In electronics, signal processing, video, ringing is oscillation of a signal in the step response. Ringing is undesirable, but not always, as in the case of resonant inductive coupling, it is known as hunting. It is related to overshoot occurring following overshoot, thus the terms are at times conflated, it is known as ripple in electricity or in frequency domain response. In electrical circuits, ringing is an unwanted oscillation of a current, it happens when an electrical pulse causes the parasitic capacitances and inductances in the circuit to resonate at their characteristic frequency. Ringing artifacts are present in square waves. Ringing is undesirable because it causes extra current to flow, thereby wasting energy and causing extra heating of the components. Ringy communications circuits may suffer falsing. Ringing can be due to signal reflection. In video circuits, electrical ringing causes spaced repeated ghosts of a vertical or diagonal edge where dark changes to light or vice versa, going from left to right.
In a CRT the electron beam upon changing from dark to light or vice versa instead of changing to the desired intensity and staying there and undershoots a few times. This bouncing could occur anywhere in the electronics or cabling and is caused by or accentuated by a too high setting of the sharpness control. Ringing can affect audio equipment in a number of ways. Audio amplifiers can produce ringing depending on their design, although the transients that can produce such ringing occur in audio signals. Transducers can ring. Mechanical ringing is more of a problem with loudspeakers as the moving masses are larger and less damped, but unless extreme they are difficult to audibly identify. In digital audio, ringing can occur as a result of filters such as brickwall filters. Here, the ringing occurs before the transient as well as after. In signal processing, "ringing" may refer to ringing artifacts: spurious signals near sharp transitions; these have a number of causes, occur for instance in JPEG compression and as pre-echo in some audio compression.
Microphonics Ripple Impedance matching Microphony with older video cameras
Digital television is the transmission of television signals, including the sound channel, using digital encoding, in contrast to the earlier television technology, analog television, in which the video and audio are carried by analog signals. It is an innovative advance that represents the first significant evolution in television technology since color television in the 1950s. Digital TV transmits in a new image format called HDTV, with greater resolution than analog TV, in a wide screen aspect ratio similar to recent movies in contrast to the narrower screen of analog TV, it makes more economical use of scarce radio spectrum space. A transition from analog to digital broadcasting began around 2006 in some countries, many industrial countries have now completed the changeover, while other countries are in various stages of adaptation. Different digital television broadcasting standards have been adopted in different parts of the world; this standard has been adopted in Europe, Asia, total about 60 countries.
Advanced Television System Committee uses eight-level vestigial sideband for terrestrial broadcasting. This standard has been adopted by 6 countries: United States, Mexico, South Korea, Dominican Republic and Honduras. Integrated Services Digital Broadcasting is a system designed to provide good reception to fixed receivers and portable or mobile receivers, it utilizes two-dimensional interleaving. It supports hierarchical transmission of up to three layers and uses MPEG-2 video and Advanced Audio Coding; this standard has been adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H.264/MPEG-4 AVC that been adopted in most of South America and is being embraced by Portuguese-speaking African countries. Digital Terrestrial Multimedia Broadcasting adopts time-domain synchronous OFDM technology with a pseudo-random signal frame to serve as the guard interval of the OFDM block and the training symbol; the DTMB standard has been adopted in the People's Republic including Hong Kong and Macau.
Digital Multimedia Broadcasting is a digital radio transmission technology developed in South Korea as part of the national IT project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones, laptops and GPS navigation systems. Digital TV's roots have been tied closely to the availability of inexpensive, high performance computers, it wasn't until the 1990s. In the mid-1980s, as Japanese consumer electronics firms forged ahead with the development of HDTV technology, as the MUSE analog format was proposed by Japan's public broadcaster NHK as a worldwide standard, Japanese advancements were seen as pacesetters that threatened to eclipse U. S. electronics companies. Until June 1990, the Japanese MUSE standard—based on an analog system—was the front-runner among the more than 23 different technical concepts under consideration. An American company, General Instrument, demonstrated the feasibility of a digital television signal; this breakthrough was of such significance that the FCC was persuaded to delay its decision on an ATV standard until a digitally based standard could be developed.
