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Since the start of digital terrestrial television in the UK, we have mainly designed our transmitters to operate in a multi-frequency network (MFN). Now, the COM7 and COM8 multiplexes are moving to single frequency networks (SFNs) on channels 55 and 56, so I thought it might be timely to explain what the main differences between MFNs and SFNs are. The UHF band that we use for terrestrial TV signals has many desirable properties, but one of the least helpful is its tendency for signals to bounce off structures. If you have been in the industry long enough to have done much with analogue television, then you may remember ghosting on pictures, and pre-echoes on MATV systems. These were common problems in the days of analogue TV, but seem to have gone away now that over-the-air TV is all digital. The problems arose because there existed more than one path from the transmitter to the aerial, and this is known as multipath. In the case of pre-echoes, the intended path was via the aerial on the roof and the cabling of the MATV system, but often there was an unintended path straight from the transmitter to the leaky fly lead at the back of the TV. The unintended path was shorter than the intended path, and in areas close to transmitters, the fly lead would let in enough signal that the TV would show a faint duplicate image to the left of the wanted image. The reason for not hearing so much about multipath these days is that the designers of the first DTT standard, DVB-T, recognised that multipath to one degree or another is a widespread phenomenon, and did something about it. This was explained in more detail in a previous article (Some Reflections on Reflections, Feedback, Autumn 2016), but to cut a long story short, it resulted in a property of the DVB-T signal called the Guard Interval. Provided that the difference in arrival times of multiple path components at the receiver do not exceed the Guard Interval, it is possible to design the receiver to be more or less immune to multipath. The Guard Interval made reception of DVB-T robust in the presence of multipath, but the designers of DVB-T had more to think about: in particular, for commercial reasons they wanted each multiplex, which occupies a single 8MHz channel, to carry around 25MBit/s of data that could be used for services. This means that the type of signal they had to use requires a C/N or MER in the region of 20dB. Normally if a transmitter in an adjacent area uses the same channel as the wanted signal, any signal power MFNs andSFNs Peter Barnett, Mandercom Ltd Reflections give rise to multiple paths of different lengths between the transmitter and the receiving aerial 38 CAI Feedback Spring 2018