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High-definition television (HDTV) is a digital television broadcasting system
with a significantly higher resolution than traditional formats (NTSC, SECAM,
PAL). While some early analog HDTV formats were broadcast in Europe and Japan,
HDTV is usually broadcast digitally, because digital television (DTV)
broadcasting requires much less bandwidth if it uses enough video compression.
HDTV technology was first introduced in the US during the 1990s by a group of
electronics companies called the Digital HDTV Grand Alliance.
History
The first television system capable of 1125 lines was created in Soviet Union in
1958. The system was dubbed "Трансформатор" (in Russian) meaning "Transformer".
The original purpose of the system was teleconferencing between high-ranked
military command offices. This system was not intended for commercial use and
was not implemented at the large scale.
Commercial High-Definition television was first developed by Nippon Hōsō Kyōkai,
and was unveiled in 1969. However, the system did not become mainstream until
the late 1990s.
In the early 2000s, a number of high-definition television standards were
competing for the still-developing niche markets.
Three HDTV standards are currently defined by the International
Telecommunication Union (ITU-R BT.709). They include 1080i (1,080 actively
interlaced lines), 1080p (1,080 progressively scanned lines), and 720p (720
progressively scanned lines). All standards use a 16:9 aspect ratio, leading
many consumers to the incorrect conclusion of equating widescreen television
with HDTV. All current HDTV broadcasting standards are encompassed within the
ATSC and DVB specifications.
Enlarge picture
Projection screen in a home theater, displaying a high-definition television
image.
HDTV is also capable of "theater-quality" audio because it uses the Dolby
Digital (AC-3) format to support "5.1" surround sound. It should be noted that
while HDTV is more like a theater in quality than conventional television, 35 mm
and 70 mm film projectors used in theaters still have the highest resolution and
best viewing quality on very large screens. Many HDTV programs are produced from
movies on film as well as content shot in HD video.
The term "high-definition" can refer to the resolution specifications
themselves, or more loosely to media capable of similar sharpness, such as
photographic film and digital video. As of July 2007, HDTV saturation in the US
has reached 30 percent – in other words, three out of every ten American
households own at least one HDTV. However, only 44 percent of those that do own
an HDTV are actually receiving HDTV programming, as many consumers are not aware
that they must obtain special receivers to receive HDTV from cable or satellite,
or use ATSC tuners to receive over-the-air broadcasts; others may not even know
what HDTV is.
HDTV sources
The rise in popularity of large screens and projectors has made the limitations
of conventional Standard Definition TV (SDTV) increasingly evident. An HDTV
compatible television set will not improve the quality of SDTV channels. To get
a better picture HDTV televisions require a High Definition (HD) signal. Typical
sources of HD signals are as follows:
* Over the air with an antenna. Most cities in the US with major network
affiliates broadcast over the air in HD. To receive this signal an HD tuner is
required. Most newer HDTV televisions have an HD tuner built in. For HDTV
televisions without a built in HD tuner, a separate set-top HD tuner box can be
rented from a cable or satellite company or purchased.
* Cable television companies often offer HDTV broadcasts as part of their
digital broadcast service. This is usually done with a set-top box or CableCARD
issued by the cable company. Alternatively one can usually get the network HDTV
channels for free with basic cable by using a QAM tuner built into their HDTV or
set-top box. Some cable carriers also offer HDTV on-demand playback of movies
and commonly viewed shows.
* Satellite-based TV companies, such as Optimum, DirecTV, Sky Digital (in the UK
and Ireland), Bell ExpressVu and Dish Network, offer HDTV to customers as an
upgrade. New satellite receiver boxes and a new satellite dish are often
required to receive HD content.
* Video game systems, such as the Xbox (NTSC only), Xbox 360, and Playstation 3,
can output an HD signal. The Xbox Live Marketplace service offers HD movies, TV
shows, movie trailers, and clips for download to Xbox 360 consoles.
* Two optical disc standards, Blu-ray and HD DVD, can provide enough digital
storage to store hours of HD video content.
Notation
In the context of HDTV, the formats of the broadcasts are referred to using a
notation describing:
* The number of lines in the vertical display resolution.
