broadcaster provides and how your receiver's program information menu is designed. Some receivers only display information about the program currently being viewed, or "now/next" information; others offer multi-hour, multi-day, or even 14-day grids. -->