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Digital TV
(HDTV) Definition
(Last update
in 2012)
Digital
Television - (DTV) -
(Often known as High Definition Television {HDTV}; may also be known as
Advanced Television {ATV}, Integrated Digital Television.) (Though
people often use DTV as a synonym for HDTV, DTV isn't always HDTV) -
"Digital Television" is a comprehensive term for digital and digitised
television. It includes HDTV, Datacasting
and Multicasting.
In the 80s, when cable programming became serious competition to the
film industry, the Japanese company NHK formally introduced High
Definition Television technology to Hollywood. At the time it was
called "NHK Hi-vision". It transmitted better picture and sound, and
had a wider screen. All of that was something the film industry very
much wanted. Unfortunately, the HDTV signal required much more
bandwidth (20 MHz) compared to a standard NTSC
analog signal (6 MHz).
In addition there were other problems dealing with the signal.
Then in 1996, things really started heating up. Over 23
well-funded international proposals from corporations and educational
institutions were submitted to the U.S. Federal Communications
Commission. These proposals tried to answer:
(1) Would a HDTV transmission be analog, a mixture of analog and
digital, or purely digital?
(2) How would the signal transmit: cable, broadcast, satellite, etc?
(3) What part of the broadcast spectrum would HDTV occupy?
(4) How would a 20 MHz signal be fitted into a 6 MHz one?
In 1990, the FCC decided a HDTV digital signal could be simultaneously
broadcasted until analog signals were phased out. In order to receive
this signal, people would need to buy either a digital TV set or a
converter such as a set-top
box.
A digital TV tuner card could work for their computer. Eventually, four
proposals seemed serious, but no one was the winner. A suggestion was
made to form a "Grand Alliance" between these contenders: MIT, Philips,
AT&T, General Instrument, Zenith Sarnoff and Thomson.
After
much discussion in 1996, the FCC adopted the Advanced Television
Systems Committee (ATSC)
Digital Television Standard based on an MPEG-2
compression scheme proposed by the "Grand Alliance." Also in 1996 the
Telecommunications Act was passed. Then in 1997 the FCC allocated pure
digital spectrum, (not analog or a blend,) to broadcasters.
In
addition they decided to require broadcasters to transmit digital
programming on a graduated schedule by 2006, (which was extended.)
During the 1990s, there were a number of important related
developments:
(1) The cable industry became a powerhouse across the country.
(2) The PC revolution gave the television and film producing industry
software tools to digitally edit and manage their work, especially
those from Macromedia, Avid and Adobe.
(3) The CDROM industry became a leader in the development of
interactive multimedia applications.
(4) Satellite companies, eager to get a bigger piece of the market,
introduced smaller 18 inch to a yard in diameter residential satellite
dishes for homes (Direct
Broadcast Satellite {DBS}). These dishes receive
transmissions of
hundreds of channels of digitally encoded NTSC broadcast signals to
digital-to-analog set-top boxes nationally and internationally.
Typically they offer more interactive
television
than their cable counterparts.
(5) A mix of analog and digital consumer electronics devices also
appeared such as CD-ROMs, VCRs, camcorders, laser disks, and digital
video disks.
(6) The success of the Internet seriously affected the television
industry. Among other things, a lot of TV viewers, and thus
advertising revenues related to them, was lost to folks who were
spending time concentrating on the Internet. Something had to
be
done.
Fully digital television requires much more technically advanced
equipment than does analog TV. Digital television receivers rely on
advanced electronic circuitry to decode the digital signals in real
time. Unlike the backward compatibility between color and black and
white analog sets of yesteryear, fully digital broadcasts cannot be
displayed on analog equipment without additional components.
The digital broadcast video (and audio) signals are encoded as a series
of pulses where the pulse height and distribution define the signal.
Rather than being directly related to the voltage applied to the
electron guns of analog TVs, digital to analog converters (DACs) are
needed to convert the signal pulses into the proper voltage levels. A
digital TV signal gets decoded into 3 simultaneous voltage levels, one
each for the 3 electron guns of the digital TV. The voltages are
applied in the same manner as for analog TV, only the method of
developing the voltages is different. In terms of scanning, even
digital TV is an analog process, since the scanning signals are analog
(at least on the standard CRT picture tube; this is not the case with
LCD displays.)
