D/A

Digital-Analog (Conversion).

DAC

Digital to Analog Conversion. Recovery of original analog signal from digital data.

DAMA

Demand-Assigned Multiple-Access. A spectrum-efficient method of dynamically allocating telephony channels in a transponder.

DAP

Double Amplitude Peak. Peak-to-Peak measurement (especially of a composite video signal)

dB

decibel. A means of expressing ratios logarithmically. Number of dB = 10 x log (base 10) of power ratio. Used to express power gain in amplifiers or power loss in passive circuits or cables.

dBi

dB antenna gain relative to an isotropic source.

dBM

dB power relative to one milliwatt.

DBS

Direct Broadcasting (by) Satellite to individual homes.

DBS/DTH

Direct broadcast satellite or direct to home television services. A form of broadcasting wherein high-powered satellites are used to broadcast a bouquet of channels directly to the homes of television viewers.

dBW

The ratio of the power to one Watt expressed in decibels.

dc Power Block

A device which stops the flow of dc power but per- rnits passage of higher frequency ac signals.

De-emphasis

A reduction of the higher frequency portions of an FM signal used to neutralize the effects of pre-emphasis. When combined with the correct level of pre-emphasis, it reduces over-all noise levels and therefore increases the signal-to-noise ratio.

Decibel (dB)

A unit to express differences of power level. Used to express power gain in amplifiers or power loss in passive circuits or cables.

Declination

The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam.

Delay

The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close on one-quarter of a second.

Dielectric

Any insulating material between two conductors which permits electrostatic attraction and repulsion to take place across it.

Declination Offset Angle

The adjustment angle of a polar mount between the polar axis and the plane of a satellite antenna used to aim at the geosynchronous arc. Declination increases from zero with latitude away from the equator.

Decoder

A circuit that restores a signal to its original form after it has been scrambled.

Decoding Margin

The extent by which SNR (or Eb/No) exceeds a value giving an unacceptable BER. (Also Eye Height)

Delta Modulation

Digital coding system based on the transmission of a change in value, rather than the absolute value of a sample. Exhibits a softer threshold than PCM.

Demodulation

The recovery of baseband information from a modulated carrier.

Demodulator

A device which extracts the baseband signal from the transmitted carrier wave.

Derived Circuit

The created circuits derived from the bearer circuit. The number of actual voice circuits being carried.

Detent Tuning

Tuning into a satellite channel by selecting a present resistance.

Deviation

The level of modulation of an FM signal - the extent by which the baseband or suboarrier signal shifts the main carrier frequency.

Dielectric Strength

The voltage which an insulation can withstand before breakdown occurs. Usually expressed as a voltage gradient (such as volts per mil).

Dielectric Constant (K)

The ratio of the capacitance of a condenser with dielectric between the electrodes to the capacitance when air is between the electrodes. Also called Permittivity and Specific Inductive Capacity.

Differential Gain/Phase

Video (color) non-linear distortion parameters.

Digital

Signals which only have two distinct conditions, corresponding to the "on" (1) and "off" (0) logic states.

Digital-to-Analog Converter

A circuit that converts digital signals into their equivalent analog form.

Digital/
digitalization

Conversion of information into bits of data for transmission through wire, fiber optic cable, satellite, or over air techniques. Method allows simultaneous transmission of voice, data or video. Data is represented by a specific sequence of off-on electrical pulses.

Direct Broadcast Satellite (DBS)

A term commonly used to describe Ku-band broadcasts via satellite directly to individual end-users. The DBS band ranges from 11.7 to 12.2 GHz.

Direct Current Resistance (D.C.R.)

The resistance offered by any circuit to the flow of direct current.

Discrimination

Measure of the ability of a component or system to separate wanted from unwanted signals, using some parameter such as polarization.

Discriminator

Type of FM demodulator.

Dish

Jargon for a parabolic microwave antenna.

Dispersion

The effect of group delay of a wideband transmission channel changing with frequency, such that different time delays are imparted to components of different frequencies. Most notice able with pulse signals. Not to be confused with Dispersal.

Dissipation Factor

The tangent of the loss angle of the insulating material. (Also referred to as loss tangent, tan &, and approximate power factor).

Distribution System

A communication system consisting of coax but occasionally of line-of-sight microwave links that carriers signals from the headened to end-users.

Domsat

Abbreviation for domestic communication satellite.

Downconversion

Translation of frequency or a block of frequencies to a lower portion of the electromagnetic spectrum, e.g. from SHF (microwavwe) frequencies to UHF or VHF.

Downconverter

A circuit that lowers the high frequency signal to a lower, intermediate range. There are three distinct types of down conversion used in satellite receivers: single down conversion; dual down conversion; and block down conversion.

Downlink

The space-to-Earth half of a two-way telecommunications satellite link.

Downlink Antenna

The antenna onboard a satellite which relays signals back to earth.

DPCM

Differential Pulse - Code Modulation

DPSK

Differential Phase - Shift Keying

Drain Wire

In a cable, the uninsulated wire laid over the component or components and sued a ground connection.

Drifting

An instability in a present voltage, frequency or other electronic circuit parameter.

DRO (or DSO)

Dielectric - Resonator (or Dielectrically-Stabilized) Oscillator. Highly stable oscillator circuit used in LNBs and BDCS.

DSI

Digital Speech Interpolation. A spectrum efficient mode for telephony transmission that take advant of the pauses that occur in conversations to insert additional signals for transmission.

Dual spin

Spacecraft configuration in which the main body spins to provide attitude stabilization, and antenna assembly (and often some of the electronics) are de-spun by means of a motor and bearing system point continuously towards the earth.

Dual-Band Feedborn

A freedhorn which can simultaneously receive two different bands, typically the C and Ku-bands.