GLOSSARY

Physical Units & Definitions | Satellite Terminology | Devices / Hardware | Signal Processing

A	B	C	D	E	F	I	J	L
M	N	P	R	S	T	U	V	W

Amplitude Modulation (AM)	One of the methods for transmitting information using radio waves by superimposing the information signal onto a radio frequency carrier wave. The amplitude of the carrier wave is varied in accordance with the time-varying amplitude of the input signal. The frequency of the carrier wave remains unchanged.
Analogue	A system in which one continuously-varying physical quantity (e.g. the intensity of a sound wave) is represented directly by another (e.g. the voltage of an electrical signal) as faithfully as possible.
ADC	Analogue-to-Digital Converter. A device for converting an analogue voltage waveform into a series of digital numbers so that the signal can be manipulated numerically ("digitally processed").
Analogue Modulation	The process of Modulation, where the modulating wave or signal is analogue and the amplitude, frequency or phase of the carrier wave or signal is varied continuously according to the content of the modulating signal.
ARQ	Automatic Repeat Request. An error detection and correction technique based on the transmission of data in discrete packets. A decoder in the receiver detects errors but cannot correct them. Instead it sends a retransmission request to the transmitter which then repeats the transmission.
Audio Subcarrier (Television)	A carrier signal modulated by a sound signal, where the carrier frequency is slightly higher than the maximum frequency encountered in a video signal. This signal is combined with a video signal and the combination is used to modulate a radio frequency carrier for subsequent transmission over a satellite link. The signal is referred to as a subcarrier because it is itself modulated onto a carrier.
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Baseband	The range of frequencies occupied by the source electrical signal that is to be transmitted over a radiocommunications link. It is the frequency band occupied by an analogue or data signal prior to modulation and frequency conversion, or after frequency conversion and demodulation. For example, the baseband of a video signal extends from 0 Hz to about 5 MHz.
Block Downconversion	The process of converting the frequency of an entire block of radio frequency signals received from the satellite to a lower intermediate frequency (of around 1 GHz). This permits subsequent processing of the signals within a satellite receiver, including selection of the desired signal from the available block of signals.
Broadband	A pure frequency signal that is used to convey information through a transmission channel. The key characteristics of the carrier signal (frequency, amplitude or phase) are varied according to the content of the information. These variations are detected at the receiver and are used to reconstruct the original signal.
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CDMA	Code Division Multiple Access. A technique allowing multiple users to simultaneously share a common transmission bandwidth. Each user transmits continuously, generating a controlled level of interference into other users. Each transmitter is assigned a unique signature, or code, which is combined with the useful information at the transmitter. The receiver is able to recover the desired information and reject unwanted information by means of this unique code.
Channel Encoding (Coding)	The process of deliberately adding redundant information to a message at the transmit end of a transmission link so that errors can be detected and corrected at the reception point. The term "channel" is used to indicate that the encoding is specifically related to the transmission channel and to distinguish it from any other encoding used in the system (e.g. for digital image compression).
Chrominance (Television)	The colour information of a television picture. It is also used to refer to the modulated colour component of a PAL, SECAM or NTSC television signal.
Code Rate	The ratio of the number of bits in a data stream that carry useful information to the total number of bits, including those added for error correction purposes. For example, a code rate of ¾ indicates that ¾ of the bits carry useful information and ¼ of the bits are used to detect and correct errors in the receiver, after which time they are discarded.
Colour Bars (Television)	A television picture consisting of several coloured vertical bars, which is used for testing the performance of colour television equipment and transmission paths. There are several variants of the colour bar signal in use worldwide.
Colour Difference (Television)	A signal obtained by subtracting the brightness (luminance) information of a television picture from the primary colour information (red or blue). Two colour difference signals (red and blue) are conveyed in a PAL, SECAM or NTSC picture. The third (green) can be deduced in the television receiver from these two colour difference signals and the brightness information.
Compression (Coding)	A digital technique for reducing the information needed to represent a still image, a moving image or an audio signal without undue impact on the subjective quality of the processed material. The most important information is retained, whilst repeated or unnecessary information ("redundant" information) is discarded. Such techniques are used to reduce the capacity needed to store and/or transmit photographic, video and audio information.
