MOMNOUGUI MASTER DEGREE WITH THESIS IN TELECOMMUNICATION ENGINEERING

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TV CLOUD ISSUE

Digital Television (DTV) is an advanced broadcasting technology that has transformed your television viewing experience. DTV has enabled broadcasters to offer television with better picture and sound quality. It also offers multiple programming choices, called multicasting, and interactive capabilities.

Digital television (DTV) is the transmission of audio and video by digitally processed and multiplexed signal, in contrast to the totally analog and channel separated signals used by analog TV. It is an innovative service that represents a significant evolution in television technology since color television in the 1950s.

There are four different digital television terrestrial broadcasting standards (DTTB) and they are: Advanced Television System Committee (ATSC) uses eight-level vestigial sideband (8 VSB) for terrestrial broadcasting. This standard has been adopted in the United States and in other countries. Digital Video Broadcasting-Terrestrial (DVB-T) uses coded orthogonal frequency-division multiplexing (C-OFDM) modulation and supports hierarchical transmission. This standard has been adapted in Europe, Australia and New Zealand. Terrestrial Integrated Services Digital Broadcasting (ISDB-T) is a system designed to provide good reception to fix receivers and also portable or mobile receivers. It utilizes OFDM 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 most of South America. Digital Terrestrial Multimedia Broadcasting (DTMB) adopts time-domain synchronous(TDS)- OFDM technology utilizing a pseudo random signal frame to serve as the guard interval (GI) of the OFDM block and the training symbol. The DTMB standard has been adopted in the People's Republic of China (PRC), including Hong Kong and Macau.

TECHNICAL INFORMATION

Formats and bandwidth

Digital television supports many different picture formats defined by the broadcast television systems which are a combination of size, aspect ratio (width to height ratio).

With digital terrestrial television (DTV) broadcasting, the range of formats can be broadly divided into two categories: high definition television (HDTV) for the transmission of high-definition video and standard-definition television (SDTV). These terms by themselves are not very precise, and many subtle intermediate cases exist.

Television pictures have differing amounts of definition (rendering of fine detail) according to how many individual picture elements are provided to reconstruct the picture. This definition is expressed as the number of horizontal lines and picture elements (pixels) in each line that are used for different formats. Thus when we say a format is 640 × 480p we mean there are 640 elements in each of 480 horizontal lines (scanned progressively) for a total of 307,200 pixels and an aspect ratio of 640÷480 or 4:3 (4 units wide by 3 units high) or SDTV.

One of several different HDTV formats that can be transmitted over DTV is: 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlaced video mode (1080i). Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 × 1080 at a 60 Hz progressive scan frame rate — known as 1080p.) HDTV cannot be transmitted over current analog television channels because of channel capacity issues.

Standard definition TV (SDTV), by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast. For 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries. For 16:9 broadcasts, the 720 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line).

Each commercial broadcasting terrestrial television DTV channel in North America is permitted to be broadcast at a bit rate up to 19 megabits per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can use the channel to include PSIP and can also subdivide across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers like National Datacast.

A broadcaster may opt to use a standard-definition (SDTV) digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple digital subchannels, (similar to what most FM radio stations offer with HD Radio), providing multiple feeds of entirely different television programming on the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's "bit budget" or multicasting. This can sometimes be arranged automatically, using a statistical multiplexer (or "stat-mux"). With some implementations, image resolution may be less directly limited by bandwidth; for example in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bitrate and make reception easier for more distant or mobile viewers.

Reception

There are a number of different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is using an antenna (known as an aerial in some countries). This way is known as Digital terrestrial television (DTT). With DTT, viewers are limited to whatever channels the antenna picks up. Signal quality will also vary.

Other ways have been devised to receive digital television. Among the most familiar to people are digital cable and digital satellite. In some countries where transmissions of TV signals are normally achieved by microwaves, digital MMDS is used. Other standards, such as Digital multimedia broadcasting (DMB) and DVB-H, have been devised to allow handheld devices such as mobile phones to receive TV signals. Another way is IPTV, that is receiving TV via Internet Protocol, relying on Digital Subscriber Line (DSL) or optical cable line. Finally, an alternative way is to receive digital TV signals via the open Internet. For example, there is P2P (peer-to-peer) Internet television software that can be used to watch TV on a computer.

Some signals carry encryption and specify use conditions (such as "may not be recorded" or "may not be viewed on displays larger than 1 m in diagonal measure") backed up with the force of law under the WIPO Copyright Treaty and national legislation implementing it, such as the U.S. Digital Millennium Copyright Act. Access to encrypted channels can be controlled by a removable smart card, for example via the Common Interface (DVB-CI) standard for Europe and via Point Of Deployment (POD) for IS or named differently

The DTV Transition

The switch from analog to digital broadcast television is referred to as the Digital TV (DTV) Transition. In 1996, the U.S. Congress authorized the distribution of an additional broadcast channel to each broadcast TV station so that they could start a digital broadcast channel while simultaneously continuing their analog broadcast channel;

An important benefit of the switch to all-digital broadcasting is that it freed up parts of the valuable broadcast spectrum for public safety communications (such as police, fire departments, and rescue squads). Also, some of the spectrum can now be auctioned to companies that will be able to provide consumers with more advanced wireless services (such as wireless broadband).
Consumers also benefited because digital broadcasting allows stations to offer improved picture and sound quality, and digital is much more efficient than analog. For example, rather than being limited to providing one analog program, a broadcaster is able to offer a super sharp High Definition (HD) digital program or multiple Standard Definition (SD) digital programs simultaneously through a process called “multicasting.”
Multicasting allows broadcast stations to offer several channels of digital programming at the same time, using the same amount of spectrum required for one analog program. So, for example, while a station broadcasting in analog on channel 7 is only able to offer viewers one program, a station broadcasting in digital on channel 7 can offer viewers one digital program on channel 7-1, a second digital program on channel 7-2, a third digital program on channel 7-3, and so on. This means more programming choices for viewers. Further, DTV provides interactive video and data services that were not possible with analog technology.

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