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DISTRIBUTION & DELIVERY Introduction to FM Single Frequency Networks by Werner Drews, 2wcom S ingle Frequency Networks (SFNs) are a form of broadcast network design where several transmitters use the same frequency channel to send the same signal. SFNs are usually used for radio and digital TV broadcasting – FM, AM and digital radio can be transmitted over SFNs – while analog television is more problematic because the multiple signal sources tend to produce ghosting. Why would a broadcaster choose this technique? There are several potential advantages. One is the greater efficiency that SFN can provide: a broadcaster’s available spectrum can be packed with more radio and TV channels than would be possible with a multi- frequency topology. SFNs are also a way of improving coverage area and signal strength, especially in positions that are more or less equidistant between transmitters, where interruptions to reception might otherwise occur. Another particular reason for choosing SFNs is to overcome difficult geographical conditions that interfere with signal delivery. These conditions can occur naturally in the case of sparsely populated areas where transmitters are widely spaced and the terrain is mountainous, but good coverage can also be difficult to obtain in urban areas where densely built-up high-rise development creates a lot of shadowing. It’s a useful technique for broadcasters to resort to, both when implementing a network infrastructure 44 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 103 JULY 2015 or – especially in urban applications – when upgrading an existing network to improve coverage and reception quality. But of course there are technical challenges to be met when creating a single frequency network. One of these is the problem of interference between signals reaching the same receiver from more than one transmitter, which can cause fading in DVB-T and DAB networks. Another key requirement for successfully operating SFNs is highly accurate timing. GPS can be used to arrange phase and frequency co-ordination between the transmitters by providing µs-accurate timing, which is within the parameters for most DVB-T SFN architectures. It’s important that the contribution is timed correctly too, so that the variance in timing between the signal leaving the origination site and reaching each transmitter is taken into account and compensated for. Each transmitter needs to wait until the signal has been received by all of the other before transmission can begin.