Contrast (Television)	The extent to which adjacent light and dark areas of a television picture differ in brightness.
Crosstalk	Interference received in one communication channel from signals conveyed by other communication channels.
CVBS (Television)	Composite Video Blanking & Synchs. A baseband television signal containing the picture information (luminance and chrominance) plus all the synchronization signals necessary to display a fully-locked television picture.
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Decoding	The process of restoring a coded signal to its original form based on knowledge of the encoding process.
De-Emphasis	A reduction in the amplitude of the higher frequency portions of a frequency modulated signal (e.g. analogue television) and its accompanying noise after transmission via a radio link. De-emphasis is used in conjunction with a complementary Pre-Emphasis device in the transmitter so that their combined effect on the signal is neutral. The pre-/de-emphasis process improves the signal-to-noise ratio for high frequency signal components and thus the overall quality of the received signal. See also Pre-Emphasis.
Demodulation	The act or process by which an output wave or signal is obtained from a carrier wave or signal, where the recovered wave or signal has the characteristics of the original modulating wave or signal. The reverse process of Modulation.
Demultiplexing	The extraction of multiple distinct messages or signals from a single composite signal ("multiplex").
Digital	A system or device in which discrete signals are used to represent continuous signals in the form of numbers or other characters. Information is represented by electrical "on / off", "high / low" or "1 / 0" pulses, instead of being represented by a continuously-varying quantity (e.g. signal voltage) as is the case in an Analogue system or device.
Digital Modulation	The process of Modulation, where the modulating wave or signal is digital and the amplitude, frequency or phase of the carrier wave or signal is varied in discrete steps according to the content of the modulating signal.
Digitisation	The transformation of a continuously varying quantity (e.g. signal voltage) into a series of discrete signals in the form of numbers or other characters.
DCT	Discrete Cosine Transform. Used principally in digital video compression systems such as MPEG-2, which are designed to remove unimportant or irrelevant ("redundant") information from television pictures, thus reducing the amount of data to be conveyed to the receiver. Redundant information could be, for example, the static background of a scene in which only a single person or object is moving, which only needs to be sent to the receiver once. The DCT is one mathematical technique for identifying and removing this redundant data without unduly degrading the picture quality.
Downconversion	The process of converting the frequency of a signal to a lower frequency. Downconversion is performed at the reception point to permit the recovery of the original signal. See also Upconversion and Frequency Conversion.
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Emission	Radiation produced, or the production of radiation, by a radio transmitting station, which can be an earth station or a satellite.
Encoding (Coding)	The process of converting a message into a code that is designed to achieve a particular purpose (e.g. error detection and correction, bit rate reduction).
Encryption	The process of "locking" a signal using secret information so that it can only be deciphered by an authorised recipient who is in possession of the appropriate secret "key". This process is used in Conditional Access systems as a mechanism for controlling and managing subscribers to a particular service or range of services.
Energy Dispersal	The process of modifying a signal before it is modulated onto a carrier wave so that the energy of the modulated carrier signal is spread as evenly as possible over its bandwidth. The purpose of this process is to reduce the potential of the signal to interfere with other radio frequency signals.
Error Correction	The process of reconstructing digital information that has been corrupted in the data transmission process. There are two basic variants of error correction: FEC and ARQ. Error correction requires the detection of erroneous data based on observation of the received data (see Error Detection).
Error Detection	The process of detecting erroneous digital information after data recovery in the receiver. Erroneous information usually results from transmission errors. Error detection exploits the properties of a code applied to the data in the transmitter. See also Error Correction.
Eurocrypt	Conditional access system used mainly with the D2-MAC television transmission standard.
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FDM	Frequency Division Multiplex. A system in which signals are each allocated a unique portion of a shared frequency range. Each individual signal is modulated and translated in frequency so that it occupies the correct frequency segment of the composite signal spectrum and does not interfere with the other signals sharing the same band of frequencies. Individual signals are recovered from the composite signal by filtering. FDM is used, for example, to convey multiple television signals in a cable distribution system.
