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Television signals are broadcast using the FM (Frequency Modulation) method; only radio signals are broadcast in AM (Amplitude Modulation).

The earliest signals were broadcast as AM signals. They are still used when long distance is required, since these signals can be bounced off the Ionosphere and therefore travel beyond the horizon, but this method of broadcasting doesn’t create as clear a signal so it is normally only used for voice.

FM signals are used for stereo radio and televisions signals because they broadcast a clearer signal. They can also support signals of a higher bandwidth, packing more of a data stream into one channel.

So what is FM and AM? A radio or television signal is sent out on a sine wave pattern. Imagine looking at a series of ocean waves from the side. You will see the water go up and down in the same regular pattern. A sine wave is very similar it starts at zero and goes up to a certain height, then it drops to zero, and goes down to that same height, then it goes back up to zero, then up to the same height and it continues. The height of the wave is called the amplitude, the length of one cycle is called the frequency, and the spot on the electromagnetic spectrum where it is broadcast at is determined by the channel or the spot on the radio dial where you have tuned in the signal (this is the broadcast band and sometimes it is called the frequency; a left over from the days of on AM signals). This is commonly called the radio or television band. The signals are considered a band because you need two signals for stereo sound, and more for all the picture data.

So an AM signal has higher wave heights and lower wave depths on the signal. The signal is encoded on the wave frequency between each wave high and low point. Send it fast enough and you can carry audio signals, like voice and mono music. If you tried to broadcast two signals on the same AM band then you don’t get the sine wave again; you end up with each signal interfering with itself creating just noise. The AM signals simply add together making stronger signals, but they cancel out the information encoded into that signal. When you use a shortwave, CB (Citizens Band) or police radio you are broadcasting with an AM signal. If two people try to talk then they either interfere and cancel each other out, or the more powerful (or closest) signal “walks” over the other signal so you only hear on voice. This is why when talking on a short wave radio you need to say “over” so the other person knows that they can talk now. The advantage with this type of system is that you can use the idiosyncrasies of our atmosphere to bounce a signal off the Ionosphere. Since there is a clear difference between each atmospheric layer it is possible to bounce the signal around (http://en.wikipedia.org/wiki/Ionospheric_reflection).

Look at this picture: http://upload.wikimedia.org/wikipedia/en/f/f1/Voice_waveform_and_spectrum.png
On the chart with the green lines the difference between the base line and the height of each green wave is the amplitude.

FM signals work in a different fashion. The signal is encoded in the height and depth of each wave so you can broadcast several signals on the same band without having them interfere with each other. This lets you broadcast a stereo signal to a FM radio.

Television signals are broadcast this way as well so that stereo sound can be sent (in the early days of TV stereo wasn’t used). This broadcast method also allows the more complex data of the television signal to be sent along with the audio signals. But, this still isn’t enough so TV signals are broadband; several signals are sent at the same time across a series of bands, close together on the spectrum, so all the information gets through. When you tune your TV you are actually selecting several bands grouped together which carry all the signals required. If you tried this with an AM band then you would have your signals canceling each other out. FM signals can be put out of phase with other signals so one band can carry more of a data stream without interfering with the rest of the data. By out of phase I mean with an offset to each other. Take your simple sine wave then copy it ten times and place them on top of each other then push each one slightly forward. You end up with a whole series of waves. If you push these waves enough so that no peaks or valleys match then you can send one piece of information on each wave. Since the signal is encoded into the amplitude (height and depth) of the sine wave then you have much higher and lower peaks and valleys than you would with an ocean wave. This lets you get more information into a tighter moment of time. Your signal is traveling just a tiny bit slower on each separate wave so they don’t interfere with each other.

Look at this picture: http://upload.wikimedia.org/wikipedia/en/f/f1/Voice_waveform_and_spectrum.png
The signals in blue are an example of dozens and dozens of sine waves running at one time.

With the improvement in technology you can pack more of a signal into less space. This lets you send more data, like a more complicated picture so you can send a High Definition (HD) signal. The three major networks (ABC, CBS, and NBC) wanted to send out more television channels with a tighter spread; so you could have 4 NBC stations in the space where only one is now broadcast. However, the US Congress said, no you have to send HD signals instead.

Still American technology was once a little better than European technology. As a result the channels overseas can’t be properly received on a North American TV and vice versa. Back then the Europeans needed a little more leeway room to fit all of the TV signal in. If you tried to watch a European TV show on an American TV (even if you got the voltage right) you would only get part of the picture signal, with a lot of static, and none of the audio signal. By now that European technology has caught up, they still use the same broadcasting method (all their cameras, receivers, televisions, broadcasting equipment and so on use the same standard) because that is the standard set. It is also why a DVD made to be shown in Europe (Region 2) won’t work on an American TV (Region 1). Both DVD players can play the media, but both models of TV can’t receive and make sense of that data. A similar standard difference exists in Africa and parts of Asia.

The only problem with and FM signal is that you can’t put as much power into it and push it through interference like you can with an AM signal. You also can’t take advantage of the Ionosphere signal bouncing ability. This limited television signals for a long time. Before the space age a TV signal had to be broadcast from an extremely tall antenna or a series of antennas. To send a TV signal from New York to Los Angeles most of it had to be sent via telephone like wires (cable). To boost each signal a lot of member stations; affiliates were needed. In many cases the show was taped and then mailed (snail mail) or physically carried to other stations for them to broadcast the show; this created a tape delay. By the time a television signal from an overseas Olympics competition could be broadcast in the US the AM radios would have already have received the results and those results would be at the newspaper office ready to be printed.