In March 1990, when it became clear that a digital standard was feasible, the FCC made a number of critical decisions. First, the Commission declared that the new ATV standard must be more than an enhanced analog signal, but be able to provide a genuine HDTV signal with at least twice the resolution of existing television images. To ensure that viewers who did not wish to buy a new digital television set could continue to receive conventional television broadcasts, it dictated that the new ATV standard must be capable of being "simulcast" on different channels; the new ATV standard allowed the new DTV signal to be based on new design principles. Although incompatible with the existing NTSC standard, the new DTV standard would be able to incorporate many improvements; the final standard adopted by the FCC did not require a single standard for scanning formats, aspect ratios, or lines of resolution. This outcome resulted from a dispute between the consumer electronics industry and the computer industry over which of the two scanning processes—interlaced or progressive—is superior.
Interlaced scanning, used in televisions worldwide, scans even-numbered lines first odd-numbered ones. Progressive scanning, the format used in computers, scans lines in sequences, from top to bottom; the computer industry argued that progressive scanning is superior because it does not "flicker" in the manner of interlaced scanning. It argued that progressive scanning enables easier connections with the Internet, is more cheaply converted to interlaced formats than vice versa; the film industry supported progressive scanning because it offers a more efficient means of converting filmed programming into digital formats. For their part, the consumer electronics industry and broadcasters argued that interlaced scanning was the only technology that could transmit the highest quality pictures feasible, i.e. 1,080 lines per picture and 1,920 pixels per line. Broadcasters favored interlaced scanning because their vast archive of interlaced
Amplitude modulation is a modulation technique used in electronic communication, most for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the carrier wave is varied in proportion to that of the message signal being transmitted; the message signal is, for example, a function of the sound to be reproduced by a loudspeaker, or the light intensity of pixels of a television screen. This technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, phase modulation, in which its phase is varied. AM was the earliest modulation method used to transmit voice by radio, it was developed during the first quarter of the 20th century beginning with Landell de Moura and Reginald Fessenden's radiotelephone experiments in 1900. It remains in use today in many forms of communication. AM is used to refer to mediumwave AM radio broadcasting. In electronics and telecommunications, modulation means varying some aspect of a continuous wave carrier signal with an information-bearing modulation waveform, such as an audio signal which represents sound, or a video signal which represents images.
In this sense, the carrier wave, which has a much higher frequency than the message signal, carries the information. At the receiving station, the message signal is extracted from the modulated carrier by demodulation. In amplitude modulation, the amplitude or strength of the carrier oscillations is varied. For example, in AM radio communication, a continuous wave radio-frequency signal has its amplitude modulated by an audio waveform before transmission; the audio waveform modifies the amplitude of the carrier wave and determines the envelope of the waveform. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency and two adjacent sidebands; each sideband is equal in bandwidth to that of the modulating signal, is a mirror image of the other. Standard AM is thus sometimes called "double-sideband amplitude modulation" to distinguish it from more sophisticated modulation methods based on AM. One disadvantage of all amplitude modulation techniques is that the receiver amplifies and detects noise and electromagnetic interference in equal proportion to the signal.
Increasing the received signal-to-noise ratio, say, by a factor of 10, thus would require increasing the transmitter power by a factor of 10. This is in contrast to frequency modulation and digital radio where the effect of such noise following demodulation is reduced so long as the received signal is well above the threshold for reception. For this reason AM broadcast is not favored for music and high fidelity broadcasting, but rather for voice communications and broadcasts. Another disadvantage of AM is; the carrier signal contains none of the original information being transmitted. However its presence provides a simple means of demodulation using envelope detection, providing a frequency and phase reference to extract the modulation from the sidebands. In some modulation systems based on AM, a lower transmitter power is required through partial or total elimination of the carrier component, however receivers for these signals are more complex and costly; the receiver may regenerate a copy of the carrier frequency from a reduced "pilot" carrier to use in the demodulation process.
With the carrier eliminated in double-sideband suppressed-carrier transmission, carrier regeneration is possible using a Costas phase-locked loop. This doesn't work however for single-sideband suppressed-carrier transmission, leading to the characteristic "Donald Duck" sound from such receivers when detuned. Single sideband is used in amateur radio and other voice communications both due to its power efficiency and bandwidth efficiency. On the other hand, in medium wave and short wave broadcasting, standard AM with the full carrier allows for reception using inexpensive receivers; the broadcaster absorbs the extra power cost to increase potential audience. An additional function provided by the carrier in standard AM, but, lost in either single or double-sideband suppressed-carrier transmission, is that it provides an amplitude reference. In the receiver, the automatic gain control responds to the carrier so that the reproduced audio level stays in a fixed proportion to the original modulation.
