Digital
TV (HDTV)
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. (That has been extended to
February, 2009.)
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.
 Digital
Television Web links
Online Demonstrations of Digital
TV:
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Interactive
TV
Dictionary & Business Index DTV links page
– www.itvdictionary.com/links.htm#dtv
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
To
see what system is used in other
countries, see the below URL. It lists what system individual countries
use:
www.dvdaust.com/world_video_standards.htm
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