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QUESTIONS AND ANSWERS
 
Television
Page 2 of 3 (24 Questions and Answers)

306. How does the television camera record the picture?
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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.

307. If black is a high current from the television's radio receiver and white is a low current, why do you get a bright spot when you increase the flow of electrons at that instant. Isn't white a bright spot?
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Yes, white is created by a strong flow of electrons. There are two separate circuits here. The current from the receiver section of the television isn't what is sent through the electron gun. Instead, that current controls the electron gun. When a large current arrives at the electron gun (actually the grid) from the receiver, the flow of electrons toward the screen is pinched off and a dark spot is created. When a small current arrives from the receiver, the electron beam remains intense and a bright spot is created.

308. If you stand between the two satellites, would you have light on you?
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When two satellites beam their radio waves at you, you are exposed to both of those waves. A normal antenna would not be able to distinguish between them and it would be hard to receive the transmissions of one and not the other. But with a satellite dish, you can easily select the transmissions of one and exclude those of the other. The satellite dish is directional, meaning that it focuses and collects radio waves from a particular direction while ignoring those from other directions. With a satellite dish aimed at a particular satellite, you can receive only transmissions from that satellite.

309. What is one doing when changing the brightness, contrast, and color adjustments on a television?
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The brightness control determines the maximum strength of the electron beam and thus the peak brightness of the phosphors on the screen. The contrast control determines the extent to which the electron beam current changes between bright regions and dim regions on the screen. If the contrast is high, then even a less-than-white spot in the image may produce full beam current and full brightness in the phosphors and a more-than-black spot in the image may be cast as full black (no beam at all). If the contrast is low, then almost the entire screen will be illuminated by a medium electron beam and the image have no full black or full white. The color adjustments control the relative intensities of the red, green, and blue guns. Because of the way color is encoded in the television signal, the traditional controls are hue and tint, which involve mixtures of red, green, and blue. All these controls involve adjustments to the voltages and currents in the electron guns (cathodes), grids, and anodes of the picture tube.

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.


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.


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.

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.

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.

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.

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