* Whether progressive scan (p) or interlaced scan (i) are used. Progressive scan
redraws all the lines (a frame) of a picture in each refresh. Interlaced scan
redraws every second line (a field) in one refresh and the remaining lines in a
second refresh. Interlaced scan increases picture resolution while saving
bandwidth but at the expense of some flicker or other artifacts.
* The number of frames or fields per second.
The format 720p60 is 1280 × 720 pixels, progressive encoding with 60 frames per
second (60 Hz). The format 1080i50 is 1920 × 1080 pixels, interlaced encoding
with 50 fields (25 frames) per second. Often the frame or field rate is left
out, indicating only the resolution and type of the frames or fields, and
leading to confusion. Sometimes the rate is to be inferred from the context, in
which case it can usually be assumed to be either 50 or 60, except for 1080p
which is often used to denote either 1080p24, 1080p25 or 1080p30 at present but
will also denote 1080p50 and 1080p60 in the future.
A frame or field rate can also be specified without a resolution. For example
24p means 24 progressive scan frames per second and 50i means 25 interlaced
frames per second, consisting of 50 interlaced fields per second. Most HDTV
systems support some standard resolutions and frame or field rates. The most
common are noted below.
Changes in notation
The terminology described above was invented for digital systems in the 1990s. A
digital signal encodes the color of each pixel, or dot on the screen as a series
of numbers. Before that, analog TV signals encoded values for one monochrome, or
three-color signals as they scanned a screen continuously from line to line. By
comparison, radio encodes an analog signal of the sound to be sent to an
amplified speaker, typically up to 20 kHz, but video signals are in the MHz
range, which is why they are much higher in the broadcast spectrum than audio
radio. Analog video signals have no true "pixels" to measure horizontal
resolution. The vertical scan-line count included off-screen scan lines with no
picture information while the CRT beam returned to the top of the screen to
begin another field. Thus NTSC was considered to have "525 lines" even though
only 486 of them had a picture (625/576 for PAL). Similarly the Japanese MUSE
system was called "1125 line", but is only 1035i by today's measuring standards.
This change was made because digital systems have no need of blank retrace lines
unless the signal was converted to analog to drive a CRT.
Standard resolutions
Enlarge picture
Visual comparison of common TV display resolutions.
When resolution is considered, both the resolution of the transmitted signal and
the (native) displayed resolution of a TV set are taken into account. Digital
NTSC- and PAL/SECAM-like signals (480i60 and 576i50 respectively) are
transmitted at a horizontal resolution of 720 or 704 "pixels". However these
transmitted DTV "pixels" are not square, and have to be stretched for correct
viewing. PAL TV sets with an aspect ratio of 4:3 use a fixed pixel grid of 768 ×
576 or 720 × 540; with an aspect ratio of 16:9 they use 1440 x 768, 1024 × 576
or 960 × 540; NTSC ones use 640 × 480 and 852 × 480 or, seldom, 720 × 540. High
Definition usually refers to one million pixels or more.
In Australia, the 576p50 format is also considered a HDTV format, as it has
doubled temporal resolution though the use of progressive scanning. Thus, a
number of Australian networks broadcast a 576p signal as their High-definition
DVB-T signal, while others use the more conventional 720p and 1080i formats.
Technically, however, the 576p format is defined as Enhanced-definition
television.
Standard frame or field rates
* 23.977p (allow easy conversion to NTSC)
* 24p (cinematic film)
* 25p (PAL, SECAM DTV progressive material)
* 30p (NTSC DTV progressive material)
* 50p (PAL, SECAM DTV progressive material)
* 60p (NTSC DTV progressive material)
* 50i (PAL & SECAM)
* 60i (NTSC, PAL-M)
Comparison with SDTV
HDTV has at least twice the linear resolution of standard-definition television
(SDTV), thus allowing much more detail to be shown compared with analog
television or regular DVD. In addition, the technical standards for broadcasting
HDTV are also able to handle 16:9 aspect ratio pictures without using
letterboxing or anamorphic stretching, thus further increasing the effective
resolution for such content.
Format considerations
The optimum formats for a broadcast depends on the type of media used for the
recording and the characteristics of the content. The field and frame rate
should match the source, as should the resolution. On the other hand, a very
high resolution source may require more bandwidth than is available in order to
be transmitted without loss of fidelity. The lossy compression that is used in
all digital HDTV storage/transmission systems will then cause the received
picture to appear distorted when compared to the uncompressed source.