FDMA	Frequency Division Multiple Access. A method allowing multiple carriers to share a single satellite transponder or range of frequencies. The transponder bandwidth is divided into sub-channels, each of which is allocated to a particular earth station (carrier). The earth stations transmit continuously and the transponder conveys several carriers simultaneously at different frequencies.
FEC	Forward Error Correction. An error detection and correction technique based on the addition of a code to the signal at the transmitter. A decoder in the receiver detects and corrects errors making use of the properties of this code. The amount of coding information added to the original signal is quantified by the Code Rate.
FM	Frequency Modulation. One of the principal methods for transmitting information using radio waves by superimposing the information signal onto a radio frequency carrier wave. The frequency of the carrier wave is varied in accordance with the time-varying amplitude of the input signal. The amplitude of the carrier wave remains unchanged.
Frame (Television)	One complete TV picture, composed of two fields and a total of 525 and 625 scanning lines in NTSC and PAL systems, respectively.
Frequency Conversion	The process of altering the frequency of a signal so that it is suitable for transmission or other processing. See also Upconversion and Downconversion.
Frequency Translation	See Frequency Conversion.
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IF	Intermediate Frequency. In radio communication systems, frequency conversion from baseband to the transmission frequency, and from the reception frequency to baseband, is usually carried out in two or more stages. Any frequency obtained after a frequency conversion that does not correspond to the baseband, the transmission frequency or the reception frequency is known as an intermediate frequency. In satellite reception systems the term IF is often used to refer to the frequency range in which the LNB delivers the signals it receives from the satellite (950 - 2150 MHz) to the IRD.
Interlaced Scanning (TV)	The process of scanning a single image ("frame") of a moving picture sequence in two sequential stages, where each stage produces a scanned image ("field") comprising one-half of the total number of horizontal lines used in the scanning process. The lines of each field alternate. It is a technique that is used in conventional television systems (e.g. PAL) to reduce the transmission bandwidth by exploiting the properties of the human eye.
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JPEG	Joint Photographic Experts Group. A group established by the International Organization for Standardization (ISO) that developed a widely-used international standard for the coding of still pictures. The term "JPEG" is often used to refer to the coding method itself, which reduces the information needed to represent the picture with good quality.
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Luminance (Television)	The brightness information of a television picture. It is also used to refer to the brightness component (Y signal) of a PAL, SECAM or NTSC television signal.
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Microwave	The frequency range from approximately 1 to 300 GHz, covering the frequency range suitable for satellite communications.
Modulation	To superimpose the amplitude, frequency or phase of a wave or signal onto another wave or signal, which is then used to convey the original signal via a transmission medium (e.g. satellite link).
MPEG	Motion Pictures Experts Group. A group established by the International Organization for Standardization (ISO) that establishes international standards for compression coding of moving pictures and audio programmes. The MPEG-2 standard is widely used for compressing video material (e.g. in the DVB standard).
MPEG-2	A widely-used video compression standard. See MPEG.
Multiple Access	The simultaneous sharing of a common transmission bandwidth by multiple users. In satellite communications, it usually refers to the shared use of one or more transponders by multiple earth stations.
Multiplex	A signal that comprises multiple distinct signals or messages, usually for the purposes of transmission via a common communications channel.
Multiplexing	The use of a common communications channel for sending two or more messages or signals (e.g. multiple digital television programmes on a single digital carrier, or "multiplex"). Multiplexing is the process of combining multiple signals into a composite signal that is suitable for transmission via the common communications channel.
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NTSC (Television)	The National Television Standards Committee, which created the North American conventional television broadcasting standard. The standard itself is also referred to as NTSC.
Non-Linear	Refers to a device or process in which the output is not directly proportional to the input. Often used in the satellite communication context to refer to the (undesirable) characteristics of practical high power amplifiers.
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PAL (Television)	Phase Alternating Line. A European conventional colour television standard, which evolved from the American NTSC standard. The term "Phase Alternating Line" refers to the technique used to overcome the colour variations that can occur in the NTSC system.
Parabola	A geometric shape formed by the intersection of a cone by a plane parallel to its side.
Parabolic	Shaped like a parabola or paraboloid.
