As the electron beam collides with the phosphor coating on the inside of the picture tube, it slowly damages that phosphor coating. Eventually the phosphors are burnt away and the inside surface of the picture tube stops being uniform. To avoid burning specific regions more than others, computers use screen savers that darken the images by turning down the electron beam and keep those images moving about randomly.
The electromagnets that control the beam are able to turn on and off very quickly. The only limit on the rate at which they can change the magnetic field comes from their inductance. They do resist changes in current passing through them. Fortunately, the television doesn’t move the beam about randomly; it sweeps the beam smoothly. Thus the changes in the current through the electromagnetic coils are also smooth. The television has no trouble ramping the field through the horizontal sweep coils back and forth every 1/15,750th of a second.
High definition televisions have more individual spots of color and brightness than the traditional sets. They may also have a somewhat different aspect ratio (horizontal width vs. vertical height). Creating high definition picture tubes is not particularly difficult since they are now rather common on computers. However, transmitting the increased information needed to paint the picture on a high definition television is a serious problem. One approach is data compression, in which redundant information is eliminated from the signal so that only new information is sent to the television. To avoid making all of the present televisions obsolete, the new high definition television standards are supposed to be downward compatible with those televisions. Unfortunately, trying to serve both types of televisions with the same transmitted signal is going to be a difficult task.
Inside the projection TV, there are three separate picture tubes that work very much like normal black and white picture tubes. One of these tubes creates an image of the red light in the television image, one creates an image of the green light, and the third creates an image of the blue light. In front of each tube, there is a color filter: red for the red tube, green for the green tube, and blue for the blue tube. There is also a projector lens that takes the light leaving the tube and filter and projects a clear image of that light on the screen in front of the projector. The light striking the screen looks exactly like the light leaving the surface of the picture tube. The three images (red, green, and blue) are carefully overlapped so that they mix and you perceive all colors.
The electrons simply deliver energy (their own kinetic energy) to the phosphors they hit. When they are hit by electrons, these phosphors emit light. They fluoresce. The picture determines which spots on the screen should be dark and which ones should be light by controlling the number of electrons that hit those spots; by controlling the current in the beam. When the current hitting a spot is low, that spot glows dimly. When the current hitting a spot is high, that spot glows brightly.
The electromagnets that steer the electron beam are very carefully designed and constructed so that they steer the beam very accurately. They are coils of wire that are built on a form and then glued together so that they cannot move. There are some adjustments made electronically inside the television set to make sure that the beam follows a very start path as it sweeps across the screen. When you adjust the horizontal and vertical sizes of the picture, you are adjusting the currents flowing through these electromagnets.
When you hold a magnet up to the front of a television, you are introducing an additional magnetic field in the system. This field exerts forces on the moving electrons inside the tube and they are deflected. The picture is distorted. With a black and white television, no harm is done because there is nothing to magnetize inside the picture tube. But color television picture tubes contain metal shadow masks that can become permanently magnetic. The picture remains distorted, even after you remove the magnet. To clear up the “damage”, you would have to demagnetize the picture tube. Although this is not a particularly difficult task, it requires a demagnetizing coil and is best done by a professional repairperson. The bottom line is, don’t play with magnets near a color television set.
The brightness information comes to the television as a steady stream. While the television knows that this information should control the brightness of adjacent spots on the screen, from left to right, it needs to be told when each horizontal line begins and when each vertical sweep begins. It knows that a new line is coming when the brightness information contains a “blacker-than-black” level. This level seems to say that the electron gun should not only stop sending electrons at the screen, it should send less than no electrons at the screen! Actually, this level is an instruction to the television’s electronics, telling the television to bring its electron beam back to the left side of the screen to begin a new horizontal line. A long “blacker-than-black” level is an instruction to the television to begin a new vertical scan down the screen.
The television can reconstruct an image from a series of brightness measurements. It takes these brightness measurement and uses them to control the electron beam as it sweeps across the screen of the picture tube. It paints the picture one dot at a time and then starts over when it has finished. Thus all that the radio wave has to send to the television is a series of brightness measurements and some synchronization information (when to start a horizontal scan and when to start a vertical scan). It uses an AM technique to send the brightness measurements on a radio wave. The transmitter’s power varies up and down to indicate brightness just as an AM radio transmitter’s power varies up and down to indicate which way to push the speaker cone.
The television and picture tube simply scans the electron beam across the screen, one horizontal row after the next as it moves “slowly” down the screen. When it gets to the bottom of the screen, the picture tube brings the beam back to the top of the screen and starts over again. While the TV is scanning the beam across the set, it uses the signal from the television station to control the intensity of the electron beam and those the brightness of the spots on the screen. It also watches for sync information to know when to begin new horizontal lines and vertical sweeps.