On the other hand, with suppressed-carrier transmissions there is no transmitted power during pauses in the modulation, so the AGC must respond to peaks of the transmitted power during peaks in the modulation. This involves a so-called fast attack, slow decay circuit which holds the AGC level for a second or more following such peaks, in between syllables or short pauses in the program; this is acceptable for communications radios, where compression of the audio aids intelligibility. However it is undesired for music or normal broadcast programming, where a faithful reproduction of the original program, including its varying modulation levels, is expected. A trivial form of AM which can be used for transmitting binary data is on-off keying, the simplest form of amplitude-shift keying, in which ones and zeros are represented by the presence or absence
A communication channel or channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders to one or several receivers. A channel has a certain capacity for transmitting information measured by its bandwidth in Hz or its data rate in bits per second. Communicating data from one location to another requires some form of medium; these pathways, called communication channels, use two types of media: broadcast. Cable or wire line media use physical wires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, fiber-optic cable is made of glass. In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is a kind of channel, which can be sent to and received from.
Examples of communications channels include: A connection between initiating and terminating nodes of a circuit. A single path provided by a transmission medium via either physical separation, such as by multipair cable or electrical separation, such as by frequency-division or time-division multiplexing. A path for conveying electrical or electromagnetic signals distinguished from other parallel paths. A storage which can communicate a message over time as well as space The portion of a storage medium, such as a track or band, accessible to a given reading or writing station or head. A buffer from which messages can be'put' and'got'. See Actor model and process calculi for discussion on the use of channels. In a communications system, the physical or logical link that connects a data source to a data sink. A specific radio frequency, pair or band of frequencies named with a letter, number, or codeword, allocated by international agreement. Examples: Marine VHF radio uses some 88 channels in the VHF band for two-way FM voice communication.
Channel 16, for example, is 156.800 MHz. In the US, seven additional channels, WX1 - WX7, are allocated for weather broadcasts. Television channels such as North American TV Channel 2 = 55.25 MHz, Channel 13 = 211.25 MHz. Each channel is 6 MHz wide; this was based on the bandwidth required by older analog television signals. Since 2006 television broadcasting has switched to digital modulation which uses image compression to transmit a television signal in a much smaller bandwidth, so each of these "physical channels" has been divided into multiple "virtual channels" each carrying a DTV channel. Wi-Fi uses 13 channels from 2412 MHz to 2484 MHz in 5 MHz steps, in the ISM bands; the radio channel between an amateur radio repeater and a ham uses two frequencies 600 kHz apart. For example, a repeater that transmits on 146.94 MHz listens for a ham transmitting on 146.34 MHz. All of these communications channels share the property; the information is carried through the channel by a signal. A channel can be modelled physically by trying to calculate the physical processes which modify the transmitted signal.
For example, in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might be added in to simulate external interference and/or electronic noise in the receiver. Statistically a communication channel is modelled as a triple consisting of an input alphabet, an output alphabet, for each pair of input and output elements a transition probability p. Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel. Statistical and physical modelling can be combined. For example, in wireless communications the channel is modelled by a random attenuation of the transmitted signal, followed by additive noise; the attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model electronic noise in the receiver. If the attenuation term is complex it describes the relative time a signal takes to get through the channel.
The statistics of the random attenuation are decided by previous measurements or physical simulations. Channel models may be continuous channel models in that there is no limit to how their values may be defined. Communication channels are studied in a discrete-alphabet setting; this corresponds to abstracting a real world communication system in which the analog → digital and digital → analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input. A channel model may either be analog. In a digital channel model, the transmitted message is modelled as a digital signal at a certain protocol layer. Underlying protocol layers, such as the physical layer transmission technique, is replaced by a simplified model.
