An element in a hierarchical division of an open system that interacts directly with elements in the next higher division or the next lower division of that open system.
The angle of the Sun above the horizon. Both the quantity (lumens) and the spectral quality of light being reflected to a remote sensor are influenced by Sun angle. Also called Sun elevation and Sun elevation angle. [Tim11]
An Earth satellite orbit in which the orbital plane remains at a fixed angle with respect to the Sun, precessing through 360°. [Tim11]
The band of cable television channels J through W lying between 216 MHz and 300 MHz. In satellite communications, the frequency band from 216 to 600 MHz used for fixed and mobile radios and additional television channels on a cable system. [Sat07]
Superheterodyne refers to a method of designing and building wireless communications or broadcast equipment, particularly radio receivers. Sometimes a receiver employing this technology is called a "superheterodyne" or "superhet." [Wha10] The superheterodyne (short for supersonic heterodyne) receiver was first evolved by Major Edwin Howard (E. H.) Armstrong. [Llo89] One of Armstrong's four principal inventions—regeneration, superregeneration, the superheterodyne, and frequency modulation (FM), he devised it during World War I, patented it shortly afterward, sold his patent to Westinghouse, who cross-licensed RCA and the radio industry. [Dou90] , in 1918. It was introduced to the market place in the late 1920s.
To “heterodyne” means to mix to frequencies together so as to produce a beat frequency, namely the difference between two frequencies. Amplitude modulation is a heterodyne process: the information signal is mixed with the carrier to produce the side-bands. The side-bands occur at precisely the sum and difference frequencies of the carrier and information. These are beat frequencies (normally the beat frequency is associated with the lower side-band, the difference between the two). When you use the lower side-band (the difference between the two frequencies), you are “superheterodyning”. Strictly speaking, the term superheterodyne refers to creating a beat frequency that is lower than the original signal. Although we have used amplitude modulation side-bands as an example, we are not talking about encoding information for transmission. What superheterodying does is to purposely mix in another frequency in the receiver, so as to reduce the signal frequency prior to processing. Why and how this is done will be discussed below.
As discussed above, superheterodyning is simply reducing the incoming signal in frequency by mixing. In a radio application we are reducing the AM or FM signal which is centered on the carrier frequency to some intermediate value, called the IF (intermediate frequency). For practical purposes, the superheterodyne receiver always reduces to the same value of IF. To accomplish this requires that we be able to continuously vary the frequency being mixed into the signal so as to keep the difference the same. Here's what the superheterodyne receiver looks like:
This is essentially the conventional receiver with the addition of a mixer and local oscillator. The local oscillator is linked to the tuner because they both must vary with the carrier frequency. For example, suppose you want to tune in a TV station at 235 MHz. The band-pass filter (which only permits signals in a small range about the center frequency to pass) must be centered at 235 MHz (or slightly higher in single side-band (SSB)). The local oscillator must be set to a frequency that will heterodyne the 235 MHz to the desired IF of 452 kHz (typical). This means the local oscillator must be set to 234.448 MHz (or alternatively to 235.452 MHz) so that the difference frequency will be exactly 452 kHz. The local oscillator must be capable of varying the frequency over the same range as the tuner; in fact, they vary the same amount. Therefore, the tuner and the local oscillator are linked so they operate together.
Advantages of Using Superheterodyning
Now, we easily see that this type of receiver can be constructed, but for what purpose? All we have accomplished is to reduce the frequency to the IF value. We still must process the signal as before. So why do many receivers use the superheterodyne method? There are three (3) main advantages, depending on the application used for:
It reduces the signal from very high frequency sources where ordinary components wouldn't work (like in a radar receiver).
It allows many components to operate at a fixed frequency (IF section) and therefore they can be optimized or made more inexpensively.
It can be used to improve signal isolation by arithmetic selectivity [FAS98]
See quad shield cable.
Originally referred to television station WTBS in Atlanta; now generally used to describe any broadcast TV station that has its signal distributed nationally by satellite.
