http://en.wikipedia.org/wiki/Digital_television
Digital television
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Digital television (DTV) refers to the sending and receiving of moving images and sound by means of discrete (digital) signals, in contrast to the analog signals used by analog TV. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new financial opportunities.
Digital television is more flexible and efficient than analog television. When properly used by broadcasters, digital television allows higher-quality images and sound and more programming choices than analog does. However, although DTV allows for superior technical quality, a digital signal does not necessarily carry a higher-quality image or sound than an analog signal.
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[edit] Technical information
[edit] Formats and bandwidth
In current practice, high-definition television (HDTV), which is usually used over DTV, uses one of two formats: 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace 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 1080p60 — but this format is not standard and no broadcaster is able to transmit these signals over the air at acceptable quality yet.)
Standard definition TV, 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 (rescaled to 768 × 576) is used in PAL countries. For 16:9 broadcasts, the 704 × 480 (rescaled to 848 × 480) format is used in NTSC countries, while 720 × 576 (rescaled to 1024 × 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).[1][2] The perceived quality of such programming is surprisingly acceptable because of interlacing—the effective vertical resolution is halved to 288 lines.
Each DTV channel is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers.
A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple subchannels, providing multiple feeds of entirely different 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.
[edit] 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 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 with guaranteed quality of service (QoS). Finally, an alternative way is to receive TV signals via the open Internet infra-structure, usually referred to as Internet TV.
Today, regardless of how viewers receive DTV, most will pick up digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand — thus using the television purely as a monitor. However, a growing number of TV sets with integrated receivers are available — these are known as iDTVs.
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 CableCard.
[edit] Protection parameters for terrestrial DTV broadcasting
[clarify]
| System Parameters (protection ratios) | Canada [13] | USA [5] | EBU [9, 12] ITU-mode M3 | Japan & Brazil [36, 37][3] |
|---|---|---|---|---|
| C/N for AWGN Channel | +19.5 dB (16.5 dB[4]) | +15.19 dB | +19.3 dB | +19.2 dB |
| Co-Channel DTV into Analog TV | +33.8 dB | +34.44 dB | +34 ~ 37 dB | +38 dB |
| Co-Channel Analog TV into DTV | +7.2 dB | +1.81 dB | +4 dB | +4 dB |
| Co-Channel DTV into DTV | +19.5 dB (16.5 dB[4]) | +15.27 dB | +19 dB | +19 dB |
| Lower Adjacent Channel DTV into Analog TV | −16 dB | −17.43 dB | −5 ~ −11 dB[5] | −6 dB |
| Upper Adjacent Channel DTV into Analog TV | −12 dB | −11.95 dB | −1 ~ −10[5] | −5 dB |
| Lower Adjacent Channel Analog TV into DTV | −48 dB | −47.33 dB | −34 ~ −37 dB[5] | −35 dB |
| Upper Adjacent Channel Analog TV into DTV | −49 dB | −48.71 dB | −38 ~ −36 dB[5] | −37 dB |
| Lower Adjacent Channel DTV into DTV | −27 dB | −28 dB | −30 dB | −28 dB |
| Upper Adjacent Channel DTV into DTV | −27 dB | −26 dB | −30 dB | −29 dB |
- ^ Latest snapshots - Freeview/DTT bitrates (Mendip transmitter, UK)
- ^ http://www.aums30.dsl.pipex.com/home.htm
- ^ ISDB-T (6 MHz, 64QAM, R=2/3), Analog TV (M/NTSC).
- ^ a b The Canadian parameter, C/(N+I) of noise plus co-channel DTV interface should be 16.5 dB.
- ^ a b c d Depending on analog TV systems used.
[edit] Interaction
Interaction happens between the TV watcher and the DTV system. It can be understood in different ways, depending on which part of the DTV system is concerned. It can be an interaction with the STB only (to tune to another TV channel or to browse the EPG).
Modern DTV systems are able to provide interaction between the end-user and the broadcaster through the use of a return path. With the exceptions of coaxial and fiber optic cable, which can be bidirectional, a dialup modem, Internet connection, or other method is typically used for the return path with unidirectional networks such as satellite or antenna broadcast.
In addition to not needing a separate return path, cable also has the advantage of a communication channel localized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite). This provides enough customizable bandwidth to allow true video on demand.
