Analog TV has a 6 MHz signal that carries intensity and color information for each scan line of the picture. An analog TV signal in the U.S. has 525 scan lines for the image, and each image is refreshed every 30th of a second (half of the scan lines are painted every sixtieth of a second in what is called an interlaced display). The horizontal resolution is something like 500 dots for a color set.
This level of resolution was amazing 50 years ago. But today, the lowest
resolution computer monitor that anyone uses today has 640x480 pixels,
and most people use a resolution like 800x600 or 1024x768. We have
grown comfortable with the great clarity and solidity of a computer
display, and analog TV technology pales by comparison.
Many of the new satellite systems, as well as DVDs, use a digital encoding
scheme that provides a much clearer picture. In these systems, the
digital information is converted to the analog format to display
it on your analog TV. The image looks great compared to a VHS tape,
but it would be twice as good if the conversion to analog didn't
happen.
There is now a big push underway to convert all TV sets from analog to
digital, so that digital signals drive your TV set directly.
When you read and hear people talking about digital television (DTV),
what they are talking about is the transmission of pure digital
television signals, along with the reception and display of those
signals on a digital TV set. The digital signals might be broadcast
over the air or transmitted by a cable or satellite system to your
home. In your home, a decoder receives the signal and uses it, in
digital form, to directly drive your digital TV set.
There is a class of digital television that is called high-definition
television, or HDTV. HDTV is high-resolution digital television
(DTV) combined with Dolby Digital surround sound (AC-3). HDTV is
the highest DTV resolution in the new set of standards. This combination
creates a stunning image with stunning sound. HDTV requires new
production and transmission equipment at the HDTV stations, as well
as new equipment for reception by the consumer. The higher resolution
picture is the main selling point for HDTV. Imagine 720 or 1080
lines of resolution compared to the 525 lines people are used to
in the United States (or the 625 lines in Europe) -- it's a huge
difference!
Of the 18 DTV formats, six are HDTV formats, five of which are based
on progressive scanning and one on interlaced scanning. Of the remaining
formats, eight are SDTV (four wide-screen formats with 16:9 aspect
ratios, and four conventional formats with 4:3 aspect ratios), and
the remaining four are video graphics array (VGA) formats. Stations
are free to choose which formats to broadcast.
The formats used in HDTV are:
· 720p - 1280x720 pixels progressive
· 1080i - 1920x1080 pixels interlaced
· 1080p - 1920x1080 pixels progressive
"Interlaced" or "progressive" refers to the scanning system. In an interlaced format, the screen shows every odd line at one scan of the screen, and then follows that up with the even lines in a second scan. Since there are 30 frames shown per second, the screen shows one half of the frame every sixtieth of a second. For smaller screens, this is less noticeable. As screens get larger, the problem with interlacing is flicker.
Progressive scanning shows the whole picture, every line in one showing, every sixtieth of a second. This provides for a much smoother picture, but uses slightly more bandwidth.
MPEG-2
Broadcasters are having to squeeze the increased picture detail
and higher quality surround sound into the same 6-megahertz (MHz)
bandwidth used by analog television. Compression software, very
similar to what is used in personal computing, allows this to happen.
Digital TV relies on a compression and encoding scheme known as
MPEG-2 to fit its stunning images into a reasonable amount of bandwidth.
In each image, the MPEG-2 software records just enough of the picture
without making it look like something is missing. In subsequent
frames, the software only records changes to the image and leaves
the rest of the image as-is from the previous frame. MPEG-2 reduces
the amount of data by about 55 to 1.
MPEG-2 already is the industry standard for DVD videos and some
of the satellite TV broadcast systems. Compression reduces image
quality from what is seen by the digital camera at the studio. However,
MPEG-2 is very good at throwing away image detail that the human
eye ignores anyway. The quality of the image is very good, and significantly
better than traditional analog TV.
The use of MPEG-2 permits an HDTV receiver to interact with computer
multimedia applications directly. For example, an HDTV show could
be recorded on a multimedia computer, and CD-ROM applications could
be played on HDTV systems. A digital TV decodes the MPEG-2 signal
and displays it just as a computer monitor does, giving it high
resolution and stability.
Next:>How is HDTV different?
