Glossary Terms Beginning D

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DAB: Digital audio broadcasting.
DAMA: Demand-assigned multiple access: highly efficient means of instantaneously assigning channels in a transponder according to immediate traffic demands.
Data communication: Transfer of digital data by electronic or electrical means.
dB: Decibel: logarithmic unit of measurement of electronic signals.
dBA: Measure of sound pressure level.
dBi: Power relative to an isotropic source expressed in decibels; the amplification factor (‘gain’) of an antenna with reference to the gain of a theoretical ‘isotropic’ antenna (the ‘i’ in dBi).
DBS: Direct broadcasting by satellite (also DTH): service that uses satellites to broadcast multiple channels of television programming direct-to-home to small consumer antennas.
dBW: Measurement of the ratio of the power to 1 W (0 dBW) expressed in decibels.
DC: Direct current.
DCE: Data communications equipment; equivalent to the TA (Terminal Adapter) in ISDN.
Decibel: dB: expression of ratios of two power levels in logarithmic form, to express either a gain or loss in power between the input and output devices.
Decoder: Device that reconstructs an encrypted signal so that it can be clearly received.
Decompression: Process of decoding information and reconstructing the data that was removed in the compression process. Both the compression and decompression processes are often referred to together overall as ‘compression’.
Dedicated: Satellite capacity committed for a particular purpose, e.g. DTH, business data services, occasional video services.
De-emphasis: Reinstatement of a uniform baseband frequency response following demodulation; the inverse of pre-emphasis.
Delay: Time taken for a signal from the uplink to travel through the satellite to the downlink; transmission delay for a single ‘hop’ satellite connection is approximately one-quarter of a second.
Demodulation; demodulator: The inverse process of modulation to recreate the original baseband message signal; the device which extracts or demodulates the ‘wanted’ signals from the received carrier.
Demultiplex: Process of recovering the individual messages from a single communications channel that carries several different messages simultaneously.
De-orbit: Process of removing a satellite from its geostationary orbit; see Graveyard orbit.
Deviation: Frequency range over which the carrier is altered to carry the information signal.
Differential coding: Simplest form of compression, sending only the data that describes the difference between two frames.
Digital: Information expressed as data; signal state expressed as on (1) and off (0) states; a binary signal.
Digitizing: Process of converting a signal to digital form.
Discrete cosine transform: Specific implementation of a mathematical process called a Fourier transform, involving trigonometry; the intra-frame coding technique that takes advantage of spatial redundancy, representing a sequence of numbers as a waveform.
Dish: Colloquial term for a satellite antenna; also generic term for an SNG uplink operation.
Dolby: AC-3 Proprietary multi-channel perceptual digital audio coding technique providing five full audio range channels in three front channels (left, centre and right) plus two surround channels, used for HDTV transmissions in the US.
Downlink: Ground station for receiving satellite communications; communications satellite link to Earth.
Drive: Information signal applied to an HPA or upconverter.
DTE: Data terminal equipment; for example, an audio codec.
DTH: Direct-to-home satellite broadcasts, also called direct broadcasting by satellite, where the satellite broadcasts a very high power so that signals can be received by small consumer antennas.
DTMF: Dual-tone multi-frequency: in-band telephone signalling commonly known as ‘touch-tone’.
Dual path: Ability to simultaneously transmit two signals from one uplink system to a single satellite.
Dual polarity: Ability to transmit or receive on horizontal and vertical polarization planes simultaneously.
Dummy load: Device that will absorb all the power from an HPA for test or standby purposes.
Duplex: Simultaneous two-way signal; a mode in which there exists two-way communication between the users.
DVB: Digital Video Broadcasting: the European-backed project to harmonize standards of digital video.
DVB-S: Digital Video Broadcasting - Satellite.