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Seller Inventory n. Gordon M. Drury ; Garik Markarian ; Keith Pickavance. Publisher: Springer , But their frequency can be in the millions of hertz—so millions of these waves arrive each second. If the waves are hundreds of meters long, how can millions of them arrive so often? It's simple. Radio waves travel unbelievably fast—at the speed of light , km or , miles per second. Photo: A radio studio is essentially a soundproof box that converts sounds into high-quality signals that can be broadcast using a transmitter.
Credit: Photographs in the Carol M. Ocean waves carry energy by making the water move up and down. In much the same way, radio waves carry energy as an invisible, up-and-down movement of electricity and magnetism. This carries program signals from huge transmitter antennas, which are connected to the radio station, to the smaller antenna on your radio set. A program is transmitted by adding it to a radio wave called a carrier.
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This process is called modulation. Sometimes a radio program is added to the carrier in such a way that the program signal causes fluctuations in the carrier's frequency. This is called frequency modulation FM. Another way of sending a radio signal is to make the peaks of the carrier wave bigger or smaller. Since the size of a wave is called its amplitude, this process is known as amplitude modulation AM.
Frequency modulation is how FM radio is broadcast; amplitude modulation is the technique used by AM radio stations. An example makes this clearer.
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Suppose I'm on a rowboat in the ocean pretending to be a radio transmitter and you're on the shore pretending to be a radio receiver. Let's say I want to send a distress signal to you. I could rock the boat up and down quickly in the water to send big waves to you. If there are already waves traveling past my boat, from the distant ocean to the shore, my movements are going to make those existing waves much bigger.
In other words, I will be using the waves passing by as a carrier to send my signal and, because I'll be changing the height of the waves, I'll be transmitting my signal by amplitude modulation. Alternatively, instead of moving my boat up and down, I could put my hand in the water and move it quickly back and forth.vipauto93.ru/profiles/trovare/come-capire-se-un-iphone-8-rubato.php
Framing structure , channel coding and modulation for digital terrestrial television
Now I'll make the waves travel more often—increasing their frequency. So, in this case, my signal will travel to you by frequency modulation. Sending information by changing the shapes of waves is an example of an analog process. This means the information you are trying to send is represented by a direct physical change the water moving up and down or back and forth more quickly. The trouble with AM and FM is that the program signal becomes part of the wave that carries it.
So, if something happens to the wave en-route, part of the signal is likely to get lost. And if it gets lost, there's no way to get it back again. Imagine I'm sending my distress signal from the boat to the shore and a speedboat races in between. The waves it creates will quickly overwhelm the ones I've made and obliterate the message I'm trying to send.
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That's why analog radios can sound crackly, especially if you're listening in a car. Digital radio can help to solve that problem by sending radio broadcasts in a coded, numeric format so that interference doesn't disrupt the signal in the same way. We'll talk about that in a moment, but first let's see take a peek inside an analog radio. Let's lift the lid on an old-style analog transistor radio and see what we can find inside!
Coding and Modulation for Digital Television | Gordon M. Drury | Springer
Click the image to see a slightly bigger photo. The big orange button in the middle lets you pause a live radio broadcast and restart it later. You're driving along the freeway and your favorite song comes on the radio. You go under a bridge and—buzz, hiss, crackle, pop—the song disappears in a burst of static. Just as people have got used to such niggles, inventors have come up with a new type of radio that promises almost perfect sound.
Digital radio , as it's called, sends speech and songs through the air as strings of numbers. No matter what comes between your radio and the transmitter, the signal almost always gets through. That's why digital radio sounds better. But digital technology also brings many more stations and displays information about the program you're listening to such as the names of music tracks or programs. Let's go back to the earlier example of sending information from a boat to the shore—but this time using a digital method. In case of emergency, I could store hundreds of plastic ducks on my boat, each one carrying a number.
If I get into trouble, as before, and want to send a distress signal, I could send you an emergency coded message "" by releasing just the ducks with those numbers. Let's suppose I do have a problem. I release ducks with the numbers 1, 2, 3, 4, and 5—but instead of sending just five numbered ducks, I send maybe 10 or 20 of each duck to increase the chances of the message arriving.
Now, even if the sea is choppy or a speedboat cuts through, there's still a high chance enough of the ducks will get through. Eventually, waves will carry ducks with the numbers 1, 2, 3, 4, and 5 ashore. You collect the ducks together and work out what I'm trying to say. To help avoid interference, a digital radio signal travels on a huge, broad band of radio frequencies about times wider than those used in analog radio.
To return to our rowboat example, if I could send a wave times wider, it would bypass any speedboats that got in the way and get to the shore more easily. This wide band allows a single digital signal to carry six stereo music programs or 20 speech programs in one go. Blending signals together in this way is called multiplexing. Part of the signal might be music, while another part could be a stream of text information that tells you what the music is, the name of the DJ, which radio station you're listening to, and so on.
Test a digital radio It takes a digital radio some time to process incoming signals—and you can hear that for yourself! Put a digital radio and an ordinary analog radio next to one another and tune them both into the same station. You'll find the sound from the digital radio lags noticeably behind the sound from the analog radio because of the time it takes to reassemble the digital signal.
Give that some thought next time you set your watch by the time signal from a digital radio! From TV broadcasts to GPS satellite navigation , radio waves zap all kinds of handy information through the air, so you might be wondering why these very different signals don't get thoroughly mixed up? Now we have digital broadcasting, it's a lot easier to keep radio signals separate from one another using complex, mathematical codes; that's how people can use hundreds of cellphones simultaneously in a single city street without hearing one another's calls.
But going back a few decades to the time when there was only analog radio, the only sensible way of stopping different types of signal from interfering with one another was to split the entire spectrum of radio frequencies into different bands with little or no overlap. Here are a few examples of the main radio broadcasting bands don't take these as exact; definitions do vary somewhat around the world, some of the bands do overlap, and I have rounded some of the figures as well :.
If you look at the table, you'll notice that the wavelength and the frequency move in opposite directions. As the wavelengths of radio waves get smaller moving down the table , so their frequency gets bigger higher. But if you multiply the frequency and wavelength of any of these waves, you'll find you always get the same result: million meters per second, better known as the speed of light. Photo: Italian Radio pioneer Guglielmo Marconi. Photo courtesy of US Library of Congress. All rights reserved.
You can extend and swivel the telescopic antenna for better reception.
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