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Television
In 1947 delegates from 60 countries attending the World Radiocommunication Conference in Atlantic City decided to name television
abbreviated TV, the remote transmission of moving images.
After the radio, the natural evolution of communications for the plurality of persons, is represented by television. A TV has the
ability to reproduce images in addition to sound, showing almost all of the information of the place where taken, with the exception
for the smell (electronic noses are studied to be used in the wine industry and in the production of perfumes) and other environmental
parameters. The purpose of this page is to explain in a more understandable way possible as such transmission is possible, referring
to the page - " principle of operation of the radio " - for basic concepts like
frequency, carrier, modulation, etc.
A little bit of history
Leaving aside the beginnings of the experiments with rotating perforated discs of the first illuminated or genes (chasing inexecutable
dreams, either for the lack of resources or simply because they conceived something was unfeasible for the technology of the time),
I would say that the basis for television have been placed with the discovery (by the Swede Jacob Berzelius) of the property that some
materials (metalloid) have when hit by light. These materials (phosphorus, selenium, silicon) basically decrease their electrical resistance
when they are enlightened, as a result a larger amount of current circulate when they are subject to an electric difference of potential
(voltage, expressed in volt). This is essentially the Ohm's law of which the mathematical formula is given below:
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V = R * I
where:
- V = voltage in Volt (V)
- R = resistance in Ohm (Ω)
- I = current intensity in Ampere (A)
and so, vice versa;
I = V / R
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Obviously, if the light that hits the sensor varies, the circulating electrical current will vary accordingly. Thus we have obtained the conversion
of a variable light signal into an electrical signal that varies with the same law. Several researchers realized that this was the way to reproduction
images at distance. Many start to accomplish the mission, from the french Selencq to american Carey, and an already famous inventor, Thomas Alva Edison,
but without great results.
Dates back to 1909 the first television transmission by the german Ernst Rhumer, in the sense that an image was reproduced at a certain distance from
the place where the original was. It was just some rough and blurry geometric shapes. But the important thing was the birth of television.
Rhumer obtained that result by means of a vertical plate, dotted with many holes containing selenium each connected to an electric wire.
The image placed in front of the plate, more or less intensely illuminated individual holes, at the ends of several wires is evidently formed an electric
"matrix" corresponding (hole to hole) to the light that fall on the plate. Without devices, Rhumer simply connected each plate's selenium element, say the
camera, to each element of the receiving plate (screen), the latter composed in a specular manner as the plate camera, with the same number of elements
in the same position. In Italy is the 3rd of January, 1954 the official date referred to as the beginning of regular television broadcasts, but the trial
started in 1939, abandoned during the Second World War.
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Principle of working
The television is different than radio for the information to be transmitted. In fact, in addition to the sound we have to transfer the image
(later also in colors). As you can imagine, the radiated signal is much more complex than that of radio, the confirmation comes from bandwidth of this signal,
which turns out to be 7 MHz. Obviously, more informations mean more difficult that engineers and developers have had to overcome, already in black and white
television. Even illustrate the operating principle has its difficulties, especially if one turns to the widest audience possible. I prefer to start with the
description of the ring end of the chain devices that make possible the television: the kinescope also known as cathode ray tube (CRT).
The CRT is the TV's electronic element set to display images. Thus, the cathode ray tube has, in other words, the task of converting the electrical signal
containing the information, transferred by means of radio frequencies in images. For the black and white TV, the cathode ray tube is made of a vacuum glass bulb,
where one side is there is a screen where images are formed, and on the opposite side the so-called electron gun.
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The physical size of the tube depends on two parameters: the value of the display rectangle diagonal, in inches (English measure), and the value of the maximum
deflection of the electron beam emitted from the electron gun, expressed in degrees. This second value in practice affects the length, horizontal, of the cathode ray
tube and then the depth of the television set. Typical values of deflection are 90° and 110°. In short, the electron gun projects a stream of electrons hitting the
phosphor which is coated on the inner side of the screen displays a light spot of varying intensity, depending on the applied signal. By means of magnetic fields
generated by coils driven by oscillating voltages vertical and horizontal, the light spot is deflected from left to right (line), starting from the upper-left composing
a line at a time until it covers the entire screen (see figure). Worldwide there are more television standards, and until the advent of color television, the standard
in use in Italy provided 625 lines repeated 50 times per second. The viewer does not notice this process of composition, for the double phenomenon of high speed of the
electron beam and image persistence on the retina of the human eye (1/16 of a second).