As you know, there are a number of differences between digital and
analog television, several are noted below:
1) The DTV hardware reads or turns the broadcast signal (depending on
the hardware involved) into bits and bytes, which is the language of
computers. This makes computers and HDTVs
compatible. As
the primary language (code) for the World Wide Web is HTML, which is a
computer code turned into bits and bytes, this makes for a much more
efficient way to experience and interact with the Internet.
2) Digital TV signals are much less susceptible to interference. With
MPEG compression (and other) technologies, an error-free picture is
possible, even if minor signal errors are present. With an analog
broadcast signal, minor signal errors can cause minor picture
degradation (ghosting etc.) As interference becomes worse,
the
picture becomes worse. With a digital signal, because of the
way
the error correction works, the picture will still look perfect until
the threshold FEC signal to noise ratio is reached. Ratios below the
threshold lead to an unacceptable picture (which is called catastrophic
degradation.)
3) The use of compression (which DTV does) means that a standard 6 MHz
TV channel bandwidth can carry around 4 or 5 separate digital TV
channels (versus one with the analog signal) and have the same good
resolution.
4) DTV can offer at least twice the picture resolution of straight
analog TVs, this making possible a cinema-quality image as well as
sound quality like that of a compact disc.
5) Signals for adjacent digital TV channels do not interfere with each
other like those in analog systems. Therefore more channels can be
occupied.
6) Unlike analog systems, the resolution of the digital TV broadcast
can be varied.
7) Because digital signals can be compressed when they're sent to the
DTV, the viewer can receive a great many more channels. This allows for
the development of channels with content that only a select group of
viewers would be interested in. With some sort of back channel the
viewer can interact with others more freely.
8) With some sort of return
path (back
channel) the viewer can interact with others more freely.
More
Definitions of
Digital TV (and Demonstrations)
Analog
TV – (a.k.a. SDTV, Standard Definition Television,
Standard
Definition) - These are by far the most widely used type of television
sets. These televisions receive signals sent as a continuously varying
analog waveform. The waveform's value determines the amount of voltage
that will be applied to the electron guns in the back of the picture
tube. This directly affects the luminance (the technical term
for
the picture brightness (a.k.a. intensity,) and chrominance (the
technical term for the color) of the picture. Analog TVs are
far
and away the dominant type of TVs used by the public and have been for
decades. In analog TV systems, the voltage applied to the electron guns
is proportional to the constantly changing video signal voltage.
Analog TVs however have limitations:
Analog TV
channels are
susceptible to interference caused by physical features (hills,
buildings etc.) This interference often takes the form
of
"ghosting" on the picture screen.
The normal TV channel occupies a bandwidth of 6 MHz. With analog TVs,
the viewer can see only a limited total number of 6 MHz channels,
obviously limiting the total number of possible channels the consumer
can receive and view, (without outside electronic help). In
addition, many of the frequency bands that could be used by TV
channels, have to be kept empty because analog TV channels spaced too
close together can interfere with one another.
Although luminance and chrominance signals are separated, they can
interfere with one another, especially in regions with low signal
strength. This can lead to problems with the color display.
To watch
digital TV on an
analog TV you need a DTV tuner (in the form of a set-top/receiver)
and some form of TV connection. Since analog televisions are not
capable of displaying the higher resolution HDTV broadcasts, the
set-top/receiver can be set to convert the signal to what is known as
480I, which is roughly equivalent to a DVD player or Direct TV
satellite image.
Analog
TV Formats - The 3 main formats used for analog TV differ
in the
number of lines in each complete picture, the frame refresh rate, and
in the detailed encoding of the color (chrominance) and brightness
(luminance) information. With each the chrominance & luminance
signals are separated. This ensures that an analog color TV signal will
display satisfactorily on a monochrome receiver (which has no
chrominance.)
The 3 main formats are:
NTSC:
National Television
Systems Committee
Countries include: USA, Canada, Japan, and Korea
Total Lines: 525
Active Lines: 484
Frame refresh rate: 6o Hz
PAL:
Phase Alternating
Line
Countries include: EU (except for France, Greece), China, India
Total Lines: 625 Lines
Active Lines: 575
Frame refresh rate: 50 Hz
SECAM:
Sequential Couleur
Avec Memoire
Countries include: France, Greece, and Russia
Total Lines: 625 Lines
Active Lines: 575
Frame refresh rate: 5o Hz
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