Digital Television

HDTV on a projector screen in a home theater

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.

	525/60 Standard	625/50 Standard
# of lines per frame	525	625
# of lines per field	262.5	312.5
# of frames per second	29.97	25
# of fields per second($f_{v}$), Hz	$2f_{h}/525=59.94$	$2f_{h}/625=50$

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.

Illustration of video scanning

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

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

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

$27\ Mwords/s\ x10\ bits/word=270Mbps$