What is a single frequency network (SFN)?

What is
a single
frequency
network
(SFN)?
Australia has switched to digital-only TV.
This means that analog TV signals have
been completely switched off and TV is
now broadcast in digital-only.
To watch digital-only TV, your TV needs to be
able to receive digital signals. You can choose
to connect a digital set-top box or digital video
recorder to your existing analog TV, or use a
digital TV, which has an in-built tuner.
Single frequency networks (SFNs) are being
used as part of the transmission arrangments
for some digital terrestrial TV services
particularly in areas where there is limited
channel capacity. An SFN is a broadcast
network where several transmitters
simultaneously transmit the same signal over
the same frequency channel. This varies from
a traditional transmission network, or multi
frequency network (MFN), where each transmitter
operates on a separate frequency channel.
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What is a Single Frequency Network (SFN)?
Operation of an SFN
Diagram 1, uses the broadcasting of ABC
services to illustrate the difference between
a multi frequency network and single
frequency network. In the example of an
MFN, the blue area is where the ABC is
broadcast on RF channel 35, the green area
is where the ABC service is broadcast on RF
channel 45 and the red area is where the
ABC is broadcast on RF channel 67.
channels, they would cause interference to
each other and most people in both coverage
areas would be unable to get good reception
from either site (though they would get good
field strengths).
A single frequency network is different.
When a network is designed to operate on
the same frequencies, each site is “locked
in” to the timing of the others, so network
In the area of overlap a viewer can tune into
designers make sure that the difference in
Between 2010 and 2013 the analog TV signals are being switched off and replaced with digital-only
any of the available RF channels for their
the time each signal arrives at each house
signals. Single Frequency Networks (SFNs) are being used with the rollout of digital terrestrial
ABC service.
is within
a defined
known as the guard
television
services particularly in areas where there
is limited
channellimit,
capacity.
interval. Provided the weaker signal arrives
An
is a broadcast
where
several transmitters simultaneously transmit the same signal
In SFN
an analog
network,network
or a digital
network
within the guard interval then receivers
over
the same frequency
This
varies from a traditional transmission network, or Multi
not designed
as an SFN,channel.
if the red
and
such
as set-top
boxes, should
successfully
Frequency
Network
(MFN),
where
operates
on a separate
frequency
channel.
green coverage
areas
were
giveneach
the transmitter
same
operate in an SFN.
What is a Single Frequency Network (SFN)?
Diagram 1, example of ABC services broadcast on a Multi Frequency Network (MFN)
and Single Frequency Network (SFN).
Single
frequency
network
Multi
frequency
network
CH 67
CH 45
CH 45
CH 45
CH 35
CH 45
Operation of an SFN
Why do broadcasters use an SFN?
Diagram 1, uses the broadcasting of ABC services to illustrate the difference between a multi
frequency network and single frequency network. In the example of an MFN, the blue area
isInwhere
ABCbroadcast
is broadcast
on RF channel
green area
is where
service
is
most the
cases,
frequencies
are 35, the
However,
unlike
analogthe
TVABC
where
ghosting
broadcast on RF channel 45 and the red area is where the ABC is broadcast on RF channel 67.
only re-used when there is a considerable
and other interference would be experienced
In
the areabetween
of overlapthe
a viewer
can tune into any ofifthe
available
RF channels
for their
service.
distance
transmitters.
nearby
transmitters
operated
on ABC
the same
In
an same
analoghigh
network,
a digital network
not designed
as anchannel,
SFN, if the
red reception
and greenproblems
coverage
The
powerorfrequencies
usually
frequency
these
areas
were
given
the
same
channels,
they
would
cause
interference
to
each
other
and
most
used in Sydney cannot be used again
are not experienced where digital TV is
people
in both coverage
areasCanberra
would be or
unable to provided
get good reception
from either site (though they
in Newcastle,
Wollongong,
using an SFN.
would get good field strengths).
anywhere in between.
A single frequency network is different. When a network is designed to operate on the same
frequencies, each site is “locked in” to the timing of the others, so network designers make sure
Factsheet
updated: March
2014time each signal arrives at each house is within a defined limit, known
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that
the difference
in the
as the guard interval. Provided the weaker signal arrives within the guard interval then receivers
such as set-top boxes, should successfully operate in an SFN.
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What is a Single Frequency Network (SFN)?
What will you notice as an installer?
In an SFN you will be able to pick up services The MER, field strength and noise figure
from each of the transmitters that potentially should be the same as usual; at the wall
serve the area. If you have test equipment
plate they should read:
that shows an echo plot, you will be able to
• MER ≥ 25 dB
see each separate service at some delayed
interval. The guard interval is not necessarily • field strength ≥ 45 dBμV/m and
the same for every service around Australia
≤ 80 dBμV/m
so as a general rule of thumb, the two
signals (when there are three or more
transmitters) should not be more than
64μsec apart.
Useful tips
Most of the time you won’t find reception
in an SFN differs much from a standard
installation. Following best practice
installation guidelines will still solve most
reception issues. You can:
• minimise use of masthead amplifiers
or boosters – in an SFN they can be
problematic
• use good quality antennas and cables;
removing additional sources of interference
can help
• re-point and/or reposition the antenna;
sometimes you will have a better result by
pointing to an alternative transmitter in the
SFN. You may also need to look at providing
some shielding from other transmitters,
perhaps by lowering the antenna and
using the roof of the house to block one
of the signals, or taking advantage of any
coverage from trees and other sources of
signal obstruction, and
• clear the memory of the digital TV receiver
to make sure that you have no stored
channels and then manually tune into the
required frequencies; this ensures you
don’t have an alternative service in the
main logical channel number set.
For more information
To find out more about the switch to digital TV and for further details on these and other
solutions, call the Digital Ready Information Line on 1800 20 10 13.
Factsheet updated: March 2014
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