- 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 will 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 13 transmitters on 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 metres).
Transmitter 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 - ten transmitters (200 watts to 2 kW).
1233 kHz - five transmitters (100 watts to 500 watts).
1242 kHz - four transmitters
(500 watts to 2 kW).
1260 kHz - two transmitters (500 watts and 1 kW).
Another example 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 used by BBC Radio 1 (275m and 285m) until the mid 1990s when Radio 1 moved
Consequently these two stations require a total of nine frequencies and 64 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.