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A
Acceptance Angle
Most domestic TV (and some FM) aerials are "directional" i.e. they receive signals better from one direction than another. Generally these aerials have their peak performance directly in front of them, however in situations where you are trying to exclude a reflected signal or include a signal from a second source you may need to know how the aerial performs at an angle. This measure is quoted as the Acceptance Angle, which is the angle from the centre line to the point where the signal falls to half power (-3dB) and is quoted in degrees. Note: the acceptance angle will vary at different frequencies within the range of the aerial. Acceptance Angle is normally quoted at peak gain.
Aerial
A device used to receive or transmit radio waves. Alternatively called
an antenna.
AM
Amplitude Modulation.
Amplifier
A device that increases the strength of a signal. This gain is usually
represented in dB.
Amplitude
The strength of a signal or the magnitude of a wave.
Analogue
A mode of transmission of information.
Antenna
See Aerial.
Array
An arrangements of elements on an aerial or an arrangement of aerials.
Aspect Ratio
The ratio of a screen width to height. Current standard 4:3. Widescreen
16:9.
Attenuator
A device that reduces the strength of a signal. This loss is usually
represented in dB.
B
Balun
Matches the impedance from the DIPOLE (~300 ohm) to the CABLE (75 ohm).
Bandpass Filter
A circuit or device which allows a limited range (or band) of
frequencies to pass whilst attenuating higher and lower frequencies.
Bandwidth
1. The range of frequencies occupied by a signal measured between half
power points e.g. an analogue UHF transmission has a bandwidth of 8 MHz
in the UK (6 MHz in the US).
2.The range of frequencies occupied by a number or group of signals.
3. The range of frequencies over which an aerial is designed to perform
e.g. "wideband" UHF = channels 21-69 = 470-860 MHz = 390 MHz.
Baseband
The band of frequencies containing the information before modulation, or
after demodulation, general term for audio and video signals in AV
links.
BER
Bit Error Ratio. A standardised measure of digital reception quality.
C
C/N
Abbreviation of Carrier to noise ratio.
CAM
Abbreviation of Conditional Access Module.
Carrier to noise ratio (C/N)
A comparison of the carrier level to the noise level accompanying it.
Channel
A frequency band of fixed width that provides a path for signals. The
UHF band has channels numbered from 21 to 68. This is not to be confused
with ‘channel’ meaning a television service. For instance, ‘Channel 5’
is actually transmitted on various ‘real’ UHF channels, including
channel 37.
Channelised
Equipment that processes each channel separately.
CI
Abbreviation of Common Interface.
Coax
Abbreviation of Co-axial.
Co-axial
Cable with an inner conductor and an outer screen.
Co-channel interference
Interference from another TV transmitter on the same channel. Usually
the interfering transmitter is distant, and in theory its signals should
not arrive at the receiving site in question. Unfortunately coverage
areas cannot be precisely defined.
Collection point
A strategically placed location in a building where a fairly large
number of downlead cables meet. At the collection point there will be
polarity switches, tap-off units, or amplifiers, to provide the signal
for all the downleads. Trunk cables will link a series of collection
points to the head-end or to a repeater. Also called a ‘node’.
Conditional Access Module (CAM)
Conditional Access Module, a module which connects to a CI slot and
allows access to encrypted channels.
Combiner
A circuit or device that allows two or more RF signals to combine and
travel along a single cable.
Common Interface (CI)
A standard interface which takes PCMCIA type cards.
Cross-modulation
If a masthead or distribution amplifier has too strong a signal at its
input, the output will be distorted. In the case of analogue TV
reception, various sorts of patterning might appear, or one channel
might be seen in the background of another. Digital reception might
suffer ‘blocking’ or complete loss of reception. The same faults can
occur if the input to a TV set or video recorder is too strong.
D
Digital Audio Broadcast
The digital broadcasting of CD-like quality radio programmes which can
also contain text, images and video.
DAB
Abbreviation of Digital Audio Broadcast.
Daisy chain
A long sequence of active components, for instance: masthead amplifier,
distribution amplifier, repeater, video recorder, satellite receiver,
set-back amplifier, TV set. Since every active component introduces a
small amount of additional signal degradation, such arrangements are
rarely satisfactory.
dB
Abbreviation of Decibel.
dBd
dBd refers to the antenna gain with respect to a reference dipole
antenna. A reference dipole antenna is defined to have 2.15 dBi of gain.
So converting between dBi and dBd is as simple as adding or subtracting
2.15 according to these formulas:
* dBi = dBd + 2.15
* dBd = dBi - 2.15
Specifying antenna gain in dBd means that the antenna in question has
the ability to focus the energy x dB more than a dipole.
dBi
dBi is the amount of focus applied by an antenna with respect to an
"Isotropic Radiator" (a dispersion pattern that radiates the energy
equally in all directions onto an imaginary sphere surrounding a point
source). Thus an antenna with 2.1 dBi of gain focuses the energy so that
some areas on an imaginary sphere surrounding the antenna will have 2.1
dB more signal strength than the strength of the strongest spot on the
sphere around an Isotropic Radiator.
