A television produces a series of tiny dots on a screen that appear as an image when they are seen as a whole. Older televisions rely on a cathode-ray tube to produce images, and operate with an analog signal. As technology has advanced and broadcast signals transitioned from analog to digital, plasma and LCD (liquid crystal display) televisions were created. These TVs are more compact and have crisper pictures than their cathode-ray counterparts because they use a thin grid of pixels to create images rather than a vacuum tube.
Vocabulary
Cathode ray tube- a beam of electrons that travel from the negatively charged to positively charged end of a vacuum tube, across a voltage difference between the electrodes placed at each end.
Analog signal- A signal in which some feature increases and decreases in the sameway as the thing being transmitted.
LCD (Liquid Crystal Display)- method of displaying readings continuously, as on digital watches, and portable computers using aliquid-crystal film, sealed between glass plates, that changes itsoptical properties when a voltage is applied.
Vocabulary
Cathode ray tube- a beam of electrons that travel from the negatively charged to positively charged end of a vacuum tube, across a voltage difference between the electrodes placed at each end.
Analog signal- A signal in which some feature increases and decreases in the sameway as the thing being transmitted.
LCD (Liquid Crystal Display)- method of displaying readings continuously, as on digital watches, and portable computers using aliquid-crystal film, sealed between glass plates, that changes itsoptical properties when a voltage is applied.
tv pIXELS AND YOUR BRAIN
Most kinds of television work from the same basic principle. The tiny dots of light produced on the TV screen, called pixels, flash in a specific pattern given by the video signal. Our eyes send this pattern to the brain, where it is viewed as an image you can recognize. The television set refreshes these patterns hundreds of times per second which gives the illusion of movement.
THE CATHODE RAY TUBE
A few older TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images. LCDs and plasma displays are other common in newer televisions.
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. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV's cathode ray tube, the stream of electrons is focused by focusing anode into a tight beam and then by using an accelerating anode. The high-speed 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.
Different kinds of phosphors produce different colors. Only red, blue, and green are needed for color televisions. Using these colors in various combinations and amounts can create all the colors the human eye can see. As energy travels from the ion gun to the phosphors, it is filtered to strike the exact point on the screen needed to produce a specific hue. All of these colored pixels create a color image.
Cathode-ray tubes weigh a lot due to the large amount of glass they contain. So they aren't very useful in large-screen televisions. So, new technologies were developed to make lighter sets with crisper images. In addition, the development of high definition (HD) digital broadcast signals made bigger screens more popular since the images were of higher quality. Plasma and LCD televisions were created as an improvement to tv's that used cathode ray tubes.
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. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV's cathode ray tube, the stream of electrons is focused by focusing anode into a tight beam and then by using an accelerating anode. The high-speed 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.
Different kinds of phosphors produce different colors. Only red, blue, and green are needed for color televisions. Using these colors in various combinations and amounts can create all the colors the human eye can see. As energy travels from the ion gun to the phosphors, it is filtered to strike the exact point on the screen needed to produce a specific hue. All of these colored pixels create a color image.
Cathode-ray tubes weigh a lot due to the large amount of glass they contain. So they aren't very useful in large-screen televisions. So, new technologies were developed to make lighter sets with crisper images. In addition, the development of high definition (HD) digital broadcast signals made bigger screens more popular since the images were of higher quality. Plasma and LCD televisions were created as an improvement to tv's that used cathode ray tubes.
PLASMA TELEVISIONS
A plasma screen television has a bunch of tiny cells that contain neon and xenon gases. Each cell is connected to an electrode that excites the gases in the cell. The gases send out charged particles that interact with phosphors coating the glass inside each cell. The phosphors light up and create the image you see on the television screen. The large number of cells in a plasma screen has a great number of pixels so it makes a clearer and brighter image than televisions that use a cathode ray tube.
Plasma Televisions produce some of deepest blacks, so the contrast ratio is very high. They also have very high refresh rates. This means that images with a lot of motion don't blur like they do on other televisions. Plasma screens can be set to be very bright but that requires a lot of electricity. They are often thicker than LCD televisions, but much thinner than Cathode Ray Tube tv's.
Plasma Televisions produce some of deepest blacks, so the contrast ratio is very high. They also have very high refresh rates. This means that images with a lot of motion don't blur like they do on other televisions. Plasma screens can be set to be very bright but that requires a lot of electricity. They are often thicker than LCD televisions, but much thinner than Cathode Ray Tube tv's.
LCD (Liquid crystal display) Televisions
LCD televisions also use cells to create images. Instead of exciting gases like plasma TVs do, it uses cells that contain a set of red, blue, and green filters covered by a layer of liquid crystals stuck between two pieces of glass. Depending on the display type, each cell is linked to either electrodes or thin film transistors (TFT), which trigger the required cells to create the image. A backlight, which is usually a cold-cathode fluorescent lamp, lights up the screen so the image can be viewed.
