Video modulation

Last updated February 27, 2024

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.^[1]

Key concepts

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

Circuit

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.^[2] 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

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.^[3] 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.^[4] 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.^[5] 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.

Decoding

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.^[6]

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.^[7] 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.^[1]

Types of video modulation

The types of video modulation can be classified by the way that a carrier wave is combined with video signal. The video signal and carrier wave both exist in waveform and when the carrier wave "carries" the video signal, the shape of carrier wave is changed and the changed carrier wave is the RF signal. Hence, how the shape of carrier wave is changed is the key to classify the types of video modulation.

Amplitude modulation (AM)

Amplitude modulation works by varying the amplitude of the carrier wave according to the bent waveform of original video signal. In other words, when the carrier wave is combined with the video signal, frequency of the combined signal is the same as the frequency of the carrier wave while the amplitude is varied. The picture above can more directly explain how the shape is changed. In the combining process, if the video signal is at its peak (highest amplitude value), the amplitude of the varied carrier wave will be the highest and as for trough (lowest amplitude value), the amplitude of varied carrier wave will be the lowest. In other words, at the point in carrier wave that corresponds to the peak of video signal, the shape of wave will be most bulged and at the point corresponding to the trough, the shape will be most sunk. In fact, the change in amplitude of carrier wave is proportional to the amplitude of the video signal.

Frequency modulation (FM)

In frequency modulation, the carrier wave is combined with video signal by varying the instantaneous frequency of the wave (the frequency of wave at a particular point). (Compare with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant.) From the picture above, if the video signal is at its peak, the instantaneous frequency of carrier wave will increase and as for the trough, the instantaneous frequency will decrease. More visually, the carrier wave becomes densest at the point corresponding to the peak in video signal and thinnest at the point corresponding to the trough.

Applications

Various types of video modulation can be applied in various areas due to their unique advantages and disadvantages.

Application of AM

Radio or television (broadcasting)
Aircraft band

AM is applied in these areas because video amplitude modulated signal can be easily coded and decoded since the change in amplitude of the carrier wave and the amplitude of original video signal is proportional. However, amplitude modulation is sensitive to noise and electromagnetic interference. Hence, the AM techniques are mainly utilized in less technical fields that tolerate noise and electromagnetic interference, such as broadcasting.

FM is applied in these fields because it is less sensitive to the noise and electromagnetic interference and when the data is collected or the signal is recorded, it is really important to minimize the effects of outside interference, such as noise and electromagnetic interference.

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In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a separate signal called the modulation signal that typically contains information to be transmitted. For example, the modulation signal might be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing a sequence of binary digits, a bitstream from a computer.

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A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized audio channel, which is received by television receivers belonging to a public audience, which display the image on a screen. A television transmitter, together with the broadcast studio which originates the content, is called a television station. Television transmitters must be licensed by governments, and are restricted to a certain frequency channel and power level. They transmit on frequency channels in the VHF and UHF bands. Since radio waves of these frequencies travel by line of sight, they are limited by the horizon to reception distances of 40–60 miles depending on the height of transmitter station.

A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses.

Radio receiver design includes the electronic design of different components of a radio receiver which processes the radio frequency signal from an antenna in order to produce usable information such as audio. The complexity of a modern receiver and the possible range of circuitry and methods employed are more generally covered in electronics and communications engineering. The term radio receiver is understood in this article to mean any device which is intended to receive a radio signal in order to generate useful information from the signal, most notably a recreation of the so-called baseband signal which modulated the radio signal at the time of transmission in a communications or broadcast system.

In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers did not have to demodulate the radio signal, but just distinguish between the presence or absence of a radio signal, to reproduce the Morse code "dots" and "dashes". The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.

References

1 2 Baer, Ralph H. "Digital video modulation and demodulation system" . Retrieved 10 November 2014.
↑ S. E., Miller (1954). "Waveguide as a communication medium". Bell System Technical Journal. 33 (6): 1209–1265. doi:10.1002/j.1538-7305.1954.tb03753.x. ISSN 0005-8580.
↑ R. L., Carbrey (Jan 2007). "Video Transmission over Telephone Cable Pairs by Pulse Code Modulation". Proceedings of the IRE. 48 (9): 1546. doi:10.1109/JRPROC.1960.287668.
↑ Graf, Rudolf F. Modern Dictionary of Electrons (7th ed.). US: Newnes. p. 162,163. ISBN 0750698667 . Retrieved 29 October 2014.
↑ Föll, Helmut. "Electronic Material".
↑ Ted, Hartson; Robert, Dickinson; Walter, Ciciora. "Expanded information capacity for existing communication transmission systems" . Retrieved 10 November 2014.
↑ W. John Head; Watson, Francis M. "Compensated light sensor system" . Retrieved 10 November 2014.

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[videomodulation-1] 1 2 Baer, Ralph H. "Digital video modulation and demodulation system" . Retrieved 10 November 2014.

[2] S. E., Miller (1954). "Waveguide as a communication medium". Bell System Technical Journal. 33 (6): 1209–1265. doi:10.1002/j.1538-7305.1954.tb03753.x. ISSN 0005-8580.

[CODING-3] R. L., Carbrey (Jan 2007). "Video Transmission over Telephone Cable Pairs by Pulse Code Modulation". Proceedings of the IRE. 48 (9): 1546. doi:10.1109/JRPROC.1960.287668.

[4] Graf, Rudolf F. Modern Dictionary of Electrons (7th ed.). US: Newnes. p. 162,163. ISBN 0750698667 . Retrieved 29 October 2014.

[5] Föll, Helmut. "Electronic Material".

[6] Ted, Hartson; Robert, Dickinson; Walter, Ciciora. "Expanded information capacity for existing communication transmission systems" . Retrieved 10 November 2014.

[7] W. John Head; Watson, Francis M. "Compensated light sensor system" . Retrieved 10 November 2014.

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