HDTV
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High-Definition
television (HDTV)
refers to the broadcasting
of television
signals with a significantly higher resolution
than traditional formats (NTSC,
SECAM, PAL) allow. Except for
early analog formats in Europe
and Japan,
HDTV is broadcast digitally, and therefore its introduction sometimes coincides
with the introduction of digital television (DTV): this technology was
first introduced in the USA during the 1990s, by the Digital HDTV Grand Alliance (grouping together AT&T, General Instrument, MIT, Philips,
Sarnoff, Thomson,
and Zenith)[1]. While a number of high-definition
television standards have been proposed or implemented on a limited basis, the
current HDTV standards are defined as 1080 active interlaced or progressive
lines, or 720 progressive lines, using a 16: 9 aspect ratio in ITU-R BT.709. The term
"high-definition" can refer to the resolution specifications
themselves, or to media capable of similar sharpness such as movie film.
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 frames (p) or interlaced
fields (i) are used.
- The number of frames or fields per
second.
For example, the format 720p60 is 1280 × 720 pixels, progressive encoding
with 60 frames per second (60 hertz
known as 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. Sometimes
the rate is then 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.
A
frame or field rate can also be specified without a resolution. For example 24p
means 24 progressive 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
It
should be noted that the terminology described above was invented for digital
systems in the 1990s. Before that, analog TV had no true "pixels" to
measure horizontal resolution, and 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 480 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
Note:
This diagram lists only a few common video resolutions based on specification. For real life video resolutions, subject to
interlace artifacts, please view the diagram at the end of this article or the display resolution article.
- NTSC is 720 x 480 non-square pixels
- PAL is 720 x 576 non-square pixels
(Note that the PAL resolution given in the image above is for square
pixels.)
NTSC
uses pixels that are narrower than square (0.912), PAL uses pixels that are
wider than square (1.094). This is referred to as the CCIR 601
standard for digital video.
Standard frame or field
rates
- 23.976p (allow easy conversion to NTSC)
- 24p (cinematic film)
- 25p
- 30p
- 50p
- 60p
- 50i (PAL/SECAM)
- 60i (NTSC)
Comparison to SDTV
HDTV
has at least twice the resolution of SDTV, thus allowing much more detail to be
shown compared to 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,
thus further increasing the effective resolution for such content.
Close-up view
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HDTV at four times the resolution of SDTV. |
SDTV resolution. |
Format considerations
The
optimum format 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 frame/s. 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%. In countries using
the NTSC
standard, (60 fps) a technique called 3:2 pull down 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.
(See also: Telecine)
Older (pre-HDTV) recordings on video tape such as Betacam
SP are often either in the form 480i60 or 576i50. These may be up converted 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
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 fields, 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, and few people possess displays
capable of displaying the 1920x1080 resolution without scaling. 720p appears at full
resolution on a common 1280x1024 LCD,
which can be found for under $250. An LCD
capable of native 1080p
resolution costs close to a thousand dollars.
In North America,
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, HBO-HD, INHD, HDNet and TNT currently broadcast 1080i content.
Technical details
One of the first DVB-S2 tuner cards.
MPEG-2
is most commonly used as the compression codec for digital HDTV
broadcasts. Although MPEG-2 supports up to 4:2:2 YCbCr chroma subsampling
and 10-bit quantization, HD broadcasts use 4:2:0 and 8-bit
quantization to save bandwidth. Some broadcasters also plan to use MPEG-4 AVC.
Some German broadcasters already use MPEG-4 together with DVB-S2 (ProSieben,
Sat1 and Three Premiere Channels). Although MPEG-2 is more widely used at
present, it seems likely that in the future all European HDTV may be MPEG-4 and
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, or
1 pixel length = 1 pixel width. New HD compression and recording formats such
as HDV use rectangular pixels for more efficient compression and to open HDTV
acquisition for the consumer market.
For more technical details see the articles on HDV, ATSC, DVB, and ISDB, respectively.
Within television studios and other production and distribution facilities,
HD-SDI SMPTE 292M
interconnect standard (a nominally 1.485 Gbit/s, 75-ohm serial digital
interface) is used to route uncompressed HDTV signals. The native bitrate of
HDTV formats cannot be supported by 6-8MHz 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), thereby one link/coax cable contains the 4:2:2
YCbCr info and the other link/coax contains the 0:2:2 additional CbCr
information.