The model may reflect channel performance measures such as bit rate, bit errors, latency/delay, delay jitter, etc. Examples of digital channel models are: Binary symmetric channel, a discrete memoryless channel with a
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal. Chrominance is represented as two color-difference components: U = B′ − Y′ and V = R′ − Y′; each of these difference components may have scale factors and offsets applied to it, as specified by the applicable video standard. In composite video signals, the U and V signals modulate a color subcarrier signal, the result is referred to as the chrominance signal. In digital-video and still-image color spaces such as Y′CbCr, the luma and chrominance components are digital sample values. Separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately; the chrominance bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier, in digital systems by chroma subsampling. The idea of transmitting a color television signal with distinct luma and chrominance components originated with Georges Valensi, who patented the idea in 1938.
Valensi's patent application described: The use of two channels, one transmitting the predominating color, the other the mean brilliance output from a single television transmitter to be received not only by color television receivers provided with the necessary more expensive equipment, but by the ordinary type of television receiver, more numerous and less expensive and which reproduces the pictures in black and white only. Previous schemes for color television systems, which were incompatible with existing monochrome receivers, transmitted RGB signals in various ways. In analog television, chrominance is encoded into a video signal using a subcarrier frequency. Depending on the video standard, the chrominance subcarrier may be either quadrature-amplitude-modulated or frequency-modulated. In the PAL system, the color subcarrier is 4.43 MHz above the video carrier, while in the NTSC system it is 3.58 MHz above the video carrier. The NTSC and PAL standards are the most used, although there are other video standards that employ different subcarrier frequencies.
For example, PAL-M uses a 3.58 MHz subcarrier, SECAM uses two different frequencies, 4.250 MHz and 4.40625 MHz above the video carrier. The presence of chrominance in a video signal is indicated by a color burst signal transmitted on the back porch, just after horizontal synchronization and before each line of video starts. If the color burst signal were visible on a television screen, it would appear as a vertical strip of a dark olive color. In NTSC and PAL, hue is represented by a phase shift of the chrominance signal relative to the color burst, while saturation is determined by the amplitude of the subcarrier. In SECAM and signals are transmitted alternately and phase does not matter. Chrominance is represented by the U-V color plane in PAL and SECAM video signals, by the I-Q color plane in NTSC. Digital video and digital still photography systems sometimes use a luma/chroma decomposition for improved compression. For example, when an ordinary RGB digital image is compressed via the JPEG standard, the RGB colorspace is first converted to a YCbCr colorspace, because the three components in that space have less correlation redundancy and because the chrominance components can be subsampled by a factor of 2 or 4 to further compress the image.
On decompression, the Y′CbCr space is rotated back to RGB. Luma Chroma subsampling
In electronics and telecommunications, a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, applied to the antenna; when excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary component parts of all electronic devices that communicate by radio, such as radio and television broadcasting stations, cell phones, walkie-talkies, wireless computer networks, Bluetooth enabled devices, garage door openers, two-way radios in aircraft, spacecraft, radar sets and navigational beacons; the term transmitter is limited to equipment that generates radio waves for communication purposes. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not called transmitters though they have similar circuits; the term is popularly used more to refer to a broadcast transmitter, a transmitter used in broadcasting, as in FM radio transmitter or television transmitter.
This usage includes both the transmitter proper, the antenna, the building it is housed in. A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and a receiver combined in one unit is called a transceiver; the term transmitter is abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance; the information is provided to the transmitter in the form of an electronic signal, such as an audio signal from a microphone, a video signal from a video camera, or in wireless networking devices, a digital signal from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, called the carrier signal; this process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitters. In an amplitude modulation transmitter, the information is added to the radio signal by varying its amplitude.
In a frequency modulation transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used; the radio signal from the transmitter is applied to the antenna, which radiates the energy as radio waves. The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, garage door openers. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, connected to the transmitter by a feed line, a transmission line. Electromagnetic waves are radiated by electric charges undergoing acceleration. Radio waves, electromagnetic waves of radio frequency, are generated by time-varying electric currents, consisting of electrons flowing through a metal conductor called an antenna which are changing their velocity or direction and thus accelerating. An alternating current flowing back and forth in an antenna will create an oscillating magnetic field around the conductor.
The alternating voltage will charge the ends of the conductor alternately positive and negative, creating an oscillating electric field around the conductor. If the frequency of the oscillations is high enough, in the radio frequency range above about 20 kHz, the oscillating coupled electric and magnetic fields will radiate away from the antenna into space as an electromagnetic wave, a radio wave. A radio transmitter is an electronic circuit which transforms electric power from a power source into a radio frequency alternating current to apply to the antenna, the antenna radiates the energy from this current as radio waves; the transmitter impresses information such as an audio or video signal onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar radio frequency currents in it; the radio receiver extracts the information from the received waves. A practical radio transmitter consists of these parts: A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.
