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10 Most Recent Questions and Answers (out of 24)

1455. My four-year-old son was fooling around with a magnet, and when I was turned away, put it right on our TV screen. I then saw him doing this, and before I could bring myself to think consequences, we were both mollified by the amazing and colorful patterns it created on the screen. He sort of moved it around the screen, like you would an eraser on a black board. Well, when he removed the magnet, the screen had been drained of its normally saturated colors, and what we now have left is a color TV with only three colors, basically green, blue, and red. And they are not solid and deep like they were before. They are rather faded, and arranged in three distinct blotches, if you will. Are we stuck with this situation forever, or will this aberration fade with time, back to normal? And, why did this happen? — E-S.B.
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Your son has magnetized the shadow mask that's located just inside the screen of your color television. It's a common problem and one that can easily be fixed by "degaussing" the mask (It'll take years or longer to fade on its own, so you're going to have to actively demagnetize the mask). You can have it done professionally or you can buy a degaussing coil yourself and give it a try (Try a local electronics store or contact MCM Electronics, (800) 543-4330, 6" coil is item #72-785 for $19.95 and 12" coil is item #72-790 for $32.95).

Color sets create the impression of full color by mixing the three primary colors of light—blue, green, and red—right there on the inside surface of the picture tube. A set does the mixing by turning on and off three separate electron beams to control the relative brightnesses of the three primary colors at each location on the screen. The shadow mask is a metal grillwork that allows the three electrons beams to hit only specific phosphor dots on the inside of the tube's front surface. That way, electrons in the "blue" electron beam can only hit blue-glowing phosphors, while those in the "green" beam hit green-glowing phosphors and those in the "red" beam hit red-glowing phosphors. The three beams originate at slightly different locations in the back of the picture tube and reach the screen at slightly different angles. After passing through the holes in the shadow mask, these three beams can only hit the phosphors of their color.

Since the shadow mask's grillwork and the phosphor dots must stay perfectly aligned relative to one another, the shadow mask must be made of a metal that has the same thermal expansion characteristics as glass. The only reasonable choice for the shadow mask is Invar metal, an alloy that unfortunately is easily magnetized. Your son has magnetized the mask inside your set and because moving charged particles are deflected by magnetic fields, the electron beams in your television are being steered by the magnetized shadow mask so that they hit the wrong phosphors. That's why the colors are all washed out and rearranged.

To demagnetize the shadow mask, you should expose it to a rapidly fluctuating magnetic field that gradually decreases in strength until it vanishes altogether. The degaussing coils I mentioned above plug directly into the AC power line and act as large, alternating-field electromagnets. As you wave one of these coils around in front of the screen, you flip the magnetization of the Invar shadow mask back and forth rapidly. By slowly moving this coil farther and farther away from the screen, you gradually scramble the magnetizations of the mask's microscopic magnetic domains. The mask still has magnetic structures at the microscopic level (this is unavoidable and a basic characteristic of all ferromagnetic metals such as steel and Invar). But those domains will all point randomly and ultimately cancel each other out once you have demagnetized the mask. By the time you have the coil a couple of feet away from the television, the mask will have no significant magnetization left at the macroscopic scale and the colors of the set will be back to normal.

Incidentally, I did exactly this trick to my family's brand new color television set in 1965. I had enjoyed watching baseball games and deflecting the pitches wildly on our old black-and-white set. With only one electron beam, a black-and-white set needs no shadow mask and has nothing inside the screen to magnetize. My giant super alnico magnet left no lingering effect on it. But when the new set arrived, I promptly magnetized its shadow mask and when my parent watched the "African Queen" that night, the colors were not what you'd call "natural." The service person came out to degauss the picture tube the next day and I remember denying any knowledge of what might have caused such an intense magnetization. He and I agreed that someone must have started a vacuum cleaner very close to the set and thus magnetized its surface. I was only 8, so what did I know anyway.

Finally, as many readers have pointed out, many modern televisions and computer monitors have built-in degaussing coils. Each time you turn on one of these units, the degaussing circuitry exposes the shadow mask to a fluctuating magnetic field in order to demagnetize it. If your television set or monitor has such a system, then turning it on and off a couple of times should clear up most or all of the magnetization problems. However, you may have to wait about 15 minutes between power on/off cycles because the built-in degaussing units have thermal protection that makes sure they cool down properly between uses.