Advantages of HDTV expressed
in non-engineering terms
- All commercial HD is digital, so the
signal will either deliver an excellent picture, a picture with noticeable
pixilation,
a series of still pictures, or no picture at all. You would never get a
snowy or washed out image from a weak signal, effects from signal
interference, such as herringbone patterns, or vertical rolling.
- Most HD programming and films will be
presented in the 16:9 proportioned, semi-widescreen
format (though some films created in even wider ratios will still display
"letterbox"
bars on the top and bottom of even 16:9 sets.) Older films and programming
that retain their 4:3 ratio display will be presented in a version of
letterbox commonly called "pillar box", displaying bars on the
right and left of 16:9 sets (rendering the term "full screen"
a misnomer).
Or, one can usually choose to enlarge the image to fill the screen;
however this option will display a distorted, stretched-out picture.
- The colors will generally look more
realistic, due to their 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 as that at which it was
originally photographed.
- Two new pre-recorded disc formats
support HDTV resolutions, namely HD DVD (which
supports 720p and 1080i but future players will support 1080p) and Blu-ray
(support up to 1080p). Players for both systems are expected to be
backward-compatible with DVDs. However the two formats are not compatible
with each other.
- The increased clarity and detail make
larger screen sizes more comfortable and pleasing to watch.
- Dolby Digital 5.1 sound is broadcast
along with standard HDTV video signals allowing full surround sound
capabilities. (standard television signals broadcast basic stereo audio
signals)
- Both designs make more efficient use
of electricity
than SDTV designs of equivalent size which can mean lower operating costs.
Early systems
The British 405-line black-and-white system, introduced in 1936, was the
first to advertise itself as "high definition," although it was high
definition only in comparison with previous mechanical and electronic
television systems, and not in the sense of the term as it is used 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, and the later European 625-line color systems (PAL and SECAM).
Contemporary systems
Asia
Hong Kong
Due to
the city's cramped living conditions, flat panel widescreen TV monitors are
very popular in
Japan
Republic of Korea
After
a long controversy between the government and broadcasters, ATSC was chosen over DVB-T.
From 2005, digital services are available across the entire country.
It is
required that at least 10 hours of HD content to be broadcast on a weekly basis
during the first year of commercial digital service.
Singapore
On
Wednesday, May 31, 2006,
Australia
Europe
North America
Canada
In
Canada, on November 22, 2003,
CBC had their first broadcast in HD, in
the form of the Heritage Classic outdoor NHL game between the Edmonton Oilers
and the Montreal Canadiens. Bell ExpressVu,
a Canadian satellite
company, Rogers Cable and Videotron
provide somewhat more than 21 HDTV channels to their subscribers including TSN
HD, SportsNet HD, Discovery HD (Canadian Edition), The Movie Network HD, and several
U.S. stations plus some PBS feeds and a couple of pay-TV movie channels. CTV Toronto broadcast in
HD along with its western counterpart, BC CTV. They were also the first to
broadcast a terrestrial HD digital ATSC signal in
Mexico
Mexican
television company Televisa
made experimental HDTV broadcasts in the early-1990s, in collaboration with
During the first half of 2005, at least one cable provider in Mexico City
(Cablevision)
has begun to offer 5 HDTV channels to subscribers purchasing a digital video
recorder (DVR).
In 2005, TV Azteca signed a deal with Harris Corporation's broadcast
communications division for digital TV transmitters and HDTV encoding equipment
to bring high-definition TV to nine Mexican cities.
The launch will be carried out in two phases. By the third quarter of 2006,
HDTV transmissions will be available in
Also, TV Azteca
has planned to broadcast the Mexican football tournament in HDTV. And almost
all retailers have started shipping televisions with HDTV tuners.
XETV
in
Guatemala
The
broadcast started on 2006 World Cup on ATSC standard and channel 3 and 7, on HD
on 17 & 19
United States
In the
United States,
HDTV specifications are defined by the ATSC. An HDTV-compatible
TV usually uses a 16:9 aspect ratio display with an integrated ATSC tuner.
Lower-resolution sources like regular DVDs may be upscaled to the native
resolution of the TV. It is estimated that by the end of 2006, 10% of American
TVs will be HDTVs, but in the same survey, 83% of Americans are not satisfied
with the HDTV they are getting. The FCC has mandated that all broadcast must be
in digital starting on February 17, 2009. The original deadline of January 1,
2006 was abolished when it was realized that TV stations and customers would
not be able to meet the earlier deadline.
