Introduction

The DAB system was designed to improve upon FM reception, particularly in cars. However, the DAB transmitter network is still under construction, offering 85% coverage of BBC national radio by the end of 2004, as opposed to 98% coverage on FM. Therefore, whether or not DAB will improve your radio reception depends on where you live.

There are two potential problems with FM radio reception - weak signal and multipath. The FM system was designed in the 1950s for reception on a directional roof aerial in mono. Stereo and portable reception were later developments. Although new transmitters have been added and the older transmitters converted from horizontal to mixed polarisation to improve in-car and portable reception, the signal is still weak in many places. The 98% coverage figure applies to stereo reception with a roof aerial. A weak FM signal manifests as background hiss, particularly in stereo mode. Switching to mono eliminates the hiss. Car radios automatically switch to mono as the signal strength degrades and also filter out the higher frequency parts of the audio to reduce hiss. Whereas FM signals fade gracefully with distance, DAB either gives perfect sound or doesn't work at all, bar making a 'bubbling mud' noise at the margins. Where a DAB and FM signal are transmitted from the same transmitter with equivalent coverage areas, DAB will give solid reception where FM is a little hissy in stereo. Further out, FM will still give a listenable mono signal where DAB does not work at all. Because DAB broadcasts at about twice the frequency of FM, it does not diffract as well around local obstacles, such as hills and tall obstacles. Thus, DAB is more susceptible to local holes in reception. In many areas, this problem is addressed by using more than one transmitter to serve a given area.

Multipath interference occurs when signals of similar strength reach the receiver via different paths from the transmitter. This is caused by reflections of tall buildings and hills and is a particular problem where the line of sight signal is weak. Multipath interference is most likely to occur in valleys and in cities with tall buildings. On FM, multipath manifests as sound distortion, particularly in stereo and at higher audio frequencies. RDS car radios may not re-tune where mulipath is encountered as the signal strength may remain high, even though the sound is poor. DAB is designed to be unaffected by multipath; signals reaching the receiver by diffferent paths reinforce each other instead of interfering. So, if your FM reception suffers from multipath interference, there is a good chance that switching to DAB will solve the problem.

At the moment, DAB offers a solution to poor FM reception in some places, but not others. Below, a guide is presented as to where DAB is and is not likely to give better reception. However, in the longer term, DAB has the major advantage that transmitters carrying the same programmes can use the same frequency. This means that reception problems can always be solved by building extra transmitters, whereas on FM, there may not be enough frequencies available to do this.

National radio

Because DAB operates at higher frequencies than FM, the maximum coverage area of a high power transmitter is smaller. Consequently, there are more main DAB transmitters than main FM transmitters, giving areas where the DAB signal is significantly stonger. DAB reception is also better in some major cities as DAB does not suffer from multipath distortion and many cities are served by more DAB than FM transmitters, giving fewer localised pockets of poor reception. On the other hand many areas have no national DAB service at all.

• London
• Hertfordshire and Bedfordshire
• North West Sussex and East Hampshire
• North Hampshire
• Dorchester

• West Wiltshire
• East Devon
• Shrewsbury and Telford
• Birmingham and Wolverhampton
• Northamptonshire
• South Warwickshire

• East Leicestershire
• SE Cambridgeshire, SW Suffolk and NW Essex
• Greater Manchester
• Merseyside
• Penrith

• Parts of the Norfolk and Suffolk coast
• South Hams, Devon
• Upper Welsh valleys
• Powys and SW Shropshire
• North Derbyshire

• Bridlington, Scarborough and Whitby area
• Rural North Pennines
• Isle of Man
• Scottish Borders

• Much of Dumfies and Galloway
• North West Scotland, Argyll and Scottish Islands
• North East and Southern Northern Ireland

Coverage of national commercial DAB is similar to that of the BBC, but does not serve Northern Ireland. DAB is more likely to improve reception of Classic FM as it has less extensive FM coverage than the BBC networks.

Local and regional radio

In most cases, local radio coverage on DAB is no more extensive than on FM. DAB is likely to give better reception where there is multipath, but not always where the signal is weak.

• London stations in Guildford, Reigate, Crawley, Luton, West Herts and West Kent
• Essex FM in Harlow and North Essex
• BBC Essex in Harlow
• Kiss 101 in Swansea, Bath and Welsh Valleys

• Galaxy 102.2, Heart 100.7, Smooth Radio and Kerrang in the West Midlands
• Smooth Radio and Century 105.4 in Wrexham, Burnley, Lancaster and Burnley
• Key 103 in East Cheshire
• Galaxy 102 in the North West

• Galaxy 105 in parts of North Yorkshire
• Real Radio in North and East Yorkshire and North Lincolnshire
• Downtown Radio and Cool FM in Northern Ireland