A North American television broadcast station, other than a Broadcast Network station, licensed by the FCC that is secondarily transmitted by a satellite carrier. [Tim11]
A cable that carries several video channels between facilities of a cable television company. [Arr11]
Supplemental Ordinary Keycodes
Keycodes other than the Mandatory Ordinary Keycodes that have not been reserved for special purposes, and thus will be treated the same as the Mandatory Ordinary Keycodes, i.e., will be available to the application that has focus.
A simple and inexpensive LED that emits light from its flat surface rather than its side with emission spread over a wide angle. [Fib111]
Surface-emitting Diode Diagrams courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/s
Optional feature for all line passives (RPI, RLS,RLDC). Shunts voltage surges to ground, protecting the active passive system components. Voltage surges can be from lightning, power sup-ply surges and power grid surges. [Arr11]
More commonly referred to as Dolby Digital, a standard for high-quality digital audio that is used for the sound portion of video stored in digital format, especially videos stored on DVD-ROMs. Dolby Digital delivers 6 channels in the so called “5:1” configuration: left, right, and center screen channels, separate left and right sounds, and a subwoofer channel. [Fib111]
Switched Virtual Circuit
Subscriber Video Device
High quality video interface, derived from Super VHS signal format.
Subscription Video On Demand
Pertaining to cable, checking frequency response by generating an RF voltage whose frequency is varied back and forth through a given frequency range at a rapid constant rate and observing the results on an oscilloscope. In cable TV applications, the structural return loss sweep-test determines internal reflections in the cable. A high structural return loss is desirable. [Arr11]
1) In communications systems, a mechanical, electro-mechanical, or electronic device for making, breaking, or changing the connections in or among circuits. 2) Synonym for central office, switching center or switched hub, a device used for connecting Ethernet LANs. 3) In communications systems, to transfer a connection from one circuit to another. [Fib111]
Switched Circuit Network (SCN)
Synonym for the Public Switched Telephone Network.
Any network in which switching is present and is used to direct messages from the sender to the ultimate recipient. Usually switching is accomplished by disconnecting and reconnecting lines in different configurations in order to set up a continuous pathway between the sender and the recipient.
A cable communications service in which each subscriber has a terminal and may communicate with any other subscriber.
Switched Virtual Circuit (SVC)
Allow an end-user to establish on-demand data connections between two end points on an ATM, Frame Relay, or X.25 network. See also Permanent Virtual Circuit (PVC).
A control which permits the selection of one image from any of several cameras to be fed into the television display or recording system.
A device permitting gradual, overlapping transition from the image of one camera to another. Sometimes incorporated as part of a special effects generator.
The cryptographic key used in a symmetric key algorithm—an algorithm for cryptography that uses the same cryptographic key to encrypt and decrypt messages.
An abbreviation for the words “synchronization”, “synchronizing”, etc. Applies to the synchronization signals, or timing pulses, which lock the electron beam of the picture monitors in step, both horizontally and vertically, with the electronic beam of the pickup tube. The color sync signal (NTSC) is known as the color burst.
The reduction in the amplitude of the sync signal, with respect to the picture signal, occurring between two points of a circuit.
A device used to supply a common or master sync signal to a system of several cameras. This ensures that their scanning pulses will be in phase. Scanning pulses out of phase produce distortion or rolling, sometimes called “sync loss.”
The level of the tips of the synchronizing pulses.
1) A signal derived from the composite or combination of the horizontal and vertical drives. 2) A pulse used to achieve or maintain synchronism, usually applied to analog signals. (The term “synchronization bit” is usually applied to digital data streams.) Commonly called the sync pulse. See also composite sync. [Fib111]
Pulses imposed on the composite baseband video signal used to keep the television picture scanning in perfect step with the scanning at the television camera. [Cha07]
A data signal that is sent along with a clock signal. A system in which events, such as signals, occur at evenly spaced time durations. Opposite of asynchronous. [Fib111]
Synchronous_Asynchronous Diagrams courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/s