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. In a color screen, there are three phosphors arranged as 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 black-and-white TV, the screen is coated with white phosphor and the electron beam "paints" an image onto the screen by moving the electron beam across the phosphor a line at a time. To "paint" the entire screen, electronic circuits inside the TV use the magnetic coils to move the electron beam in a " raster scan " pattern across and down the screen.
The beam paints one line across the screen from left to right. It then quickly flies back to the left side, moves down slightly and paints another horizontal line, and so on down the screen.
In this figure, the blue lines represent lines that the electron beam is "painting" on the screen from left to right, while the red dashed lines represent the beam flying back to the left. When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen, as represented by the green line in the figure.
When the beam is "painting," it is on, and when it is flying back, it is off so that it does not leave a trail on the screen.
The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top. As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, gray and white across the screen.
Because the lines are spaced very closely together, your brain integrates them into a single image. A TV screen normally has about lines visible from top to bottom. In the next section, you'll find out how the TV "paints" these lines on the screen. Standard TVs use an interlacing technique when painting the screen.
In this technique, 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 -- for example, every odd-numbered line.
Then, the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning , which paints every line on the screen 60 times per second. Most computer monitors use progressive scanning because it significantly reduces flicker.
Because the electron beam is painting all lines 30 times per second, it paints a total of 15, lines per second.
Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on. 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 accurately paint the picture that the TV station or VCR sends.
A signal that contains all three of these components -- intensity information, horizontal-retrace signals, and vertical-retrace signals -- is called a composite video signal. One line of a typical composite video signal looks something like the image on this page. The horizontal-retrace signals are 5-microsecond abbreviated as "us" in the figure pulses at zero volts. Electronics inside the TV can detect these pulses and use them to trigger the beam's horizontal retrace.
The actual signal for the line is a varying wave between 0. This signal drives the intensity circuit for the electron beam. In a black-and-white TV, this signal can consume about 3. A vertical-retrace pulse is similar to a horizontal-retrace pulse but is to microseconds long. The vertical-retrace pulse is serrated with horizontal-retrace pulses in order to keep the horizontal-retrace circuit in the TV synchronized.
When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.
A color TV signal starts off looking just like a black-and-white signal. An extra chrominance signal is added by superimposing a 3. Right after the horizontal sync pulse, eight cycles of a 3. Following these eight cycles, a phase shift in the chrominance signal indicates the color to display. The amplitude of the signal determines the saturation. Here is the relationship between color and phase:.
A black-and-white TV filters out and ignores the chrominance signal. A color TV picks it out of the signal and decodes it, along with the normal intensity signal, to determine how to modulate the three color beams.
Now you are familiar with a standard composite video signal. Note that we have not mentioned sound. If your VCR has a yellow composite-video jack, you've probably noticed that there are separate sound jacks right next to it. Sound and video are completely separate in an analog TV. The first four signals use standard NTSC analog waveforms as described in the previous sections.
As a starting point, let's look at how normal broadcast signals arrive at your house. A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum , chopped into 6-MHz slices, to accommodate TV channels:. The composite TV signal described in the previous sections can be broadcast to your house on any available channel.
To the left of the video carrier is the vestigial lower sideband 0. The sound signal is centered on 5. As an example, a program transmitted on channel 2 has its video carrier at The tuner in your TV, when tuned to channel 2, extracts the composite video signal and the sound signal from the radio waves that transmitted them to the antenna.
The picture on the screen is dependent on the electrons precisely hitting phosphors on the back of the screen, which emit different colours of light when impacted. The electrons are thus forced to land in the wrong place, causing the distortion of the image and the psychedelic colours. Download all demo cheatsheets and background infosheets PDF ». Enter the terms you wish to search for. Department of Physics Accelerate! Resources Demonstrations Cathode ray tube. Cathode ray tube. Download cheatsheet PDF ».
Resources Accelerate! Air is withdrawn from the CRT so that air molecules don't interfere with the passage of electrons between the electron gun and the screen. Phosphor is the name given to any substance that emits visible light when exposed to radiation, such as ultraviolet light, or, in this case, a beam of electrons. The electrons collide with the phosphor atoms, causing them to gain energy or become "excited.
Older CRT TVs use just a single color phosphor and so can only produce black-and-white, or monochrome, pictures. Later CRT TVs use phosphors colored in the three primary colors -- red, green and blue -- and therefore can produce full color pictures.
In the latter case, manufacturers apply multiple color coatings to the screen through a device known as an aperture mask, made from perforated metal, to create thousands of narrow, colored lines of phosphor.
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