With the coming of the Space Age and satellites FM signals could be sent to a satellite and to almost anywhere. In the 1960s the Olympics were broadcast via satellite so that people all around the world could watch it with only a few seconds delay. This was amazing, for its time. Now days all of the big TV stations broadcast via satellites so you can watch an event like the Olympics before the wire services can spoil it.

Check out these Wikipedia articles: http://en.wikipedia.org/wiki/Broadcast_band
FM Broadcasting: http://en.wikipedia.org/wiki/FM_Broadcasting
AM Broadcasting: http://en.wikipedia.org/wiki/AM_broadcasting
Mediumwave on AM: http://en.wikipedia.org/wiki/Mediumwave
Types of Radio Emissions: http://en.wikipedia.org/wiki/Types_of_radio_emissions
Frequency Spectrum: http://en.wikipedia.org/wiki/Frequency_spectrum
Sideband: http://en.wikipedia.org/wiki/Sideband
This chart: http://en.wikipedia.org/wiki/Electromagnetic_spectrum#Radio_frequency
Shows where the various radio and TV signals lie on the electromagnetic spectrum; the lower on the chart the smaller the distance between each wave (the smaller the frequency); the higher on the chart the more energy in the signal. You see that the strongest and most damaging energy is at the top of the chart; Gamma Rays (Bruce Banner had better watch out: http://en.wikipedia.org/wiki/Incredible_Hulk). You will also see the narrow visible spectrum and notice that the spot used by radio and TV signals are well below that of light so it is safe for humans to be exposed to them. However, if you get too close to the antenna then radio signals can cook you like a microwave (A little higher on the spectrum, microwaves are safe at a distance since they have less energy than the light we can see. This is why a simple metal screen can protect most people from the damage done by a microwave).

2006-11-11 12:54:54 · answer #1 · answered by Dan S 7 · 1 1

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VSB is or was the type of am modulation used for the transmission of the video component of analogue television. All systems 405, 525, and 625 line, used it. Double sideband with center carrier is basic am, or amplitude modulation as Medium Wave AM. Single sideband, SSB transmits only one sideband, blocking or suppressing the other and the carrier. There is Upper Sideband and distinct from Lower Sideband is used. Weather, Aircraft, Amateurs and CB'ers use SSB. VSB is used for TV because of the difficulty and cost of making linear broadband brickwall filters. By gradually sloping-off the filter so its center passed through the carrier frequency from say upper sideband to lower sideband, any loss in the upper sideband would be made up for by the intrusion in the lower sideband. This part of the modulation represents the parts of a video spectrum which have little change, walls , clear sky, or any uniform area. From the use of a small part uf the lower sideband the name is derived. A vestige or remaining portion of the mainly unused sideband is used. Its almost SSB but not quiet, so its VSB.

2016-04-10 11:26:28 · answer #2 · answered by ? 4 · 0 0

Picture Transmission In Tv

2016-12-18 07:55:20 · answer #3 · answered by Anonymous · 0 0

Packrat2 is right. AM is used for video transmission, and FM is used for the audio transmission in a TV signal. On top of what packrat2 said, it is also because a FM signal takes much more bandwidth (I don't remember the math for this but it shouldn't be to hard to find and/or figure it out) in the form of harmonics, in fact the harmonics are proportional to the bandwidth of the signal, FM is feasible for a low bandwidth transmission such audio (around 15kHz), but imagine a signal with a bandwidth of around 4MHz (6MHz is the whole bandwidth for a channel, audio plus video) for a video signal, the harmonics would be huge, so they could pack less channels in the same chunk of spectrum. FM generally means higher quality than AM, that's why you may have noticed that when you are setting your antenna, is much easier to get the audio of a channel, but to get the video of the channel gets a little bit trickier because video its AM and it much more susceptible to noise, compared to FM where the data is encoded in the frequency rather than the amplitude. Hope this helps :)

2006-11-11 16:32:17 · answer #4 · answered by Gamatronics 3 · 1 0

Actually, the TV visual signal is transmitted as "vestigial sideband" AM. With that technique, the upper sideband is transmitted in its entirety, but the lower one is "chopped off." The purpose of this procedure is to make the signal fit within the allocated 6 MHz channel width. For more info, go to: http://en.wikipedia.org/wiki/NTSC and scroll down to "Transmission modulation scheme." As for WHY this is how it's done, is all goes back to the standards adopted by the NTSC, back in the 40s. Bear in mind that FM radio transmission was in its infancy back then, and digital techniques in communication (like pulse code and pulse width modulation) were nonexistent. So why are we still using a 60 year-old modulation scheme? !) It works. 2) People would scream bloody murder if they woke up one day and found their TVs didn't work, because a new standard was adopted.

2006-11-11 13:33:12 · answer #5 · answered by pack_rat2 3 · 1 0

Amplitude modulation is not used in TV transmission but Frequency modulation. Reason being it uses Double Side Band transmission with high frequency of Data and Low freqency of image. To inclued the whole bandwidth of lower and higher freqencies FM is used. AM cannot accomodate higher freqencies.

2006-11-13 04:33:39 · answer #6 · answered by Mathew C 5 · 0 2

Your best source would be an ARRL ( Amateur Radio Relay League) handbook that you can order online or buy at an electronics store or borrow from a library. Their section on TV will help.

2016-03-19 06:38:23 · answer #7 · answered by ? 4 · 0 0

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