Decibel (dB)
A convenient method of measuring an increase or decrease as a ratio,
regardless of the values involved. A logarithmic ratio usually used to
express the difference between two power levels. (dBs = 10log P2/P1)
Aerial gain is normally expressed as a ratio of the signal power
increase over a half-wave dipole (dB or dBd) or over an isotropic source
(dBi). (dBi = dBd + 2.1 approx.)
Digital
A transmission method that will eventually take over from analogue for
many applications.
Diplexer
A device that splits a collection of signals into two groups according
to the frequency range in which they are located, or combines two groups
of signals, each occupying a separate frequency range, into a single
collection of signals.
Dipole
The part of the aerial which is connected to the cable, sometimes
referred to as the "driven" element. The point onto which the signal is
focused by the other elements on the aerial.
Director
A shorter element in front of the dipole so called because it adds to
the directivity of the aerial. Adding directors increases the gain and
narrows the acceptance angle.
DiSEqC
Digital Satellite Equipment Control. A standardised method for two-way
communication between devices in satellite reception systems.
Information is exchanged between devices interconnected by standard
coaxial cable by means of a modulated 22 kHz tone. DiSEqC™ is a
trademark of Eutelsat
Distribution amplifier
An amplifier that provides the strong signals necessary to feed a
distribution system network.
Distribution network
The cables, tap-off units, trunk splitters, downleads, and outlet plates
that carry the signal from the head-end or repeaters to the TV sets.
Distribution system
A system that receives incoming TV and radio signals and supplies them
to a number of receivers.
Downlead
The final cable running to the wall outlet, usually from a tap-off unit.
The cable linking the aerial to the head-end.
DTT
Digital terrestrial television. Digital TV from an aerial. Marketed as
‘Freeview’.
DVB
The Digital Video Broadcasting Project, an industry-led consortium of
over 300 broadcasters, manufacturers, network operators, software
developers, regulatory bodies and others in over 40 countries committed
to designing global standards for the delivery of digital television and
data services.
E
Earth bonding
Secure, very low resistance, conductive path to earth.
EPG
Abbreviation of Electronic Program Guide.
Electronic Program Guide (EPG)
A guide showing programmes, which can be displayed on a conventional
television (via a STB) or iDTV as the now and next programmes or the
television schedule for a day or more at a time.
Equalisation
Signal loss on cable increases with frequency. To compensate for this an
equaliser, or slope filter, is used. This reduces the strength of the
lower frequencies. Equalisers are often fitted at an intermediate point
on a system, at a repeater.
F
FEC
Abbreviation of Forward Error Correction.
Filter
A device that passes signals within a nominated frequency or group of
frequencies and rejects others.
Flylead
The short cable that connects the TV set (or VCR) to the wall plate.
FM
A transmission system in which the modulating waveform is made to vary
the carrier frequency.
Forward Error Correction (FEC)
Is a system of error control for data transmission wherein the receiving
device has the capability to detect and correct fewer than a
predetermined number or fraction of bits or symbols corrupted by
transmission errors.
Forward Gain
The measure of an aerials ability to increase signal power in a given
direction expressed in dB (relative to a dipole or isotropic radiator).
Free To Air (FTA)
An unencrypted television signal.
Free To View (FTV)
An encrypted television signal which can be decoded using a FTV card
(free of charge). No subscription charge is required to receive a FTV
channel.
Frequency
The number of cycles (oscillations) per second, measured in Hz.
Front to Back Ratio
The measure of an aerials ability to reject signals from behind
(relative to Forward Gain).
FTA
Abbreviation of Free To Air.
FTV
Abbreviation of Free To View.
G
Gain
The amount an amplifier increases the signal that passes through it.
Ghosting
The appearance on the (analogue) TV screen of a secondary image,
normally to the right of the main image. This is caused by multipath
reception, in which TV signals are reflected back to the aerial from a
building or other large object.
H
Head-end
The collection of filters, amplifiers, power supplies, etc., that takes
in all the signals from aerials, satellite receivers, and other signal
sources, and sends them out to the system network.
I
Interference
An unwanted signal that disturbs reception.
Intermediate frequency (IF)
An intermediate frequency (IF) is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception.
Isolating
An isolating outlet plate includes components that prevent the passage
of high voltages.
L
Leveller
A device used to adjust the strength of individual channels or
multiplexes.
LNB
The electronics on a satellite dish. The unit at the front of the dish
that receives amplifies, and frequency-converts the incoming signals.
The LNB output is ‘satellite IF’
Loss
The amount by which a component or cable reduces the signal that passes
through it.
