Glossary of digital broadcasting


Amplitude modulation (AM) Varying the height, or amplitude, of the carrier wave to carry the programme data. It is used for analogue broadcasting.

Azimuth This is the compass setting of a satellite, in other words the direction in the horizontal plane. You need to know the azimuth of a satellite from your location so you can aim the dish at it. See also elevation, declination and polarisation.

BER (Bit Error Rate) This is how many bits are received before there is an error bit. Ideally at the LNB this should be 2 x 10^5 to 6 x 10^6. 10^2 will cause stop-start pictures, 10^3 ranges from very dubious to acceptable and 10^4 is solid. Note that 10^2 means '10 to the power of two' or 100. BER is displayed as 'signal quality' on some receivers.

Boot Flexible cover for F connectors that prevents water getting in.

CAM (Conditional Access Module) Device, similar to a laptop PCMCIA card, that plugs into a receiver to allow viewing of encrypted programmes.

Cascade multiswitch See multiswitch

CB (Corrected Bits) How many bits were corrected by Viterbi, as shown in a meter readout. See also UCB

Constellation Diagram Diagram used to represent PSK, plotting I against Q that allows you to see quality of the transmission. See IQ for more information. The complete phase cycle is divided into four. For QPSK there will be one symbol in each quadrant. The sharper the dot, the lower the noise and error rate. If there is a cloud then there are errors. Gain compression causes the dot to move towards the origin.

c/n ratio (Carrier to Noise ratio) Must be at least 8 dB and ideally better than 10dB. It strongly affects BER and depends on signal strength above LNB noise and polarity setting. This ratio is the most important. The signal is usually much higher than needed. As it goes down in the distribution cables it is the noise that will be critical.

CT100 High quality satellite cable made by Webro/Belkin with hollow (air-spaced) dielectric. (Signal loss per 100m: 20 dB at 950 MHz and 30 dB at 2050 MHz).

CT125 High quality satellite cable made by Webro/Belkin, with hollow (air-spaced) dielectric, used for runs of cable of 30m or more due to its lower signal loss ( Signal loss per 100m: 17 dB at 950 MHz and 25 dB at 2050 MHz)

CTF100/WF100 High quality satellite cable made by Webro/Belkin with a foam dielectric, used when sharp bends are likely. It has similar signal loss to CT100.

Declination This is the angular difference between magnetic North as reported by a compass and true North. It varies with your location on the earth but these values change slowly as the magnetic poles of the Earth shift. You need this value to be able to successfully point a satellite dish at the correct azimuth setting.

dB (Decibel) This is a scale that shows how big one voltage, power, loudness etc is compared with another. For satellite use we are interested in voltage. Most are compared with 1 microvolt (1 uV). 0dBu means they are the same, that is the measured voltage is 1uV. A positive value, e.g. 12dB, means that the voltage is bigger than the one you are comparing it with. A negative value, e.g. -32dB, means it is smaller. Here are some useful values: e.g. 60dBu means a voltage of 1 mV, that is 1000 times more than 1 microvolt

How many times bigger

Digital cliff Point at which error correction is overwhelmed and the picture disappears or locks.

DiSEqC (Digital Satellite Equipment Control) Pronounced DyeSeck Allows control of motors and LNBs.

DVB (Digital Video Broadcasting) See also DVB-C, DVB-S and DVB-T.

DVB-C (Digital Video Broadcasting by Cable) For example Virgin Media

DVB-S (Digital Video Broadcasting from Satellite) For example freesat.

DVB-T (Digital Video Broadcasting - Terrestrial) Digital broadcasting from ground-based aerials, for example freeview.

Earth bonding Metal strip holding F connectors allowing all of the connector and cable screens to be connected together and to earth. It reduces interference and improves safety.

Elevation This is the angle of a satellite above the horizon. You need to know the elevation of a satellite from your location so you can aim the dish at it.

Endpoint connection Cable to a receiver or socket from an LNB, a multiswitch or a tap box.

FEC (Forward Error Correction) Data is added to the signal to allow data errors to be corrected. It is repair data and has nothing to do with pictures or sound. In perfect conditions it is not needed. FEC is shown as a ratio of used data to total data e.g. 7/8 where seven bits are used picture or sound data and 1 is for FEC. Common values are 2/3, 3/4 and 4/5. It is also known as the “Viterbi Rate”. This is because Viterbi and Reed Solomon correction is used.

Frequency modulation (FM) Varying the frequency of the carrier wave to carry the programme data. This is used for VHF/FM radio and other analogue broadcasting.

FTA (Free To Air) Digital broadcasts that are encrypted but decoders do not have a subscription fee.

FTV (Free To View) Digital broadcasts that are not encrypted.

