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Digital TV & HDTV Tutorial
The basics of Digital Television & High Definition
Television
Vancouver - BC: High Definition
Television (HDTV), if you have ever seen it, promises
to dazzle even the most jaded home electronics fan.
The picture, as many would say, is like looking through
a clear glass window. The high resolution digital
picture is so detailed that many will forget they are
looking at a television screen. So what is HDTV?
What is it all about? What are the basic facts
that you must know?
In this tutorial, we cover the basics of HDTV and how
it compares with Digital Television (DTV) as a whole,
what programs are available in HDTV, how you could view
HDTV in your own home and make recordings of HDTV programs,
and what's in store in the near future.
Digital Television, The Formats
High Definition Television (HDTV) is actually
a subset of the Digital Television (DTV) family of formats,
as defined by the
Advanced Television Systems Committee (ATSC).
DTV uses digital data (1's and 0's) transmission of
the picture and sound information, as opposed of the
traditional analog signals used for what we
know as analog television, devised by the National Television
System Committee (NTSC). The relatively new DTV
picture formats are generally characterized by the horizontal
and vertical resolutions, aspect ratio, interlaced or
progressive scanning, and refresh rate.
Vertical and Horizontal Resolution.
How many pixels (picture elements) each dimension of
the picture holds. For example, 480 lines of vertical
resolution means there are 480 horizontal lines of information
in the vertical axis. Each horizontal line consists
of 640 or 704 pixels lined up.
Aspect Ratio. The
ratio of the picture's width to height is expressed
as “width:height”. For example, “4:3” aspect ratio
means that the picture width is 4 units wide by 3 units
high. Another way to express this aspect ratio
is “1.33:1”, meaning it is 1.33 times wider than it
is high. This traditional aspect ratio is commonly
called “full screen”, since it fills the traditional
TV screen. In contrast, “16:9” aspect ratio calls
for a picture that is 16 units wide by 9 units high,
or 1.78 times wider than it is high, or “1.78:1” aspect
ratio. This new aspect ratio used by some DTV
formats, and by all HDTV formats, is usually called
“widescreen” or “16 x 9”. The widescreen format
is closer to the movie aspect ratios of 1.78:1 and 2.35:1.
Widescreen aspect ratios take advantage of the physiological
fact that our eyes have wider horizontal field-of-view
than in the vertical direction. By filling more
of our natural vision, directors and content producers
can better draw us into the action. That's why
movie screen have gone to the 1.78:1 and 2.35:1 aspect
ratios decades ago. It's a more visually involving
experience.
Interlaced or Progressive Scanning.
The television picture can be “drawn” in one of two
ways. Traditionally, the picture is drawn with
two passes, one for the odd-numbered horizontal lines
(first frame update), and another for the even-numbered
horizontal lines (second frame update). So it
takes two passes (or two frame updates) to refresh the
entire picture. This is called interlaced scanning.
An analog TV picture is completely refreshed about 30
times a second (or 30 Hz). To put it another way,
the entire picture is redrawn 30 times every second,
with the odd- and even-numbered lines redraw cycle repeated
30 times per second. Some of the new DTV formats
call for progressive scanning, where the entire
picture (both odd-numbered and even-numbered horizontal
lines) is updated in a single pass or scan. Progressive
scanning results in a brighter image with no visible
TV scan lines and fewer motion artifacts (the stair-step
edges that you see on moving objects). Progressive
scan correlates better with the film medium, where the
entire film cell is protected onto the screen one cell
at a time.
Refresh Rate. This
is the rate at which the entire picture is redrawn,
expressed in number of times per second (or Hz, short
for Hertz). DTV supports interlaced scanning at
30 Hz and progressive scanning at 24, 30, and 60 Hz.
The 24 Hz refresh rate corresponds nicely with film
projection's 24 frames per second (fps) rate.
The table below summarizes all 18
of the ATSC Digital Television formats.
There are a total of six (6) HDTV formats, of which
720p/30 and 1080i/30 are the most common. Again,
It is important to realize that HDTV is only a subset
of the DTV standards, and so DTV is the more general
term, while HDTV specifically references the six high
definition formats of the 18 DTV formats. The
DTV formats are most frequently referred by their horizontal
lines of resolution and whether they scan in progressive
or interlace (e.g., 480p, 720p, 1080i). The suffix
“p” stands for progressive scan, while the suffix “i”
stands for interlaced scan. Sometimes, they are
further distinguished by their refresh rate, as designated
with a slash (“/”), followed by the refresh rate.
For example, “1080i/30” refers to 1080 horizontal lines
of resolution with interlaced scanning at 30 Hz refresh
rate.
Next:
Digital HDTV Resolution-->
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