1161. What function does the Degauss button actually perform on computer monitors and why is it not available for televisions? — JF, San Francisco, California
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Both color monitors and color televisions create their color images by combining the three primary colors of light—red, green, and blue. Each display has an intricate pattern of red, green, and blue phosphor dots or stripes on the inside surface of its picture tube and it produces full color images by adjusting the brightness balance of these tiny glowing spots. Beams of electrons are directed at these phosphors from the back of the picture tube and their impacts with the phosphors cause the phosphors to fluoresce—emit light.

Because the picture tube can't direct its electron beams accurately enough to hit specific red, green, or blue phosphor regions, it needs help from a shadow mask that's located a short distance before the phosphor layer. This thin metal grillwork shades the light-producing phosphors from the wrong electrons. The picture tube has three separate beams of electrons, one for each primary color, and the grillwork ensures that electrons in the red beam are only able to strike phosphors that produce red light. The same goes for the blue beam and the green beam.

The grillwork must stay in perfect registry with the pattern of phosphors on the inside of the picture tube, even as their temperatures change. That's why this grillwork is made of Invar, a special steel alloy that doesn't change size when its temperature changes. Unfortunately, Invar can be magnetized and its magnetic fields can then steer the electrons so that they strike the wrong phosphors. If you were to hold a strong magnet near the face of a computer monitor, you would probably magnetize the Invar shadow mask and spoil the color balance of the images on the monitor.

To demagnetize the Invar, you must expose it to a magnetic field that fluctuates back and forth and gradually diminishes to zero. The Invar's magnetization would also fluctuate back and forth and would dwindle to nothing by the time the demagnetizing field had vanished. Traditionally, this demagnetizing was done with a large wire coil that was powered by alternating current so that its magnetic field fluctuated back and forth. This coil was gradually moved away from the picture tube so that the influence of its magnetic field slowly diminished to zero, leaving the Invar completely demagnetized. In good computer monitors, this coil and an automatic power source for it are built in. When you push the degauss button, you see a burst of colors as the demagnetizing coil's fluctuating magnetic field erases the magnetization of the shadow mask and also steers the electrons wildly.

Apparently, degaussing circuitry has been built into all color televisions sets for the past 20 or 30 years. When you turn on your television, a demagnetizing coil activates briefly and removes minor magnetization from the television's invar mask.


1128. I read a recent article about the FCC requiring all TV stations to switch to digital signals instead of analog ones by 2006. How are digital signals different from analog signals, and will they work with our current TV's? — JP
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Current video signals use continuous physical quantities to represent the brightness and color of the spots on a television screen. For example, the current in a video cable can take any value and that value is used to represent the brightness and color of the spots. This use of a continuous physical quantity (such as current) to represent a continuous physical quantity (such as brightness) is called analog representation.

In a digital video signal, a physical quantity first represents numbers and then these numbers represent the brightness and color of the spots. The physical quantity representing the numbers doesn't have to be continuous. For example, a current that's on could represent the number 1 while a current that's off could represent the number 0. A certain pattern of on and off currents could represent larger numbers and these numbers could then represent brightness and color. This use of a continuous or non-continuous physical quantity (such as magnetization, charge, or current) to represent numbers and then these numbers to represent a continuous physical quantity (such as brightness) is called digital representation.

One advantage of digital representation is that it's relatively immune to noise. In analog representation, any disturbance in the continuous physical quantity representing the information leads directly to a disturbance in the recovered information. For example, if the strength of a radio wave is representing brightness and color on your television (the current technique), then any disturbance of the radio wave leads directly to a damaged image on your television. But in digital representation, small changes in the physical quantity that's carrying the information won't change the numbers that are obtained from that physical quantity and will thus have absolutely no effect on the recovered information. For example, if the strength of a radio wave is representing numbers in digital format, using binary (base two) encoding, then a small disturbance of the radio wave will not affect the binary numbers that are recovered from the radio wave. To see why that's true, imagine representing the number 1 as a powerful radio wave and a 0 as no radio wave at all. It's pretty easy to tell a powerful radio wave from an absent one so that, even if there is some radio interference around, it's unlikely to confuse the receiver. Moreover, even if noise does occasionally confuse the receiver about a number or two, the digital scheme can include redundant information that allows the receiver to identify errors and to fix them! That's why a compact disk is so immune to noise—even if there is a flaw or dirty spot on the disk, there is enough redundant digital information to reproduce the music flawlessly.