IF (Intermediate Frequency) The LNB converts the two satellite transmission bands of 10.70 to 11.9 GHz and 11.55 to 12.75 GHz to 950 to 2150 MHz to allow it to go down cables.

IQ The two signals used in QPSK. I is the inphase signal and Q the quadrature one which is 90 degrees out of phase with the I.

IQ screen Another name for Constellation Diagram.

LNB (Low Noise Block) This picks up the signal and is at a focus of the dish. It has a feedhorn that collects the signal and an amplifier that makes it larger and down-converts it to IF.

Modulation Varying something about the carrier wave to allow it to carry the programme data. Examples of modulation are amplitude, frequency and phase.

Modulation Error Vector This number indicates quality for digital transmissions. It is the sum of all signal degradations such as noise and modulation errors. It can be seen as displacements on the constellation diagram.

Multiswitch Device to allow many satellite receivers to connect to one quatro LNB. Each output, called an Endpoint Connection, can then connect to any of the four band/polarisations. Cascade multiswitches have four trunk outputs to go to another multiswitch.

NID (Network IDentifier) The Network Identifier identifies a certain Network Provider. It allows a receiver to search for just those channels provided by that network provider.
s/n ratio see c/n ratio

Phase modulation Modulation, used for digital information, where the phase of the carrier wave is changed, and each size of change represents one digital code of two, three or more bits.

Polarisation The rotation of LNB in its clamp to correct for curvature of the earth. It is also known as skew adjustment. Accurate setting is essential to prevent the signal whose polarisation is at right-angles from interfering with the chosen signal.

Polarity trunks Four cables going from a quatro LNB to a multiswitch, or from a cascade multiswitch to the next one. Each cable carries one of the four band/polarisation combinations.

PSK (Phase Shift Keying) A phase modulation method which allows more than one bit to be transmitted for each physical change. In PSK a carrier wave is made to jump forward by a fraction of the cycle. For example this might be 45, 135, 225, 315 degrees. In this case four possible digital codes can be represented: 00, 01, 10, 11. The use of eight different jumps, or states, allows three bits per phase jump. These are also called constellation points when they are displayed on a diagram showing the complete cycle. Common uses are QPSK and 8PSK.

QPSK (Quadrature Phase Shift Keying) PSK with four different jumps or states commonly used for satellite TV transmissions.

Quad LNB Universal LNB with four outputs.

Quatro LNB LNB with four outputs each of which is fixed to one of the frequency/polarisation options. It is used to feed a multiswitch through polarity trunks.

Rain fade 6 dB, or greater, loss in signal in heavy rain or snow, that can result in the signal being too poor to work.

Reed-Solomon (RS) An algorithm for Forward Error Correction (FEC). It was devised by Irving S. Reed and Gustave Solomon of MIT Labs in 1960.

SID (Service IDentifier) This is used by the receiver to find a certain service within a transmission.

Signal quality See BER

Spot-beam Most satellites transmit the same signals over a single wide area. Now a satellite can have several narrower focussed beams, called spot-beams. These enable the same frequency to be used several times for different data, so reducing the cost.

SR (Symbol rate) The symbol rate is the rate of state changes on a communications circuit, typically 22000 or 27500. It is similar to baud rate on a modem.

Tap box Box which allows a series of endpoint connections to tap off a fixed amount of the trunk signal in a distribution system.

Terminator 75 ohm resistor in an F connector, plugged into unused sockets and cable ends. This absorbs the signal and prevents it reflecting back and causing data corruption.

TID ( Transponder IDentifier) This is a unique identifier for a certain transponder within a given network (see NID). It enables the receiver to search for only those channels on a particular transponder.

TP (TransPonder) From transmitter/responder. A satellite transponder receives signals from the earth on one frequency and transmits signals back to the earth on another.
Trunks See polarity trunk

UCB (UnCorrected Bits) How many bits could not be corrected by Viterbi, as shown in a meter readout. See also CB

Universal LNB LNB that can set by the receiver to any combination of the two frequency bands (0/22 kHz) and polarisations (13/18V) for a satellite.

Viterbi The Viterbi algorithm, which selects the most likely option, was conceived by Andrew Viterbi as an error-correction scheme for noisy digital communication links. It is used after the QPSK decoder with RS.

VPID (Video Programme Identifier) This identifies the video component of a broadcast. Radio broadcasts have a VPID value of 8191 meaning empty.

8PSK Version of PSK where there are eight possible phase changes, so three bits can be sent per phase change. It is used for high definition digital broadcasting but is less tolerant of poor signal quality. 16PSK (4 bits) and 32PSK (5bits) can also be used.

16PSK See 8PSK

32PSK See 8PSK


(C) Peter Scott 2009

Last edit 26 December 2015