- 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 mutual interference.

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 multi-path reception.

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).

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 - 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).

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 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.

DRM background
SDC data 1
DRM future
DRM audio
FAC sync
DRM multiplex
DRM modes
SDC data 2