- Single Frequency Network -
Single Frequency Network
Using COFDM modulation enables a Single Frequency Network (SFN) to be created where many transmitters are
broadcasting on the same frequency and it does not matter if individual transmitter coverage area overlaps.
This is not possible with
analogue transmissions as in areas that receive signals from different transmitters broadcasting on the same frequency there is considerable
For SFN to work each transmitter must be broadcasting exactly the same data, on exactly the same frequency and
all transmitters must broadcast at the same time. The cyclic prefix, or guard interval, (see COFDM page) slightly reduces the effective
data throughput as this duplicates data already present but the result is a robust signal that is immune to data errors caused by
SFN could have a profound effect on the medium wave bandplan as in the UK and Europe there are many AM ‘synchronised
networks’ where several frequencies are needed so that a radio station can have national or regional coverage.
Here are just two examples
of AM synchronised networks, but there are many more -
In the UK commercial rock music station Absolute Radio (formerly Virgin Radio)
has a total of 21 transmitter with 10 transmitters on the primary frequency of 1215 kHz (for older surfers this frequency and
transmitter network was used to broadcast BBC Radio 1 on 247 metres and then BBC Radio 3 when Radio 1 moved to 275/285 m).
power ranges from a few hundred watts to a powerful 200 kW (Moorside Edge, West Yorkshire), this transmitter provides the main coverage
along with three 100 kW transmitters. To achieve national coverage, however, requires additional transmitters to cover areas where
reception of the main frequency is poor or non-existent.
1197 kHz - five transmitters (200 watts to 2 kW).
1233 kHz -two transmitters
(100 watts to 500 watts).
1242 kHz - three transmitters (500 watts to 2 kW).
1260 kHz - one transmitter (1 kW).
of a synchronised network is commercial radio station talkSPORT (the station’s name describes the format). To get national coverage
talkSPORT requires -
1053 kHz - twelve transmitters (including 500 kW Droitwich).
1071 kHz - two 1 kW transmitters.
1089 kHz - seven
transmitters (including two 400 kW).
1107 kHz - six transmitters (500 watt to 2 kW).
These are the frequencies previously used by
BBC Radio 1 (275m and 285m) until the mid 1990s when Radio 1 moved to VHF-FM.
Consequently these two stations require a total of nine
frequencies and 41 AM transmitters to provide national coverage and in some areas reception is possible on more than one frequency.
This makes any analogue synchronised network wasteful in terms of spectrum usage and the number of transmitters needed.
DRM only requires
one frequency and as many transmitters as needed to achieve any desired broadcast area. This makes DRM much more efficient in terms
of radio spectrum use.