Группы методов модуляции для передачи видеосигнала

Group of video transmitting modulation modes
Video modulation is a strategy of transmitting video signal in the field of radio modulation and television technology. This strategy enables the video signal to be transmitted more efficiently through long distances. In general, video modulation means that a higher frequency carrier wave is modified according to the original video signal. In this way, carrier wave contains the information in the video signal. Then, the carrier will "carry" the information in the form of radio frequency (RF) signal. When carrier reaches its destination, the video signal is extracted from the carrier by decoding. In other words, the video signal is first combined with a higher frequency carrier wave so that carrier wave contains the information in video signal. The combined signal is called radio frequency signal. At the end of this transmitting system, the RF signals stream from a light sensor and hence, the receivers can obtain the initial data in the original video signal. There are many application of video modulation:
Broadcasting music and speech
Two way radio systems (a radio that can both transmit and receive)
Aircraft band (radio communication in civil aviation)
Computer modems (a device that modulates Wi Fi)
These applications all utilized the efficiency of video modulation in order to minimize costs.

There are some important concepts needed to be understood in order to fully comprehend the video modulation.

Video signals are usually encoded into binary digit (0, 1) or seven digit pulse code modulation (PCM), which is a method used to digitally represent original video signal. The video signal converted to binary digit PCM at the point of origin can be then transmitted over existing telephone cable or wire directly to the destination. If the distance between origin and destination is too long, a repeater is needed to receive the signal and retransmit it at higher power so that the signal can cover longer distances. The circuit designed for the transmission should gather as many advantages as possible in order to be efficient. In other words, the circuit should be built with high quality and reliability and with minimum cost. The most common cable used to build the circuit is the 51 pair cable. This type of cable is often utilized in house phone because of the low price and good functionality. Also, signals can be transmitted efficiently since the distance between two adjacent repeaters can be 6000 feet rather than only 3000 feet compared to other cable.

Coding is the process during which the video inputs are transferred into binary digit (0, 1) because digits can be much more easily transmitted. A crystal oscillator is used in coding. It is an electronic oscillator that uses the mechanical resonance of a vibrating crystal or piezoelectric material to create an electrical signal with a very precise frequency. Inside the crystal oscillator, there is usually a crystal or elastic material whose constituent atoms, molecules, or ions are in a regular order. For example, the quartz is often used in crystal oscillators due to its elasticity. After a crystal is accurately installed, electric field can be distorted by a change in the voltage due to an electrode near or on the crystal. This characteristic is called electrostriction or inverse piezoelectricity. When the video signals are inputted into the oscillators, the crystal with electrodes attaching to it starts to vibrate as a resonator. The crystal's piezoelectric property converts the mechanical vibrations into an oscillating voltage and then the voltage is picked up by the attached electrodes. Thus, the electric field is distorted due to the potential differences between the electrodes. Since the original video signal exists as a time varying voltage on a wire, the distorted electric field is also time varying. The changing electric field is converted into waveform, a signal in the shape/form of wave. Finally, the output signals are combined with a carrier wave and transferred into RF signals, then transmitted to the receiver.

After the RF signals arrive the destination, the receiver can not obtain the data directly from the RF signals because there are coded and multiplied RF signals existing at the same time. As for the multiple RF signals problem, an electric filter is used. The frequency of each RF signal is usually different from other RF signals. An electronic filter can 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 electric filter, only the one corresponding video signal is received, with no interference from the other RF signals. For the coded RF signals, a light sensor is utilized. Generally, a light sensor is a device that detects changes of light in quantities and provides a corresponding output. One application of light sensors is the compensated light sensor system. This system can control the level of light at a workplace. The ratio of the light at the light sensors to the level of controlled light at the workplace is maintained substantially constant as daylight entering the room varies. In decoding process, a light sensor is used to detect the RF signal and then output the decoded signal. In general, the RF signals steamed from electric filters are first converted from binary digit to octal digit (0, 1, 2, 3, 4, 5, 6, 7). The octal signals are then detected by the light sensors. The sensors respond by developing corresponding digital, electrical signals which are hence decoded to apprise the viewer of the transmitted message or are applied to external equipment such as a calculator, a computer, a hard copy machine or other devices appropriate for the particular occasion. tags, these references will then appear here automatically >