An electronic oscillator circuit to generate the radio frequency signal. This generates a sine wave of constant amplitude called the carrier wave, because it serves to "carry" the information through space. In most modern transmitters, this is a crystal oscillator in which the frequency is controlled by the vibrations of a quartz crystal; the frequency of the carrier wave is considered the frequency of the transmitter. A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator; this is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal which represents moving images, or for data in the form of a binary digital signal which represents a sequence of bits, a bitstream. Different types of transmitters use different modulation methods to transmit information: In an AM transmitter the amplitude of the carrier wave is varied in proportion to the modulation signal.
In an FM transmitter the frequency of the carrier is varied by the modulation signal. In an FSK transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two binary digits, 0 and 1. Many oth
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light and water waves; the law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection. In acoustics, reflection is used in sonar. In geology, it is important in the study of seismic waves. Reflection is observed with surface waves in bodies of water. Reflection is observed with many types besides visible light. Reflection of VHF and higher frequencies is important for radar. Hard X-rays and gamma rays can be reflected at shallow angles with special "grazing" mirrors. Reflection of light is either diffuse depending on the nature of the interface. In specular reflection the phase of the reflected waves depends on the choice of the origin of coordinates, but the relative phase between s and p polarizations is fixed by the properties of the media and of the interface between them.
A mirror provides the most common model for specular light reflection, consists of a glass sheet with a metallic coating where the significant reflection occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their skin depths. Reflection occurs at the surface of transparent media, such as water or glass. In the diagram, a light ray PO strikes a vertical mirror at point O, the reflected ray is OQ. By projecting an imaginary line through point O perpendicular to the mirror, known as the normal, we can measure the angle of incidence, θi and the angle of reflection, θr; the law of reflection states that θi = θr, or in other words, the angle of incidence equals the angle of reflection. In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, the remainder is refracted. Solving Maxwell's equations for a light ray striking a boundary allows the derivation of the Fresnel equations, which can be used to predict how much of the light is reflected, how much is refracted in a given situation.
This is analogous to the way impedance mismatch in an electric circuit causes reflection of signals. Total internal reflection of light from a denser medium occurs if the angle of incidence is greater than the critical angle. Total internal reflection is used as a means of focusing waves that cannot be reflected by common means. X-ray telescopes are constructed by creating a converging "tunnel" for the waves; as the waves interact at low angle with the surface of this tunnel they are reflected toward the focus point. A conventional reflector would be useless as the X-rays would pass through the intended reflector; when light reflects off a material denser than the external medium, it undergoes a phase inversion. In contrast, a less dense, lower refractive index material will reflect light in phase; this is an important principle in the field of thin-film optics. Specular reflection forms images. Reflection from a flat surface forms a mirror image, which appears to be reversed from left to right because we compare the image we see to what we would see if we were rotated into the position of the image.
Specular reflection at a curved surface forms an image which may be demagnified. Such mirrors may have surfaces that are parabolic. If the reflecting surface is smooth, the reflection of light that occurs is called specular or regular reflection; the laws of reflection are as follows: The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane. The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal; the reflected ray and the incident ray are on the opposite sides of the normal. These three laws can all be derived from the Fresnel equations. In classical electrodynamics, light is considered as an electromagnetic wave, described by Maxwell's equations. Light waves incident on a material induce small oscillations of polarisation in the individual atoms, causing each particle to radiate a small secondary wave in all directions, like a dipole antenna. All these waves add up to give specular reflection and refraction, according to the Huygens–Fresnel principle.
In the case of dielectrics such as glass, the electric field of the light acts on the electrons in the material, the moving electrons generate fields and become new radiators. The refracted light in the glass is the combination of the forward radiation of the electrons and the incident light; the reflected light is the combination of the backward radiation of all of the electrons. In metals, electrons with no binding energy are called free electrons; when these electrons oscillate with the incident light, the phase difference between their radiation field and the incident field is π, so the forward radiation cancels the incident light, backward radiation is just the reflected light. Light–matter interaction in terms of photons is a topic of quantum electrodynamics, is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter; when light strikes the surface of a mate