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Conventional television is also known as fast-scan TV (FSTV). This is the TV that brings you sports events, newscasts, and all the other programming with which you re familiar. In FSTV, the frames come at the rate of 30 per second. The human eye/brain perceives bursts of motion down to a time resolution of about 1/20 second. Therefore, in FSTV, the sequence of still images blends together to give the appearance of continuous motion. The FSTV image has 525 lines per frame. In recent years, technological advances have been made that promise to make high-resolution TV widely available and affordable. This mode has more than 525 lines per frame. The quick frame time, and the increased resolution, of FSTV make it necessary to use a much wider frequency band than is the case with fax or SSTV. A typical video FSTV signal takes up 6 MHz of spectrum space, or 2000 times the bandwidth of a fax or SSTV signal. Fast-scan TV is almost always sent using conventional AM. Wideband FM can also be used. With AM, one of the sidebands can be filtered out, leaving just the carrier and the other sideband. This mode is called vestigial sideband (VSB) transmission. It cuts the bandwidth of an FSTV signal down to about 3 MHz. Because of the large amount of spectrum space needed to send FSTV, this mode isn t practical at frequencies below about 30 MHz (10 times the bandwidth of a VSB signal). All commercial FSTV transmission is done above 50 MHz, with the great majority of channels having frequencies far higher than this. Channels 2 through 13 on your TV receiver are sometimes called the VHF (very-high-frequency) channels; the higher channels are called the UHF (ultra-high-frequency) channels. An amplitude-versus-time graph of the waveform of a TV signal is illustrated in Fig. 26-13. This represents one line of one frame, or 1/525 of a complete picture. The highest instantaneous signal amplitude corresponds to the blackest shade, and the lowest amplitude to the lightest shade. Thus, the FSTV signal is sent negatively. The reason that FSTV signals are sent upside down is that retracing (moving from the end of one line to the beginning of the next) must stay synchronized between the transmitter and receiver. This is guaranteed by a defined, strong blanking pulse. This pulse tells the receiver when to retrace; it also shuts off the beam while the CRT is retracing. You ve probably noticed that weak TV signals have poor contrast. Weakened blanking pulses result in incomplete retrace blanking. But this is better than having the TV receiver completely lose track of when it should retrace! Weak TV signals are received better when the strongest signals correspond to black, rather than to white. This was discovered, as things so often are, by experimentation. When you tune your TV set to a vacant channel, you see snow, or white-and-gray, fast-moving dots. If a TV signal comes on the air without modulation, the screen goes dark. Only when there is modulation do portions of the screen get light again. Color FSTV works by sending three separate monocolor signals, corresponding to the primary colors red, blue, and green. The signals are literally black-and-red, black-and-blue, and black-and-green. These are recombined at the receiver and displayed on the screen as a fine, interwoven matrix of red, blue, and green dots. When viewed
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from a distance, the dots are too small to be individually discernible. Various combinations of red, blue, and green intensities result in reproduction of all possible hues and saturations of color.
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In a radio or television transmitting antenna, electrons are moving back and forth at an extreme speed. Their velocity is constantly changing as they speed up in one direction, slow down, reverse direction, speed up again, and so on. Any change of velocity is acceleration. When electrons move, a magnetic field is created. When electrons accelerate, a changing magnetic field is produced. An alternating magnetic (M) field gives rise to an alternating electric (E) field, and this generates another changing M field. The process has come full circle. Thus it repeats, the effects propagating through space at the speed of light. The E and M fields expand alternately outward from the source in spherical wavefronts. At any given point in space, the E flux is perpendicular to the M flux. The direction of wave travel is perpendicular to both the E and M flux lines (Fig. 26-14). The EM flux field can oscillate at any conceivable frequency, ranging from many years per cycle to quadrillions of cycles per second. The sun has a magnetic field that oscillates with a 22-year cycle. Radio waves oscillate at thousands, millions, or billions of cycles per second. Infrared, visible light, ultraviolet, and X rays oscillate at many trillions of cycles per second. All of these effects are electromagnetic fields, and as such, they all
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