Anthony Rossini

5/3/2008

Extra Credit term Paper

How a Television Works

Nearly every home in America contains a television. The TV has become a major part of out lives by giving us access to news, sports, movies, and other pictures. The average American spends two to five hours in front of the TV and 99% of Americans don’t know how a TV really works. The birth of the TV is just about impossible to pinpoint because there were many different contributors that helped to create the TV.

In 1862 Abbe Giovanna Caselli invented the Pan telegraph and became the first person to send a still image over wires. Then in 1873 scientists may and smith experiment with selenium and light which revealed the possibility for inventors to change images into electronic signals. In 1877 Boston George Carey put made drawings of what he called a selenium camera that would allow people to see by pictures through the use of electricity. In 1880 Inventors Alexander Graham Bell and Thomas Edison started sending sound through the use of electrical wires. In 1884 Paul Nipkow sent images over wires using a rotating metal disk technology calling it the electric telescope with 18 lines of resolution. In 1890 At the World's fair in Paris the first International Congress of Electricity was held. That is where Russian Constantin Perskyi made the first known use of the word television. In 1906 Lee de Forest invents the Audion vacuum tube that proved essential to electronics. The Audion was the first tube with the ability to amplify signals. This was crutial to the development of the TV because an un amplified signal can only travel so far befor it loses its energy. Boris Rosing combines Nipkow's disk and a cathode ray tube and builds the first working mechanical TV system. In 1923 Vladimir Zworkin patents his iconoscope a TV camera tube based on Campbell Swinton's ideas. The iconoscope, which he called an electric eye, becomes the cornerstone for further television development. In 1926 John Baird operates a television system with 30 lines of resolution system running at 5 frames per second. IN 1927 Philo Farnsworth, files for a patent on the first complete electronic television system, which he called the Image Dissector.

The television that we know and use today uses many different types of technology that work together to give us a picture and sound. The basic components of the average television include the cathode ray tube (CRT), TV steering coils, TV phosphors, capacitors, and electro magnets.

CATHODE RAY TUBE

In a cathode ray tube the cathode is a heated filament. The heated filament is in a vacuum created inside a glass tube. The ray is a stream of electrons that naturally pour off a heated cathode into the vacuum.

Electrons are negative and the anode is positive so it attracts the electrons coming out of the cathode. In a TV's cathode ray tube the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.

A phosphor is any material that, when exposed to radiation, emits visible light. The radiation might be ultraviolet light or a beam of electrons. In a CRT phosphor coats the inside of the screen. When the electron beam strikes the phosphor it makes the screen glow. In a black and white screen there is one phosphor that glows white when struck and the other glows black. In a color screen there are three phosphors arranged as dots or stripes that glow red green and blue light. There are also three electron beams to make the three different colors glow together. There are thousands of different phosphors that have been created. They are characterized by the color that they glow when electrons hit it, and the length of time that it glows after it is hit by the electron beam.

Soon after the cathode ray tube was invented scientists discovered that a magnet could be used to deflect the electron beam inside of the CRT. Now in order for the cathode ray tube to produce a picture the electron beam inside the cathode ray tube needs to be controlled and directed toward the end of the tube where the phosphorus is located. Copper magnetic coils (deflection coils) produce magnetic fields inside the CRT are used to deflect the beam to create and image. (See illustration on bottom of page) One magnet controls the electrons on a vertical y axis while the on the controls the beam on a horizontal x axis. The CRT monitor creates a picture out of many rows or lines of tiny colored dots. These small dots that are produced on the CRT screen are called pixels. The more lines of dots per inch on the cathode screen, the higher and clearer the resolution is. Therefore a 1024 x 768 resolution will be sharper than 800 x 600 resolution because the former uses more lines creating a denser and more detailed picture.

Most TVs use a crisscross technique when creating an image on the screen. This means that the image on the screen is redrawn by the cathode ray tube 60 times per second but only half of the pixels are colored each time. The beam colors every other line or the odd numbered lines as it moves down the screen. Then the next time it moves down the screen it colors the even numbered lines changing back and forth between even numbered and odd numbered lines on each pass. This means the entire screen in two passes is painted 30 times every second. The alternative to method of coloring the screen is called progressive scanning. Progressive scanning colors every pixel and every line on the screen 60 times per second.

Now that we know how the TV creates a picture the next question to be answered is how does the television know what picture to create. The answer is that the television receives a composite video signal and this signal has three components. The components of the composite video signal are intensity information, horizontal-retrace signals, and vertical-retrace signals. These signals are the input that the television uses to deflect the electron beam inside the CRT against the phosphorus screen to produce a picture. If a TV is turned on and there is no composite video signal a random pattern of flickering pixels will be seen. Composite video signals can come from such inputs as a VCR or video games. It can also be broadcasted and picked up by the televisions antenna in the same way a radio picks up its signal with and antenna.

This method of receiving picture on a television has become outdated. It was found to be inefficient because in some areas it was hard to get reception and only a few channels could be broadcasted. These problems gave rise to cable television which is basically a cable that runs from your television to a large antenna that receives signal in microwaves. As the popularity of TV increased more and more stations were added to cable by giving each station a different frequency. The FCC or Federal Communications Commision established channels 14 to 69 using a block of frequencies between 470 MHz and 812 MHz. Now most televisions receive their signals from a cable.

Inside your television there are many different capacitors that control which station plays on the TV. When you change the channel on your TV you are actually switching to a different capacitor inside the TV that is designated to pick up the frequency of a specific channel. On older televisions changing the channel meant that you had to physically turn a knob on the television. As the knob was turned you could hear the loud clicks as the television switched from one capacitor to another. Modern televisions that have remotes transfer from capacitor to capacitor automatically depending on the signal that the television picks up from the remote control.

A capacitor is a small electronic device that when charged stores electricity in the electric field between the plates of the capacitor. The capacitor does not store charge for long periods of time in the same way a battery does but I does hold charge for some time. Different capacitors hold different amounts of charge and therefore they can be used to detect different channels in the TV that operate at different frequency.

Todays modern televisions have even outdated the cathode ray tube. New televisions a made with plasma. Plasma Televisions are made up of plasma gas which is the same thing as Neon Gas. These gases are placed in tiny cells like pixels across the screen of the television. The insides of the pixels are coated with phosphor, like in inside screen of the cathode ray tube. Each pixel has three pixel behind it and each is for a primary color. The cathode ray tube operated on the same principle except different phosphors are used for different colors. These pixels are set between two pieces of glass. Connected to each plate is an electrode. These electrodes cover the whole plate of glass forming a grid on both plates of the glass. These electrodes are then connected to a computer inside the television. To create a picture the computer sends electricity to the electrode and to each pixel the amount of charge needed to make the neon gas inside the sub pixels give off ultra violet light which strikes the phosphor making it glow and thus creating a picture. With all of these cells or pixels lighting up at the same time they create a much clearer image than that produced by a television using a cathode ray tube.