Even though LCDs are very light and thin, they can have "dead" pixels, where one or more cells on the screen do not change. If you view LCD screens from an angle that can also lower the picture quality. They have slower response times than plasma or CRT televisions, so images can blur in movement.
More recent versions of the LCD television use light-emitting diodes (LEDs) as the light source rather than cold-cathode fluorescent lamps. LED televisions require less electricity than regular LCD screens, and take up even less space. LEDs usually emit a brighter white light that makes these screens especially vivid and bright.
Even though LCDs are very light and thin, they can have "dead" pixels, where one or more cells on the screen do not change. If you view LCD screens from an angle that can also lower the picture quality. They have slower response times than plasma or CRT televisions, so images can blur in movement.
More recent versions of the LCD television use light-emitting diodes (LEDs) as the light source rather than cold-cathode fluorescent lamps. LED televisions require less electricity than regular LCD screens, and take up even less space. LEDs usually emit a brighter white light that makes these screens especially vivid and bright.
TV PHOSPHORS
A phosphor is any material that, when exposed to radiation, gives out light. The radiation might be ultraviolet light or a beam of electrons. Any fluorescent color is really a phosphor. Fluorescent colors absorb invisible ultraviolet light and send out visible light at a certain color.
In a Cathode Ray Tube, phosphor coats the inside of the screen. When the electron beam hits the phosphor, the screen glows. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged in dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together.
There are thousands of different phosphors that have been formulated. They are characterized by their emission color and the length of time emission lasts after they are excited.
In a Cathode Ray Tube, phosphor coats the inside of the screen. When the electron beam hits the phosphor, the screen glows. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged in dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together.
There are thousands of different phosphors that have been formulated. They are characterized by their emission color and the length of time emission lasts after they are excited.
Standard TVs use an interlacing technique when they paint the screen. The screen is painted 60 times per second but only half of the lines are painted per frame. The beam paints every other line as it moves down the screen So it would paint every odd numbered line. Then, the next time it moves down the screen it would paint the even-numbered lines. It would alternate back and forth between even-numbered and odd-numbered lines. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning. It paints every line on the screen 60 times per second. Most computer monitors use progressive scanning because it reduces flicker.
When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can correctly paint the picture that the TV station or VCR sends. The TV station or VCR then sends a well-known signal to the TV that contains three different parts:
When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can correctly paint the picture that the TV station or VCR sends. The TV station or VCR then sends a well-known signal to the TV that contains three different parts:
- Intensity information- for the beam as it paints each line
- Horizontal-retrace signals- tells the TV when to move the beam back at the end of each line
- Vertical-retrace signals- 60 times per second to move the beam from bottom right to top left
COMPOSITE VIDEO SIGNAL
A signal that contains all three of these components: intensity information, horizontal-retrace signals, and vertical-retrace signals is called a composite video signal. A composite-video input on a VCR is normally a yellow RCA jack. The horizontal-retrace signals are 5-microsecond pulses at zero volts. Electronics inside the TV can detect these pulses and use them to trigger the beam's horizontal retrace.
COLOR TV SCREENS
A color TV screen differs from a black-and-white screen in three ways:
- There are three electron beams that move simultaneously across the screen. They are named the red, green and blue beams.
- The screen is not coated with a single sheet of phosphor as in a black-and-white TV. Instead, the screen is coated with red, green and blue phosphors arranged in dots or stripes. If you turn on your TV or computer monitor and look closely at the screen with a magnifying glass, you will be able to see the dots or stripes.
- On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen.
Survey questions
Do you know how televisions work?
Student 1- No.
Student 2- No.
Student 3- No.
Student 4- No.
Student 5- No.
Adult 1- No
Adult 2- No
Adult 3- No
Adult 4- No
Adult 5- Kind of. (Yes)
Student 1- No.
Student 2- No.
Student 3- No.
Student 4- No.
Student 5- No.
Adult 1- No
Adult 2- No
Adult 3- No
Adult 4- No
Adult 5- Kind of. (Yes)
SOURCES
http://techtips.salon.com/need-digital-tv-20808.html
http://www.wisegeek.com/how-does-a-television-work.htm#
http://www.overstock.com/Electronics/LG-32CS560-32-1080p-LCD-TV-16-9-HDTV-1080p/6652689/product.html
http://earth911.com/recycling/electronics/television/
http://allthingsd.com/20120727/google-fiber-amazing-internet-same-old-tv/
http://condor.depaul.edu/sjost/gph205/documents/images/crt.htm
http://www.wisegeek.com/how-does-a-television-work.htm#
http://www.overstock.com/Electronics/LG-32CS560-32-1080p-LCD-TV-16-9-HDTV-1080p/6652689/product.html
http://earth911.com/recycling/electronics/television/
http://allthingsd.com/20120727/google-fiber-amazing-internet-same-old-tv/
http://condor.depaul.edu/sjost/gph205/documents/images/crt.htm
http://www.televisions.com/tvs/panasonic-televisions/panasonic-plasma-tvs/Panasonic-TX-P-46-G-15.php
http://electronics.howstuffworks.com/tv6.htm