March 1, 1995

One Minute Papers - Questions and Answers

How do the electrons know which spots on the screen to color darker and which to leave lighter?

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.

How does the television camera record the picture?

Like the television picture tube, the camera generates a signal that indicates the brightnesses of individual spots one at a time. It first measures the brightness of light reaching it from the upper left hand spot, then the spot to its immediate right and so on horizontally across the field of view. It then moves down to a low horizontal line and repeats this sweep. It eventually records the light levels from the entire scene in front of it and begins again. It detects this light using an optical system that forms an image of the scene on a light sensitive surface. This surface may be part of an imaging vacuum tube (sort of a reverse picture tube), or it may be a semiconductor device that resembles a vast array of tiny photocells.

How does the picture get to the TV itself? How does a radio wave make a picture?

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.

How do projection or rear projection televisions work?

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.

How do color televisions produce colors?

That is a topic for the next lecture.

How does a magnet change the picture on a television -- does this hurt the TV?

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.

How do high definition televisions differ from traditional ones?

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.

How can computer monitors and televisions have images burnt into them over time?

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.

How do the magnets that redirect the electron beam in the picture tube move it to the exact point that it's supposed to?

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.

How can the magnets be manipulated in such a way that they can do this moving of the electron beam in such an incredibly small amount of time?

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.

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