Photographic film destined for the theater typically has a high resolution and
is photographed at 24 frames per second. Depending on the available bandwidth
and the amount of detail and movement in the picture, the optimum format for
video transfer is thus either 720p24 or 1080p24. When shown on television in
countries using PAL, film must be converted to 25 frames per second by speeding
it up by 4.1 percent. In countries using the NTSC standard, 30 frames per
second, a technique called 3:2 pulldown is used. One film frame is held for
three video fields, (1/20 of a second) and then the next is held for two video
fields (1/30 of a second) and then the process repeats, thus achieving the
correct film rate with two film frames shown in 1/12 of a second.
Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in
the form 480i60 or 576i50. These may be upconverted to a higher resolution
format (720i), but removing the interlace to match the common 720p format may
distort the picture or require filtering which actually reduces the resolution
of the final output.
Non-cinematic HDTV video recordings are recorded in either 720p or 1080i format.
The format used depends on the broadcast company (if destined for television
broadcast); however, in other scenarios the format choice will vary depending on
a variety of factors. In general, 720p is more appropriate for fast action as it
uses progressive scan frames, as opposed to 1080i which uses interlaced fields
and thus can have a degradation of image quality with fast motion.
In addition, 720p is used more often with Internet distribution of HD video, as
all computer monitors are progressive, and most graphics cards do a poor job of
de-interlacing video in real time. 720p video also has lower storage and
decoding requirements than 1080i or 1080p.
In North America, Fox, My Network TV (also owned by Fox), ABC, and ESPN (ABC and
ESPN are both owned by Disney) currently broadcast 720p content. NBC, Universal
HD (both owned by General Electric), CBS, PBS, The CW, HBO, Showtime, Starz!,
MOJO HD, HDNet ,TNT, and Discovery HD Theater currently broadcast 1080i content.
In the United Kingdom on Sky Digital, there are BBC HD, Sky One HD, Sky Arts HD,
Sky Movies HD1 & 2, Sky Sports HD1,2 & X, Discovery HD, National Geographic
Channel HD, The History Channel HD & Sky Box Office HD1 & 2. With MTV HD, FX HD,
Living HD Rush HD, Ultra HD & Eurosport HD to come in the near future. BBC HD is
also available on Virgin Media. The BBC Trust has given provisional approval for
a BBC HD channel, which would be broadcast satellite, cable and DTT.
* Public consultation on the Trust's provisional conclusions on the proposed BBC
HD service is open until 23 October 2007.
Technical details
MPEG-2 is most commonly used as the compression codec for digital HDTV
broadcasts. Although MPEG-2 supports up to YCbCr chroma subsampling and 10-bit
quantization, HD broadcasts use and 8-bit quantization to save bandwidth. Some
broadcasters also plan to use MPEG-4 AVC, such as the BBC which is trialing such
a system via satellite broadcast, which will save considerable bandwidth
compared to MPEG-2 systems. Some German broadcasters already use MPEG-4 AVC
together with DVB-S2 (Pro 7, Sat.1 and Premiere). Although MPEG-2 is more widely
used at present, it seems likely that in the future all European HDTV may be
MPEG-4 AVC, and Ireland and Norway, which have not yet begun any digital
television broadcasts, are considering MPEG-4 AVC for SD Digital as well as HDTV
on terrestrial broadcasts.
HDTV is capable of "theater-quality" audio because it uses the Dolby Digital
(AC-3) format to support "5.1" surround sound. The pixel aspect ratio of native
HD signals is a "square" 1.0, in which each pixel's height equals its width. New
HD compression and recording formats such as HDV use rectangular pixels to save
bandwidth and to open HDTV acquisition for the consumer market. For more
technical details see the articles on HDV, ATSC, DVB, and ISDB.
Television studios as well as production and distribution facilities, use HD-SDI
SMPTE 292M interconnect standard (a nominally 1.485 Gbit/s, 75-ohm serial
digital interface) to route uncompressed HDTV signals. The native bitrate of
HDTV formats cannot be supported by 6-8 MHz standard-definition television
channels for over-the-air broadcast and consumer distribution media, hence the
widespread use of compression in consumer applications. SMPTE 292M interconnects
are generally unavailable in consumer equipment, partially due to the expense
involved in supporting this format, and partially because consumer electronics
manufacturers are required (typically by licensing agreements) to provide
encrypted digital outputs on consumer video equipment, for fear that this would
aggravate the issue of video piracy.
