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Digital television

Digital television (DTV) is the sending and receiving of moving images and sound by 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 because it offers new financial opportunities.

Digital television is more flexible and efficient than analog television. When properly used by broadcasters, digital television can allow higher-quality images, sound, and more programming choices than analog does. However, a digital signal does not necessarily carry a higher-quality image or sound than an analog signal.

After February 19, 2009, full-power television stations in the USA will broadcast in digital only.

While the majority of the viewed TV broadcast stations are full-power stations, about 1800 in number, there are three other categories of TV stations that exist:“low-power” stations, “Class A” stations, and “TV translator” stations. There is presently no deadline for these stations, about 7100 in number, to convert to digital broadcasting.

Technical information

Formats and bandwidth

With digital television, two formats--HDTV and SDTV--of TV programs are broadcast.

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(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 (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). 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 (similar to what most FM stations offer with HD Radio), 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 are 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 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 DSL or optical cable line. Finally, an alternative way is to receive digital TV signals via the open Internet. For example, there is a lot of P2P Internet Television software that can be used to watch TV on your 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 CableCard.

Protection parameters for terrestrial DTV broadcasting

In order for digital television to be broadcast, it must initially interoperate with analog television. When analog television ceases to exist, digital television signals must not interfere with each other. Propagation research carried out by several important digital television regulators has derived a table of acceptable parameters for tolerable interference margins. This table below provides all the important acceptable interference margins.

System Parameters
(protection ratios)
Canada [13] USA [5] EBU [9, 12]
ITU-mode M3
Japan & Brazil [36, 37]
C/N for AWGN Channel +19.5 dB
(16.5 dB)
+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)
+15.27 dB +19 dB +19 dB
Lower Adjacent Channel DTV into Analog TV −16 dB −17.43 dB −5 ~ −11 dB −6 dB
Upper Adjacent Channel DTV into Analog TV −12 dB −11.95 dB −1 ~ −10 −5 dB
Lower Adjacent Channel Analog TV into DTV −48 dB −47.33 dB −34 ~ −37 dB −35 dB
Upper Adjacent Channel Analog TV into DTV −49 dB −48.71 dB −38 ~ −36 dB −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

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 also 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.

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.

Disadvantages to conversion

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 can be added to non-digital televisions to lengthen their useful lifespan. Several of these devices have already been shown and, while few were initially available, they are becoming more available by the day. In the United States, a government-sponsored coupon is available to offset the cost of an external converter box. Once connected to the converter unit, operation of non-digital units is achievable and, in most cases, rich in new features (in comparison to previous analog reception operation). At present, analog switchoff is scheduled for February 17, 2009 in the United States and August 31, 2011 in Canada.

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. ATSC-capable VCRs are likely to be far less common than their NTSC counterparts, with most current offerings being VCR/DVD combo units. 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, except one: The Artec T3A.

Portable radios that are able to listen to television audio on VHF channels 2-13 would also lose this ability, while television stations which formerly broadcast on Channel 6 TV stations in the United States (with analog FM audio on 87.75 MHz) would no longer be heard on standard FM broadcast band radios. These stations would lose the ability for commuters to listen to their broadcasts.

If any new TVs contained only an ATSC tuner, this could prevent 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 of what is currently being sold. Such a box would also likely introduce additional delay into the video signal. Fortunately, analog inputs suitable for connection to VCRs have remained available on all current digital-capable TV's.

Compression artifacts and allocated bandwidth

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 hazes.

Broadcasters attempt to balance their needs to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services.

Buffering and preload delay

Unlike analog televisions, digital televisions have a significant delay when changing channels, making "channel surfing" more difficult.

Different devices need different amounts of preload time to begin showing the broadcast stream, resulting in an audio echo effect when two televisions in adjacent rooms of a house are tuned to the same channel.

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. The nature of digital TV results in a perfect picture initially, until the receiving equipment starts picking up noise or losing signal. Some equipment will show a picture even with significant damage, while other devices may go directly from perfect to no picture at all (and thus not show even a slightly damaged picture). This latter effect is known as the digital cliff or cliff effect.

For remote locations, distant analog channels that were previously acceptable in a snowy and degraded state may be anything from perfect to completely unavailable. In areas where transmitting antennas are located on mountains, viewers who are too close to the transmitter may find reception difficult or impossible because the strongest part of the broadcast signals pass above them. The use of higher frequencies will add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available. Many intermittent signal fading conditions, such as the rapid-fade effect caused by reflections of UHF television signals from passing aircraft, will not produce intermittently-snowy video, but potential intermittent loss of the entire signal.

Multi-path interference is a much more significant problem for DTV than for analog TV and affects reception, particularly when using simple antennas such as rabbit ears. This is perceived as "ghosting" in the analog domain, but this same problem manifests itself in a much more insidious way with DTV. Unlike the problems of the preceding paragraph, multi path can in fact be worse for DTV under high signal conditions. It is perceived by the viewer as a spotty loss of audio or picture freezing and pixelation as people move about in the vicnity of the antenna and is often worse in wet weather due to increased reflection re-polarization of the DTV signal arriving from multiple paths. In extreme cases - the signal is lost completely. The cure is to employ a directional antenna outdoors, aligned with the transmitting location.

Limitations

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) is not part of the ATSC specification. High frame-rate 1080p may become an option in the near future, as a result of recent technology advances such as H.264/MPEG-4 AVC video coding, allowing more detail to be sent via the same channel bandwidth allocations that are 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 frame buffers to eliminate scanline crawling.

Conversion

As of late 2007, six countries had completed the process of turning off analog terrestrial broadcasting. Many other countries had plans to do so or were in the process of a staged conversion. IEEE in Jamaica also decided to switch over the same time as the US.

See also

References

External links

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