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.
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.
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.
| System Parameters|
|Canada ||USA || EBU [9, 12]|
|Japan & Brazil [36, 37]|
|C/N for AWGN Channel|| +19.5 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|
|+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|
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.
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.
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:
Broadcasters attempt to balance their needs to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services.
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.
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.
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.
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.
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