The other advantage to digital representation is that digital compression techniques become possible. A typical video signal contains lots of unnecessary and duplicated information. For example, when two people are standing in a room and the only things that are changing with time are the images of those two people, there is really no reason to keep sending an image of the room itself from the broadcast station to your home. Digital compression can identify redundant information and remove it from the transmission. In doing so, it can use the communication channel more efficiently.

By adopting a digital transmission scheme, the FCC has recognized that broadcasters will be able to send much clearer, more detailed images using digital representations than with the current analog representations, while still occupying the same portions of the electromagnetic spectrum. However, there is a cost—current televisions will not work directly with these new digital signals. To fix that shortcoming, there will be inexpensive converters that receive the new digital signals and recreate the analog signals needed for current televisions. This conversion will allow older televisions to keep working, but the new digital televisions will be designed to make better use of the enhanced details in the transmissions. The new transmissions will contain about 4 times the detail of current transmissions so that the images will be sharper as well as more immune to noise than the current transmissions.


1069. How did the sniperscopes used in World War II work? They seem to have used an invisible light to illuminate the target and the sniper then looked through the scope and was able to see the target. — ND, Florence, Montana
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These sniperscopes used infrared light to illuminate their targets and then detected this infrared light with the help of an infrared-sensitive photocathode. Producing infrared light is easy; any incandescent bulb produces large amounts of it. The sniperscope simply filtered out the visible light from an incandescent bulb, leaving only the invisible infrared light to illuminate the target.

Understanding the photocathode system requires an examination of the interactions of light and metal. Whenever a particle of light—a photon—strikes a metal surface, there is the possibility that the photon will eject an electron from that metal surface. However, each type of metal requires a certain minimum photon energy before it will release an electron. Because infrared light photons carry very little energy, they can only eject electrons from very special metals. The sniperscope contained a very thin layer of one such infrared-sensitive metal.

Actually, this metal layer was deposited on a transparent glass window that formed the front end of a vacuum tube. Light from the scene in front of the sniper passed through a converging lens that formed a real image of the scene on the metal layer. The metal layer was so thin that light striking its front surface through the glass window caused electrons to emerge from its back surface. Electrons ejected from the back of the metal layer were accelerated by a high voltage that was applied between this metal photocathode layer and a phosphor-coated anode layer located very nearby. Each electron acquired so much energy during its brief flight that it caused the phosphors on the anode to glow brightly when it hit them. The electron flight path was short so that electrons emitted by a certain spot on the photocathode would hit a corresponding spot on the phosphor anode and the sniper would see a clear image of the scene in front of the sniperscope.

Because one infrared photon striking the photocathode could lead to the release of dozens of photons from the phosphors on the anode, this sniperscope provided a modest amount of "image intensification." But modern starlight scopes go far beyond this level of amplification. Like the old sniperscope, these modern devices also use a photocathode to turn a pattern of light from the real image of a lens into a pattern of free electrons. But the starlight scope then amplifies these electrons by sending them through narrow channels that have highly charged walls. As the electrons bounce their ways through the channels, they knock out hundreds, then thousands, then even millions of other electrons so that each original photon can release more than a million electrons from the amplifying system. When these electrons strike the phosphor-coated anode, the image they produce is bright and visible, so that the person looking at the anode can effectively see when each photon of light strikes the photocathode and initiates one of these electron cascades. With such incredible light sensitivity, there is no longer any need to actively illuminate the target with infrared light—even starlight is enough illumination to make the target visible through the starlight scope's image intensification system.


951. Please explain the "Wagon Wheel Effect." How can the wheel appear to move forward, then backward, then stop, just by viewing it differently? — J, Davenport, IA
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This effect is the result of viewing a series of stop-action frames in rapid sequence as a movie or video. Even though a wagon wheel is turning forward, its orientation during sequential frames of a movie may make it appear to be stopped or turning backward. For example, if the wagon wheel completes exactly one full turn between each frame of the movie, the wheel will appear to be stopped—its orientation in each frame will be the same. If it completes slightly less than one full turn between each frame, it will appear to be turning backward! As you can see, a tiny change in wheel rotation rate, from slightly more than one full turn per frame to slightly less than one full turn per frame, is enough to make the wheel appear to switch from turning forward, to stopped, to turning backward. So it's no wonder that the wheels appear to change speeds abruptly from no apparent reason.