Newer dual-link HD-SDI signals are needed for the latest 4:4:4 camera systems
(Sony HDC-F950 & Thomson Viper), where one link/coax cable contains the 4:2:2
YCbCr info and the other link/coax cable contains the additional 0:2:2 CbCr
information.
Advantages of HDTV expressed in non-engineering terms
High-definition television (HDTV) potentially offers a much better picture
quality than standard television. HD's greater clarity means the picture on
screen can be less blurred and less fuzzy. HD also brings other benefits such as
smoother motion, richer and more natural colors, and the ability to allow a
variety of input devices to work together.
Almost all commercially available HD is digital, so the system cannot produce a
snowy or washed out image from a weak signal, effects from signal interference,
such as herringbone patterns, or vertical rolling.
With HDTV the lack of imperfections in the television screen often seen on
traditional television is another reason why many prefer high definition to
analog. As mentioned, problems such as snow caused from a weak signal, double
images from ghosting or multi-path and picture sparkles from electromagnetic
interference do not occur with HDTV digital broadcasts.
The colors will generally look more realistic, due to greater bandwidth. The
visual information is about 2-5 times more detailed overall. The gaps between
scanning lines are smaller or invisible. Legacy TV content that was shot and
preserved on 35 mm film can now be viewed at nearly the same resolution at which
it was originally photographed. A good analogy for television quality is looking
through a window. HDTV offers a degree of clarity that is much closer to this.
The "i" in these numbers stands for "interlaced" while the "p" stands for
"progressive". With interlaced scan, the 1,080 lines are split into two, the
first 540 being painted on a frame, followed by the second 540 painted on
another frame. This method reduces the bandwidth and raises the frame rate to
50-60 frames per second. A progressive scan displays all 1,080 lines at the same
time at 60 frames per second, using more bandwidth. (See: An explanation of HDTV
numbers and laymen's glossary)
Dolby Digital 5.1 surround sound is broadcast along with standard HDTV video
signals, allowing full surround sound capabilities. (Standard broadcast
television signals usually only include monophonic or stereophonic audio. Stereo
broadcasts can be encoded with Dolby Surround, an early home video surround
format.) Both designs make more efficient use of electricity than SDTV designs
of equivalent size, which can mean lower operating costs. LCD is a leader in
energy conservation.
Disadvantages of HDTV expressed in non-engineering terms
However, there are a variety of reasons why the best HD quality is not usually
achieved. The main problem is a lack of HD input. Many cable and satellite
channels and even some "high definition" channels are not broadcast in true HD.
Also, image quality may be lost if the television is not properly connected to
the input device or not properly configured for the input's optimal
performance.HD digital signals will either deliver an excellent picture, a
picture with noticeable pixelation, a series of still pictures, or no picture at
all. Any interference will render the signal unwatchable. As opposed to a
lower-quality signal one gets from interference in an analogue television
broadcast, interference in a digital television broadcast will freeze, skip, or
display "garbage" information.
HDTV is the answer to a question few consumers were asking. Viewers will have to
upgrade their TVs in order to see HDTV broadcasts, incurring household expense
in the process. Adding a new aspect ratio makes for consumer confusion if their
display is capable of one or more ratios but must be switched to the correct one
by the user. Traditional standard definition TV shows and feature films (mostly
movies from before 1953) originally filmed in the standard 4:3 ratio, when
displayed correctly on an HDTV monitor, will have empty display areas to the
left and right of the image. Many consumers aren't satisfied with this unused
display area and choose instead to distort their standard definition shows by
stretching them horizontally to fill the screen, giving everything a too-wide or
not-tall-enough appearance. Alternately, they'll choose to zoom the image which
removes content that was on the top and bottom of the original TV show.
As of 2007, broadcasters may demand, or cable-television operators may elect, to
place HD signals in a premium band that requires higher cable fees. That some
satellite companies offer the local HD channels as a service at additional cost
(transmission comes from satellite) suggests to some broadcasters that on-air
broadcasts of local HD signals must be a premium service to subscribers. Viewers
may be denied some television channels that they expected, be allowed only
access to the non-digital, and obviously sub-standard non-digital signal, or to
install an antenna to receive the digital broadcasts. Such issues more entail
economic and legal disputes than they entail technology.
