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- Video_modulation abstract "Video modulation is the process of combining a video signal with a carrier wave, resulting in a combined signal, called a radio-frequency (RF) signal, where the carrier wave "carries" the video signal. Unlike the original video signal, the RF signal, also known as a "television signal", can be economically transmitted long distance through air, space or cables. A receiver recovers the original video signal by effectively stripping the video away from the carrier wave in an operation called video demodulation.Multiple RF signals can exist in the same space or in the same cable if the carrier wave is a different frequency for each of the RF signals. The receiver uses filtering to select only one RF signal on the basis of its carrier frequency, while rejecting all other RF signals. One example of this operation is selecting a channel on a television receiver. With only one RF signal that passes through the receiver's filter, only the one corresponding video signal is recovered, with no interference from the other RF signals.In some digital systems, many carriers are utilized for one video signal. These carriers are adjacent to one another in the frequency spectrum, and are all simultaneously passed by the receiver's filter for recovery of the one video signal. The many carriers of another channel are of course all different in frequency.The original video signal exists as a time-varying voltage on a wire. Video modulation takes place when some aspect of a high frequency carrier wave is made proportional to the time varying voltage of the original video signal. If the amplitude modulation (AM) technique is utilized, then its the amplitude of the carrier wave that is made proportional to the video signal voltage. If the frequency modulation (FM) technique is utilized, then its the frequency of the carrier wave that is made proportional to the video signal voltage. With FM, the carrier wave frequency changes only a small percent of its normal (center) frequency in the modulation process.Video modulation effectively moves (translates) the frequency spectrum of the video signal to a much higher frequency range where it exists as the above-mentioned RF signal.For modulating analog video signals using the amplitude modulation technique, the amplitude of the carrier is made proportional to the value of the video signal. The RF spectrum is no more than double the spectrum width of the original video signal because the RF signal consists of not only a carrier, but lower and upper sidebands as well. In analog broadcast television (NTSC, PAL, SECAM) that in the U.S. was in service up until 2009 for full-power stations, most of the lower sideband was removed in order to save spectrum space. Since all of the original video signal information fits in either upper or lower sideband, none of the video signal information was lost.Another analog modulation technique, frequency modulation, is used by about 4 remaining "free to air" satellite television channels (2014). The spectrum width of the resulting RF signal is typically 4 or more times wider than the spectrum of the original video signal. The extra spectrum width reduces background noise that appears as snow or sparkles, and also reduces other radio-frequency interference from other radio waves.Video modulation was also utilized for analog video tape. The original un-modulated video signal frequency modulates a carrier. However, this carrier is only 30 percent to 300 percent (depending on tape format) higher in frequency than the highest frequency component in the original video signal. The reason is that this RF signal is recorded directly onto the tape, and if the RF carrier wave was too high in frequency, the magnetic head gap would need to be too small, or too much tape would be consumed. The chief benefit of the FM recording used with video tape is that during recording, the RF signal saturates the magnetic particles on the tape for a stronger signal on playback. The tape saturation does not appreciably alter the frequency of the carrier wave recorded directly to the tape, so the video signal carried by the RF carrier is reasonably well preserved. For playback, the RF signal is routed to an FM demodulator to recover the original video signal. In its most important respects, this FM demodulator is similar to the FM demodulator in a car radio tuned to the FM radio band.A digital video signal represents a list of numbers, which in turn represent the picture information. "Digital compression" creates another list of numbers, which is a much shorter list. With digital video, it's this list of numbers that must be transmitted, one after the other. However, because of the limited frequency spectrum allowed for modulated digital video signal, the digital video signal must become a continuous-time digital video signal. In other words, the digital information is encoded as a video signal voltage in a wire where that voltage undergoes