950. How do I make my own satellite descrambler/decoder?
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Even if I knew, I'm sure that I'd get in trouble for telling. The encoding schemes are proprietary information and not available to the general public. To my knowledge, most of the descrambling/decoding in a satellite receiver is done by custom integrated circuits that are extremely difficult to reverse engineer (i.e., to open up, examine, and duplicate) so that pirating satellite signals is nearly impossible without insider information.

866. When TV screens or computer monitors are shown on television shows, they flicker or bars of light wave across them. Why does this happen? — SY, Halifax, Nova Scotia
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Although you can't tell it by looking at a television screen, the image on that screen is formed one dot at a time by beams of electrons that are scanning back and forth across its surface from inside. The image is built one line at a time, from the top of the screen to the bottom of the screen, and each line is itself built one dot at a time, from the left side of the screen to the right side of the screen. You can't see this sequential construction process because your persistence of vision prevents you from seeing any changes in intensity that occur in less than about 1/100 of a second. In any short period of time, the screen will only have had time to produce a few horizontal lines of dots. When a camera or television camera observes a television screen, it often makes its observation in such a short period of time that only part of the screen is built. When you then look at the recorded image, you see a horizontal bar of image—the portion of the image that was built during the observation.

743. How does a TV or VCR remote control work? Is it infrared light or a laser? How does the TV or VCR know what to do with the light it receives from the remote? — FC, Lafayette, CA
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The remote unit communicates with the TV or VCR via infrared light, which it produces with one or more light emitting diodes (LED). The most remarkable feature of this communication is that the TV or VCR is able to distinguish the tiny amount of light emitted by the LED from all the background light in the room. This selectivity is made possible by blinking the LED rapidly at one of two different frequencies. Since it's unlikely that any other source of light in the room will blink several hundred thousand times per second and at just the right frequency, the TV or VCR can tell that it's observing light from the remote. The remote sends information to the TV or VCR by switching back and forth between the two different frequencies. For example, it may use the higher frequency to send a "1" bit and the lower frequency to send a "0" bit. The remote sends a long string of these 1's and 0's, and the TV or VCR detects and analyzes this string of bits to determine (1) whether it's directed toward the TV or VCR (an address component in the information) and (2) what it should do as the result of this transmission (a data component in the information). Assuming that the string of bits was intended for the TV or VCR, its digital controller (a simple computer) takes whatever action the data component of the transmission requested.

722. What is the principle of the Trinitron Sony TV system? — JPD, Spiennes, Belgium
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To form a color image, a color television illuminates a dense pattern of tiny spots—some red, some green, and some blue. By mixing various amounts of these three primary colors of light, the color television can make us perceive any color. But the television must control the amounts of these three colors at each spot on the screen, a very difficult task. A typical color television does this by shining three separate beams of electrons through a mask with holes in it and onto a screen that's covered with tiny phosphor spots. Because the three beams approach the mask at different angles, they illuminate different portions of the screen after passing through the holes. Thus the "blue" beam only illuminates spots of blue phosphor, the "red" beam illuminates red spots, and the "green" beam illuminates green spots.

However, the Sony Trinitron system uses a line mask rather than one containing holes and the phosphors are coated onto the screen in stripes rather than spots. Again, three separate electron beams are used but they now illuminate specific stripes of phosphor rather than spots of phosphor. The advantage of the stripe approach is that there is more active phosphor on the screen (fewer dark places between spots) so the image is brighter.


700. Suppose you have two electric currents, one consisting of electrons and the other of protons, moving in the same direction at the same velocity. Will the magnetic fields that these currents produce have identical magnitudes and directions? The right hand rule describes the direction of the magnetic field in terms of the direction of current, so it appears that it should be independent of the current's charge. — ABD, Petersburg, VA
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Current is defined as flowing in the direction of positive charge motion. Because electrons are negatively charged, the current they are carrying is flowing in the direction opposite their motion! In your question, you describe two beams, one of electrons and one of protons, and note that both beams are heading in the same direction at the same speed. The proton beam's current is heading in the same direction as the beam while the electron beam's current is heading in the opposite direction from the beam. Assuming that the two beams have equal numbers of particles per second, they will produce magnetic fields of equal magnitudes. But the magnetic field produced by the electron beam will be directed opposite that of produced by the proton beam!

A beam of hydrogen atoms—each of which consists of one proton and one electron—is a perfect example of this situation. The electrons in that atomic beam produce a magnetic field in one direction while the protons in that atomic beam produce a magnetic field in the opposite direction. The two fields cancel one another perfectly, as they must because a beam of neutral hydrogen atoms can't produce any magnetic field.


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