Low loss coax
The name given to the cables introduced in the 1960s for UHF
installations. Nowadays ‘low-loss’ is likely to mean domestic-quality
semi-airspaced cable with only a braid screen – quite unsuitable for
modern distribution systems.
M
Mast
The pole (usually aluminium), that supports the aerial.
Masthead amplifier
A small amplifier fitted at the aerial to increase signal levels.
Modulator
A device that converts baseband signals to RF. In other words, it
converts video and audio signals (possibly from a camera and microphone)
into a form that appears, to a TV set, to be a standard broadcast
channel. A modulator can be thought of as a very low powered TV
transmitter. Any analogue channel carried on a system, except analogue
terrestrial broadcasts, originates from a modulator.
Multipath
If the transmitted signal finds more than one route to the receiving
aerial—usually by being reflected from a building—various reception
faults can be caused. In the case of analogue TV, the visible effect is
called ‘ghosting’.
Multiplex
A group of digital signals transmitted within one channel. A terrestrial
digital TV multiplex usually carries approximately six programme
services. Digital satellite transmissions are also in multiplex form, as
is digital radio.
N
Noise
Undesired disturbance, received by an aerial or satellite dish, or
accidentally generated within a system. See S/N ratio.
O
Outlet
The wall plate (or wall socket) that provides the connection
point for the TV set.
P
Padded outlets
Wall plates designed to be connected with several in a series along a
cable. This was a technique used in the 1960s and early 1970s, but is
unsuitable for modern requirements.
Polarity switch
Takes the four outputs from a ‘four fixed output’ LNB and allows each
satellite receiver connected to select from the four. To each receiver
it appears that it has its own dish.
Pre-echo
In areas of high field strength, the TV set, and other components, can
pick up signal directly off air. This direct signal can be strong enough
to compete with that from the aerial system. This is pre-echo. The most
obvious symptom is a ghost image. Since the interfering signal is
received directly at the TV set it arrives before the aerial signal so
the shadow is to the left. The shadow can be very strong, resulting in a
double picture, which may pull sideways. Teletext and Nicam may be
garbled. Moving the TV set or its leads even slightly will have a great
effect on reception, because this affects the strength of the signal
which they pick up, and its phase relationship with the main signal.
R
RF
Radio frequency. All signals carried on a distribution system are RF, as
opposed to baseband.
Repeater
A collection of filters, amplifiers, power supplies, etc., situated on
the system network some distance from the head-end. The usual function
is simply to amplify (strengthen) the signals, but in some cases there
is also equalisation or other processing.
RSL
Restricted Service License. A TV or radio channel broadcast to a small
local area, either permanently or for a limited time. Transmissions are
usually low powered and the coverage area might or might not be the same
as a local relay station carrying the main channels.
S
Satellite IF
The signals that leave the LNB (on the dish) and feed the receiver
input. ‘IF’ is ‘intermediate frequency’ – the signals have had one
frequency conversion at the LNB and will be further converted in the
receiver.
Satellite receiver
The ‘set-top’ box. Includes the decoder.
Screening
Conductive material, normally earthed, that surrounds a signal conductor
and prevents radiation either to or from that conductor.
Set-back amplifier
A small DIY device fitted behind the TV set to increase signal strength
or provide signals for further receivers.
Signal
A waveform that conveys information, such as a TV picture.
S/N ratio
(Signal to noise ratio) The ratio between the wanted signal and the sum
of all the unwanted (interfering) signals.
Splitter
A device used to originate two or more signal feeds from one input.
Spur unit
A device that taps off a secondary feed from a trunk, of lower level
than the primary feed.
Stacking
A technique by which the outputs of two identical aerials are combined.
An increase in gain can be achieved. By exploiting the phase
relationship of the outputs the combined array can be used to
discriminate against interference, including ghosting.
T
Tap-off line
A large size co-ax cable that runs along a route convenient to feed a
large number of outlets, with tap-off units positioned strategically
along its length.
Tap-off unit
Fitted at intervals along a tap-off line, these units extract a small
percentage of the signal and feed it, via the downlead, to a wall
outlet. The bulk of the signal is passed on to the next tap-off unit,
or, in the case of the last unit on the line, is absorbed by a line
terminator.
Terrestrial
A television station that broadcasts from the ground, as opposed to from
a satellite.
Trunk cable
Links the head-end to a repeater. Usually a large size co-ax.
U
UHF
The frequency band used in the UK for terrestrial television
broadcasting, both analogue and digital. The UHF TV channels are
numbered 21 to 68.
V
VCR
Videocassette recorder, or ‘video’.
VHF
The frequency band used in the UK for terrestrial radio broadcasting,
both analogue and digital.
Video
(i) Used to identify a vision (as opposed to sound) baseband signal.
(ii) Abbreviation for videocassette recorder.
W
Wall plate
The fitting (usually found near a mains socket) that allows the
connection of the aerial signal to a TV set or other receiver.
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