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costruzione immagine video
segnale video
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Deflection
In more detail, the cathode, constituted by an incandescent filament, emits a beam of electrons that are accelerated by a high voltage
applied to the CRT (tens of KV). Open parenthesis, that is why you see the symbol of high voltage on the cover with the warning
"do not open the TV", remembering that you can receive a strong shock even with the power off and unplugged, because the capacitors
that, for the uninitiated, are energy storage element, closed parenthesis. If there was no deflection, we would see only a light point on
the center of the screen, but with magnetic fields generated by coils driven by appropriate signals (horizontal and vertical) we can
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cinescopio
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reproduce the image on the screen captured by the camera. The horizontal frequency, which is what moves the electron beam horizontally, and is
the value resulting from the product of lines for the number of frames to be produced in a second. The vertical frequency or frame rate, is the
number of paintings that reproduce the electron beam in a second: that is usually equal to mains frequency (electric power). So far we have seen how
the electron beam is controlled to cover the whole screen, now describing how we get the picture. The flow of electrons emitted from filament is
always the same so if we do not vary its intensity according to the transmitted signal the screen would always be completely white. The electronic flow,
however, is mitigated by plates placed in front of the filament to which the received signal is applied. As you can see in figure showing a television
signal of a complete line, that signal has a negative sign. Continuing in the description, you can notice that the signal containing the image
information is between 10% and 75% of modulation, while the rest is used for the synchronization signal. Finally, the dashed line of 10% level represents
the white level while the 75% represents the black level, blanking is the period during which the electron beam goes back at the start line and
can not be otherwise that black level.
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Sound
The sound, broadcast in FM, is added to the composite signal (image + sync) shifted by 5.5 MHz from the carrier, thus beyond the bandwidth of that signal
so as not to interfere with it. You can see the offset of the sound from the video when you try to manually fine tune a transmission that comes with a weak signal.
Even if you see the pictures you can not hear the related sound until exactly centered on the channel. It has recently been introduced sound with stereo capability,
which means having two channels, left and right, so you have a sound space.
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cassa acustica
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The Colors
Let us now turn to the description of the color television system, which is essentially an evolution of broadcasting in black and white. As for the black and white TV
let's start with the final link in the chain, the kinescope. A CRT for color television is different from a black and white ones, for the number of electron guns.
In a CRT for color television there are three electron guns. At this point it's necessary to open a parenthesis on colors. You have to know that if three beams of equal
intensity of primary colors (Red, Green and Blue) are projected to a point, as a result we would get a spot of white. Well known is the experiment constituted by a circle
divided in three equal areas, one colored red, one colored green and one blue colored placed in rotation at a sufficient speed, the disc will look completely white.
By mixing the primary colors red, green and blue we can get any other color by adjusting the intensity of one or more colors. In particular, by mixing the red
and green, and eliminating the blue, we get the yellow. Similarly, by mixing red and blue, eliminating the green, we get magenta. Mixing green and blue, with no
red, we get cyan. The colors yellow, magenta and cyan are called secondary colors. Returning to our picture tube, you might have guessed that the three electron guns
reproduce the red, green and blue. Another difference between a CRT for black and white TV and the one for TV color, is the phosphors that coat the part inside of the screen, and
a shadow mask placed immediately before the coating. The shape of the holes of the mask depends on the construction technique adopted by the producer of cathode ray tube.
Perhaps you would be thinking that the color television signal is composed by the signals of the three primary colors plus the usual timing, but it is not. The color television
signal transmission is always made of black and white signal, called luminance, plus the so-called chroma. The chroma is the signal that carries color information, but only
about the red and blue. Green is obtained, through electronic circuit, by the difference between the luminance and chrominance. Without going into deep details, remember,
as we saw earlier, that the sum of the three primary colors get the white! With that system the compatibility of television sets is preserved in the sense that owning a
black and white TV one can still follow a broadcast in color.
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The standard
In the world there are 3 color TV systems: American NTSC, German PAL, French SECAM. In Italy, after a period of experimentation, we adopted the PAL (Phase Alternating Line).
As the system name suggests, the transmission of a frame is obtained by serializing the image on alternate lines, odd lines first, followed by all the even lines. The vertical
frequency is always 50 Hz, so we have 50 fields per second, and then 25 images every second. We have chosen this lines alternate path to remedy the problems introduced by
noise signal. In black and white TV these disorders are irrelevant and couldn't noticed, but in the color reproduction causes significant variations in color, with very
unpleasant effects. In summary, the system provides changing phase of 180 degrees between two contiguous rows thereby compensating any noise played on a line with a reproduction
of the disorder but of opposite sign in the next line, the eye then does the remain.
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As can be seen in the picture, the signal of a broadcast in color does not differ much from that of a broadcast in black and white except for the chrominance signal inseted in
sync signal. That signal, oscillating at a frequency of 4.43 MHz, carries the color information which, in a TV color unit, a particular circuit filters. Another set of circuits,
by mixing and subtraction operations, obtained, from the luminance and the chroma signals, the three primary colors that will drive the three electron guns.
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video color signal
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