South America
Argentina
While
HDTV-ready TVs sales are increasing in this country, no single HD feed is
available right now. The government is still deciding which format will be
used, but chances are American's ATSC will be chosen. DirecTV and local cable
company "Cablevision" may also offer HDTV decoders and channel pack
later this year.
Brazil
Since 2001
native 480p
TVs have been sold in
The Brazilian government took a while to ponder which standard to choose,
but on June 29th 2006, President Luis Inácio Lula da Silva signed a decree
choosing ISDB as a national standard, strangely based on something not so
transmission-related - the promise of construction of a semiconductor factory
in the country financed by NEC, Sony, Panasonic and Toshiba.
The transition to ISDB will take 9 years, the manufacturers will adopt the
new standard for the new TVs and will also make converters available so the
people can continue using their ISDB-incompatible TV's with the new
transmission system.
The
Japanese government is studying the implementation of some improvements on the
standard as suggested by Brazilian researchers. These new features are very unlikely
to be adopted in Japan due to incompatibility problems, but are being
considered for use in future implementations in other countries, including
Brazil itself.[2]
Uruguay
While
HDTV-ready TV sets are available at the country, a few factors seem to
constraint the development of the new technology in the near term:
- Prices for LCD, Plasma and DLP based
TV sets can be two times more expensive in
- The cable industry has then few
incentives to provide other services beyond basic TV services:
Internet-by-cable and cable-telephony have been either strictly prohibited
by law or by high taxes on equipment that make a business case for newer
technologies unfeasible. Digital Cable has stared rolling out, with an
initial 100% increase of monthly cost for the SD digital service. High
HDTV TV-set prices do not help. Some of the cable companies for the
largest markets are also owned by the largest local TV content providers,
which also as of 2006 have not started producing any HDTV content
publicly.
- DirecTV might be in the better
standing to provide HDTV content, given that they have experience and
content from the
Recording, compression
HDTV
can be recorded to D-VHS (Digital-VHS), W-VHS (analog MUSE only), to an
HDTV-capable digital video recorder such as DirecTV's
high-definition TiVo or
Dish Network's
DVR 921, 942 or VIP622, or to a computer equipped with an HDTV capture card. In
the
However, 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.
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. [3].
This encryption can prevent someone from recording content at all or simply
limit the number of copies.
Table of terrestrial HDTV
transmission systems
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Main characteristics of three DTTV systems |
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Systems |
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Source coding |
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Video |
Main Profile syntax of
ISO/IEC 13818-2 (MPEG-2
– Video) |
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Audio |
ATSC Standard A/52 (Dolby AC-3) |
ISO/IEC 13818-2 (MPEG-2 – Layer II Audio) and
Dolby AC-3 |
ISO/IEC 13818-7 (MPEG-2 – AAC Audio) |
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Transmission system |
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Channel coding |
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Outer coding |
R-S (207, 187, t = 10) |
R-S (204, 188, t = 8) |
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Outer interleaver |
52 R-S block |
12 R-S block |
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Inner coding |
rate 2/3 Trellis code |
PCC: rate 1/2, 2/3, 3/4, 5/6, 7/8;
constraint length = 7, Polynomials (octal) = 171, 133 |
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Inner interleaver |
12 to 1 Trellis code |
bit-wise, frequency |
bit-wise, frequency,
selectable time |
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Data randomization |
16-bit PRBS |
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Modulation |
COFDM |
BST-COFDM
with 13 frequency segments |
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Designation |
Usage examples |
Definition (lines) |
Rate (Hz) |
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Interlaced (fields) |
Progressive
(frames) |
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Low; MP@LL |
240; 288 (SIF) |
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Standard; MP@ML |
60; |
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Enhanced |
480; 576 |
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High; MP@HL |
720 |
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1080 |
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This table illustrates
total horizontal and vertical detail via box size. It does not accurately
reflect the screen shape (aspect ratio) of these formats, which is always stretched
or squeezed to 4:3 or 16:9. The table assumes an average vertical detail loss
of .75x due to interlace. The actual loss is variable due to content, motion,
opinion on acceptable levels of flicker, and possible success of
deinterlacing. 1920 × 1080i is not included because all common use of 1080i
is filtered to 1440 or less. |
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