Parabolic Antenna	An antenna having a main reflector surface that is a paraboloid or is shaped like a paraboloid. It has the property of reflecting parallel incoming signals to a single focal point.
Paraboloid	A geometric surface whose sections parallel to two co-ordinate planes are parabolic and whose sections parallel to the third plane are either elliptical or hyperbolic.
Power	The rate at which electrical energy is fed into or taken from a device or system, expressed in watts or dBW. The signal strength on the uplink or downlink of a satellite communications system is quantified by the power of the radio wave radiated by the transmit antenna.
Power Flux Density	The signal power received over a surface area of one square metre, expressed in dBW/m2. Used to quantify the strength of a radio wave at the reception point of an earth-space link.
Pre-Emphasis	An artificial increase in the amplitude of the higher frequency portions of a baseband signal prior to frequency modulation and transmission via a radio link. Used in conjunction with the proper amount of de-emphasis at the receiver, this results in an improved signal-to-noise ratio for the demodulated FM signal. See also De-Emphasis.
Progressive Scanning	The process of scanning a single image ("frame") of a moving picture sequence progressively from top to bottom, producing a scanned image containing all of the horizontal lines used in the scanning process. Used in PC monitors and some advanced television systems. See also Interlace Scanning.
Propagation	To transmit in the form of a wave.
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Radiation	The outward flow of energy from any source in the form of radio waves.
Radio	The use of electromagnetic waves, lying in the radio frequency range, for communications purposes.
Radiocommunication	Telecommunication by means of radio waves.
RF	Radio Frequency. The 10 kHz to 300 GHz frequency range that can be used for wireless communication. The term RF is usually used to distinguish signals transmitted to and from the satellite from signals processed at other frequencies within the same communication system (e.g. intermediate frequencies).
Radio-Frequency Links	Communication links established by means of radio waves.
Radio Waves	Electromagnetic waves lying in the radio frequency range, propagated in space without artificial guide.
RGB	Red Green Blue. Primary colours that, when suitably combined, produce the same visual effect as almost any other colour. These primary colours are used in colour television systems, which reproduce colour images by controlling the intensity of red, green and blue light sources on the television screen.
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Scanning	The process of moving the electron beam in a television camera tube simultaneously in the horizontal and vertical directions so that an image is scanned from left to right and top to bottom. The electrical signal generated by this process is converted into an image on the television screen using the same scanning sequence.
Scrambling	A process that renders a signal unintelligible and/or randomises its content. It is used either to protect the content of the signal from unauthorised access, or for Energy Dispersal purposes.
Sparklies (Television)	The visual effect of impulsive noise arising at the output of an FM demodulator due to a weak signal, slight mistuning of the receiver or interference. The noise appears as randomly-distributed, momentary black and white flecks on the picture.
Subcarrier	Any signal carrying information that is transmitted within the bandwidth of another signal which itself modulates a carrier. Used in analogue TV transmission systems, for example, to convey colour and audio information.
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TDM	Time Division Multiplexing. A system in which the bits of more than one digital signal are interleaved in time to form a single digital bit stream that carriers all of the information contained in the original signals.
TDMA	Time Division Multiple Access. A method allowing multiple carriers to share a single satellite transponder or range of frequencies. The earth stations transmit sequentially in unique time slots at the same carrier frequency, so that only a single station transmits at any given time.
Telecommunications	The science and technology of communication by artificial means (radio, television, telephony, etc.).
Terrestrial Interference	Interference between a satellite system and entirely earth-based microwave communication systems.
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UHF	Ultrahigh Frequency. The frequency range from 300 MHz to 3 GHz.
Upconversion	The process of converting the frequency of a signal to a higher intermediate frequency or to the transmission frequency. Upconversion is performed at the transmission point to prepare the signal for transmission over the satellite link. See also Downconversion and Frequency Conversion.
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VHF	Very High Frequency. The frequency range from 30 MHz to 300 MHz.
Videocrypt	Proprietary conditional access system.
Videoguard	Proprietary conditional access system.
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Widescreen	A television picture or television screen that is wider than a conventional (4 by 3) television picture or screen, usually with an aspect ratio of 16 by 9.
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