no abrupt changes over time, which might represent the transition from one number to the next. Instead, the video signal voltage changes smoothly (gradually) over time, hence the term "continuous-time".At the receiving end, these numbers must be recovered in a computer (again, one after the other in discrete-time) without alteration from the original list. With very good reception, this is what actually happens. The replication of the list of numbers is exact. This is one of the few cases in life where perfection is actually achieved, at least in the limited sense that a list of numbers is copied perfectly from transmitter to receiver. As the signal exists in the air or in a cable, the signal does not physically change abruptly from one number to the next. Therefore careful modulation techniques need to be used. Like with analog video, a synch signal needs to be transmitted so that the receiver can discern where one number ends and the next number begins. In this way the continuous-time digital signal can arrive in the receiver's computer memory once again as a discrete-time list of numbers. This list of numbers in the receiver's computer memory is the recovered discrete time compressed video signal. As these numbers are further processed by the computer program to eventually yield a picture on the receiving screen, new numbers are arriving all the time.In the case of digital television (8VSB video modulation) in the U.S., the digital numbers of the compressed digital video signal are converted from binary digital (base 2) over to octal digital (base 8). Each number is therefore represented by only 8 different possible digits, zero through seven. Each of these digits is represented by a voltage in a wire that is proportional to the value of the 8 digits. This is the '8' in 8VSB. The 8 possible voltage values are proportional to: -7, -5, -3, -1, +1, +3, +5, +7. The video signal voltage changes for each new number in the stream (or list). This digital video signal voltage then amplitude modulates a radio-frequency wave as with analog video. In this video modulation process, the resulting RF signal amplitude becomes proportional to the changing voltage of the original digital video signal. Now the RF wave takes on only 8 different possible amplitudes in response to each number in the stream of numbers. Between numbers, the RF wave amplitude changes gradually (continuous-time) between the previous number and the new number. In the demodulation process when reception is strong, it is easy enough for the receiver to discern which digit it's receiving without making errors. The receiver hardware converts each octal (base 8) digit to 3 binary (base 2) digits, and processing continues from there.As in analog video broadcasting that ended in 2009 for full-power stations, the lower sideband of the 8VSB RF signal is mostly removed, leaving only a vestige of the original lower sideband. So the "VSB" in "8VSB" stands for vestigial sideband. As with analog before it, either sideband carries all the information, so losing most of the lower side band to save spectrum space does not loose information, which in this case is a continuous stream of numbers.Much of the time, reception is less than perfect. So some of the numbers transmitted in digital video broadcasting is utilized for forward error correction, and represent redundancy of the numbers. All the numbers representing picture information are scrambled in the order of delivery in such a way that a burst error, the most common reception caused error, is spread over a large portion of the viewing screen, so any one area only loses a small number of digits, allowing easier error correction. Of course, the scrambling of the order in which the numbers arrive is also reversed during processing inside the receiver.".
- Video_modulation wikiPageID "566523".
- Video_modulation wikiPageRevisionID "606566437".
- Video_modulation auto "yes".
- Video_modulation date "December 2009".
- Video_modulation hasPhotoCollection Video_modulation.
- Video_modulation subject Category:Radio_modulation_modes.
- Video_modulation type Abstraction100002137.
- Video_modulation type Attribute100024264.
- Video_modulation type Manner104928903.
- Video_modulation type Property104916342.
- Video_modulation type RadioModulationModes.
- Video_modulation comment "Video modulation is the process of combining a video signal with a carrier wave, resulting in a combined signal, called a radio-frequency (RF) signal, where the carrier wave "carries" the video signal. Unlike the original video signal, the RF signal, also known as a "television signal", can be economically transmitted long distance through air, space or cables.".
- Video_modulation label "Video modulation".
- Video_modulation sameAs m.02qqgh.
- Video_modulation sameAs Q7927954.
- Video_modulation sameAs Q7927954.
- Video_modulation sameAs Video_modulation.
- Video_modulation wasDerivedFrom Video_modulation?oldid=606566437.
- Video_modulation isPrimaryTopicOf Video_modulation.