3. How television works
Cathode ray tube...................................................................................................................................2
The Black-and-White TV Signal .........................................................................................................2
Painting the TV Screen.........................................................................................................................3
Satellite TV signal................................................................................................................................3
HOW TELEVISION WORKS
F or underestand how television works we have to start expleining why television are
designed the way they are.
If you divide an image into a collection of small colored dots, your brain will reassemble the
dots into a meaningful image. On a TV or computer screen, the dots are called pixels.
Cathode ray tube
In electronics we use the terms anode and cathode such as positive and negative.
All the oldest Tv's, were composed by cathode ray tube, to display their images. The
cathode ray tube is a created insaide a glass “tube” in a vacuum. “Ray” is the part of
electrons that put the cathodes in vacuum.
Electrons are negative. The anode is positive, so it attracts the electrons off the cathode.
In a TV's cathode ray tube, the part of the electrons focused by foucusing anode in a
beam and the acelerated. This screen is coated with phosphor.
The Black-and-White TV Signal
In these TV's, the screen is coated with white phophor and the electron beam “paints” an
image moving the electron beam across the phosphor line. The beam paints one line per
line across the screen from left to right..
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4. The red lines represent the beam flying back to the left. When the las line is painted the
beam (blue line) have to return to the left corner. This line is represented by the green line.
When the beam paints each line , the intensity of the beam can change to create different
colors: black, gray and white across the screen.
Painting the TV Screen
Standard TVs use technique when painting the screen. The screen is painted 60 times per
second but only half of the lines are painted per frame. It paints every other lines, for
example, every “odd-numbered line”. Then, it paints the even-numbered lines, alternating
back and forth between even-numbered and odd-numbered lines on each pass.
When a television station wants to reproduce a signal to your TV, or when your VCR wants
to display the movie on a video on your TV, the signal needs to mesh with the electronics
beam, then the TV can paint the picture in the screen that the TV station or VCR sends.
But what about the signal sended by the satellite? How can be see a comedy or the news?
Is in our TV's little people actuing? Of usually, not.
Satellite TV signal
There are two tipes:
• Large-dish satellite antennas pick off unencoded or encoded signals sended to Earth
bysatellites. You point in a particular satellite, and then you select a particular channel it is
transmitting.
•Small-dish satellite systems are digital. The TV programs are in MEPG-2 format and
transmitted to Earth. Then the set-top box makes a lot of work and converts it in an anolog
TV signal, wich is reproduced by your Tv.
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