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").
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
Interference received in one communication channel from signals conveyed by other communication channels.
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.
The process of restoring a coded signal to its original form based on knowledge of the encoding process.
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.
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.
The extraction of multiple distinct messages or signals from a single composite signal ("multiplex").
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.
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.
Radiation produced, or the production of radiation, by a radio transmitting station, which can be an earth station or a satellite.
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).
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.
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).
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.
Conditional access system used mainly with the D2-MAC television transmission standard.
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.
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.
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.
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.
One complete TV picture, composed of two fields and a total of 525 and 625 scanning lines in NTSC and PAL systems, respectively.
The process of altering the frequency of a signal so that it is suitable for transmission or other processing. See also Upconversion and Downconversion.
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.
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.
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.
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.
The frequency range from approximately 1 to 300 GHz, covering the frequency range suitable for satellite communications.
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).
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).
A widely-used video compression standard. See MPEG.
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.
A signal that comprises multiple distinct signals or messages, usually for the purposes of transmission via a common communications channel.
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.