Some viewers of HDTV will notice a type of distortion during faster scene
movement or total scene change. This "pixelation" or "blockiness" distortion is
caused by aggressive video compression in the source material. Technically this
is not a fault of HDTV, which is generally just displaying what is provided by
the tuner or source (like a cable tv or satellite tv decoder box). The higher
the compression used on the video, the more pronounced the pixelation. Since
each content provider can choose bitrate (hence, the amount of bandwidth) used
for the video, providers who devote more bandwidth will have higher video
quality HDTV content and less distortion. This type of pixelation did not occur
using traditional, broadcast, analog TV.
Another disadvantage of HDTV compared to traditional television has been
consumer confusion stemming from the different standards and resolutions, such
as 1080i, 1080p, and 720p. Complicating the matter have been the changes in
television connections from component video, to DVI, then to HDMI. Finally, the
HD-DVD vs. Blu-ray Disc high definition storage format war engenders even more
animosity for consumers. The confusion has led to slower uptake of the
technology as many people wait to see what becomes the "ultimate" de-facto
standard.
Early systems
The term "high definition" was used to describe the electronic television
systems of the late 1930s and 1940s beginning with the former British 405-line
black-and-white system, introduced in 1936; however, this and the subsequent
525-line U.S. NTSC system, established in 1941, were high definition only in
comparison with previous mechanical and electronic television systems, and NTSC,
along with the later European 625-line PAL and SECAMs, is described as standard
definition today.
On the other hand, the 819-line French black-and-white television system
introduced after World War II arguably was high definition in the modern sense,
as it had a line count and theoretical maximum resolution considerably higher
than those of the 625-line systems introduced across most of postwar Europe.
However, it required far more bandwidth than other systems, and was switched off
in 1986, a year after the final British 405-line broadcasts.
Japan was the only country where commercial analog HDTV was launched and had
some success. In other places, such as Europe, analog (HD-MAC) HDTV failed.
Finally, although the United States experimented with analog HDTV (there were
about 10 proposed formats), it soon moved towards a digital approach.
In addition to an HD Ready television, other equipment is often needed for the
home user to view High Definition Television. Cable Ready TV sets can display HD
content without the use of an external box. They provide a card slot that
accepts a CableCARD. There are several sources of high definition content and
the equipment used to control each of these must be HD compatible.
High Definition picture sources include terrestrial broadcast, direct broadcast
satellite, digital cable, high definition discs (BD and HD DVD), internet
downloads and the latest generation of games consoles. The availability of
television pictures broadcast in the HD format varies by region and country.
HDTV can be recorded to D-VHS (Data-VHS), W-VHS (analog only), to a HDTV-capable
digital video recorder (for example DirecTV's high-definition Digital video
recorder, Sky HD's set-top box, or TiVo's Series 3 or HD recorders), or an
HDTV-ready HTPC. Some cable boxes are capable of receiving or recording two
broadcasts at a time in HDTV format, and HDTV programming, some free, some for a
fee, can be played back with the cable company's on-demand feature. The massive
amount of data storage required to archive uncompressed streams make it unlikely
that an uncompressed storage option will appear in the consumer market soon.
Realtime MPEG-2 compression of an uncompressed digital HDTV signal is also
prohibitively expensive for the consumer market at this time, but should become
inexpensive within several years (although this is more relevant for consumer HD
camcorders than recording HDTV). Analog tape recorders with bandwidth capable of
recording analog HD signals such as W-VHS recorders are no longer produced for
the consumer market and are both expensive and scarce in the secondary market.
In the United States, as part of the FCC's "plug and play" agreement, cable
companies are required to provide customers that rent HD set-top boxes with a
set-top box with "functional" Firewire (IEEE 1394) upon request. None of the
direct broadcast satellite providers have offered this feature on any of their
supported boxes, but some cable TV companies have. As of July 2004, boxes are
not included in the FCC mandate. This content is protected by encryption known
as 5C. This encryption can prevent duplication of content or simply limit the
number of copies permitted, thus effectively denying most if not any fair use of
the content.
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