[edit] Advantages to conversion
DTV has several advantages over analog TV, the most significant being that digital channels take up less bandwidth (and the bandwidth needs are continuously variable, at a corresponding cost in image quality depending on the level of compression). This means that digital broadcasters can provide more digital channels in the same space, provide high-definition television service, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages, spoken or subtitled. The sale of non-television services may provide an additional revenue source. In many cases, viewers perceive DTV to have superior picture quality, improved audio quality, and easier reception than analog.
[edit] Disadvantages to conversion
[edit] Impact on existing analog technology
The analog switch-off ruling, which so far has met with little opposition from consumers or manufacturers, would render all non-digital televisions obsolete on the switch-off date, unless connected to an external off-the-air tuner, analog or digital cable, or a satellite system. An external converter box (an ATSC tuner) can be added to non-digital televisions to lengthen their useful lifespan. Several of these devices have already been shown, and while few are currently available, they are becoming more available by the day.
Some existing analog equipment will be less functional with the use of a converter box. For example, television remote controls will no longer be effective at changing channels, because that function will instead be handled by the converter box. Similarly, video recorders for analog signals (including both tape-based VCRs and hard-drive-based DVRs) will not be able to select channels, limiting their ability to automatically record programs via a timer or based on downloaded program information. Also, older, handheld televisions, which rely primarily on over-the-air signals, and battery operation, will be rendered impractical, since the proposed converter boxes are not portable, nor powered with batteries. Portable radios which feature the ability to listen to television audio on VHF channels 2-13 would also lose their ability to function, while television stations which formerly broadcast on Channel 6 and were able to have their analog audio heard on common radios using a quirk in the system where their audio could be heard on the far end of the FM band at 87.7, would lose the ability for commuters to listen to their broadcasts.
If new TVs contain only an ATSC tuner, it prevents older devices, such as VCRs and video game consoles with only an analog RF output, from connecting to the TV. Connection would require an analog to digital converter box, which is the opposite as what is currently being sold. Such a box would also likely introduce additional delay into the video signal.
[edit] Compression artifacts and allocated bandwidth
DTV picture technology is still in its early stages. DTV images have some picture defects that are not present on analog television or motion picture cinema, because of present-day limitations of bandwidth and compression algorithms such as MPEG-2.
When a compressed digital image is compared with the original program source, some hard-to-compress image sequences may have digital distortion or degradation. For example:
- quantization noise,
- incorrect color,
- blockiness,
- a blurred, shimmering haze.
Broadcasters attempt to balance their needs to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services.
[edit] Buffering and preload delay
Unlike analog televisions, digital televisions have a significant delay when changing channels, making "channel surfing" difficult.
Different devices need different amounts of preload time to begin showing the broadcast stream, resulting in an undesirable and annoying audio echo effect when two televisions in adjacent rooms of a house are tuned to the same channel.
[edit] Effects of poor reception
Changes in signal reception from factors such as degrading antenna connections or worsening weather conditions may gradually reduce the quality of analog TV. However, the nature of digital TV results in a rapid failure of the ability of the receiving equipment to generate a watchable picture. This effect is known as the digital cliff or cliff effect.
For rural locations, distant analog channels that were previously acceptable in a snowy and degraded state will now be unavailable.
[edit] Hype vs reality of picture quality
Making the switch from analog to digital will provide television viewers with the potential for a movie-quality picture, and better HD for those who own an HDTV, but initially most broadcasters simply transmit a low-quality non-widescreen 480i digital version of their old existing analog services.
[edit] Limitations of digital television
The greatest DTV detail level currently available is 1080i, which is a 1920x1080 interlaced widescreen format. Interlacing is done to reduce the image bandwidth to one-half of full-frame quality, which gives better frame update speed for quick-changing scenes such as sports, but at the same time reduces the overall image quality and introduces image flickering and crawling scanlines because of the alternating field refresh.
Full-frame progressive-scan 1920x1080 (1080p) requires up to twice the data bandwidth currently available in the DTV channel specification. 1080p may become an option in the future, as image compression algorithms improve, allowing more detail to be sent via the same channel bandwidth allocations to be used now.
The limitations of interlacing can be partially overcome through the use of advanced image processors in the consumer display device, such as the use of Faroudja DCDi and using internal framebuffers to eliminate scanline crawling.
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