DAB Digital Radio, what's really going on?

Although MW and LW signals can suffer from various forms of interference based on addition and cancellation, multipath as such doesn't afflict them -- in essence because they use wavelengths which are much greater than those of FM radio. Their main limitation is what's known as restricted bandwidth. Imagine the radio signal as something solid being sent down a pipe. In the case of MW and LW (also collectively known as the AM bands) the diameter of the pipe isn't very large and so the signal has to be trimmed around the edges so that it can pass through. In real terms this means the high and low frequencies of the music are lost, leaving a relatively low-fi sound that's acceptable for speech but poor when reproducing music. In this respect FM is far superior, because it has the bandwidth to provide reasonably good high-frequency reproduction -- and in stereo, not just mono.

MW and LW transmissions are also affected by the time of day and some other natural factors. This often results in varying reception quality at night. So it's clear why FM transmission has been king when it comes to stereo music radio. And FM also has the advantage of being able to carry an additional data channel (known as RDS) used to supply radio-station information and automatically switch to traffic bulletins. But while it's very good, FM is certainly not perfect. In a vehicle it's difficult to avoid background noise, and in poor reception areas the signal may deteriorate to such a point that the radio receiver (also sometimes called a radio tuner) has to give up on it and find you another station. Given the inherently short range of an FM transmitter, it is also necessary to re-tune the radio at frequent intervals in the course of a lengthy journey. RDS can help here but it is not always reliable.

The Digital Breakthrough

It could be said that all these problems arise from the fact that AM and FM radio are analogue technologies. Until about 1960 the vast majority of electronic systems were analogue in nature but the advent of the integrated circuit in the late 1960s and the microprocessor in the early 1970s has brought about the widespread adoption of digital techniques. In simple terms the difference is that digital methods make the high-speed processing of massive amounts of complex information possible in a way which would be either impractical or uneconomic in analogue systems. A modern personal computer (PC) consists of a small box on the desktop whereas an analogue version of the same thing would require an enclosure the size of an aircraft hangar. Digital radio systems make it possible to detect and correct errors introduced during reception. As we'll see, this is a major step forward.

The other major point about digital systems is that they can be very much more efficient than analogue techniques. This is particularly important in radio where the amount of usable frequency space is very limited. Taking VHF/FM broadcasting as an example, the band between 88 and 108MHz is allocated worldwide for this purpose. In the UK it is only possible to fit five national networks and some 200 low-power local stations into this space. This is why BBC Radio 5 is not available on FM. However, if digital techniques were used it would become possible to fit some 50 national networks and an essentially infinite number of local stations into the same allocation.

The pressure on the radio spectrum is severe and will only increase in the future. For this reason - and also to address some issues around mobile reception - it has long been a goal of broadcasters to move to digital techniques. Work on DAB (standing for Digital Audio Broadcasting) as a replacement for analogue FM services began in 1981, and in 1987 the Eureka 147 group of broadcasters and manufacturers was formed under the auspices of the EU. It was hoped that the specification adopted would form a future world standard. DAB makes use of very sophisticated encoding and error-correcting techniques and also embodies so-called data reduction. This uses psychoacoustic techniques to discard programme material considered likely to be inaudible to the listener and hence to maximise the efficiency of the transmission. The principle involved is exactly the same as that used for converting audio to a format such as MP3, and in fact DAB radio uses an earlier version called MP2.

Inaccurate Claims & Audible Benefits: DAB Wins on Points

DAB is not claimed to represent an improvement over FM in sonic terms. The main reason for its introduction was to alleviate pressure on radio spectrum space although -- as we will see -- it offers considerable benefits to mobile reception. The audio specification of DAB is actually slightly inferior to that of CD, and the 'lossy compression' necessary to maximise efficiency further degrades the audio quality. Advertising and marketing material claiming that DAB offers 'CD-quality' radio are factually inaccurate: it can do no such thing. The available quality is more like that available from an MP3 player.

What DAB does very well is to improve the quality of mobile reception. DAB receivers are insensitive to multipath and in fact can take advantage of multipath reception rather than suffer from it. This means that the characteristic fluttering distortion which often accompanies mobile FM listening simply ceases to be an issue. You will probably notice the lack of multipath most when using a DAB radio for the first time in a moving vehicle. Transmitters carrying the same DAB networks can operate on the same frequencies everywhere in the country, so no re-tuning is required. In addition, the fact that all broadcasts are realised and transmitted as interleaved data streams (explained in a moment) makes 'tuning' a DAB radio impossible in the conventional sense. Current DAB radios usually display the available stations next to 'soft' presets, which the user merely presses to receive the required station.

How DAB is Transmitted: Multiplexes, Bit Rate & Data Services

In DAB, separate signals from different broadcasters are combined into what is known as a multiplex or ensemble. This can be thought of as a continuous data-stream into which the output of several broadcasters are interleaved. In the UK there were originally to be seven multiplexes with one being for BBC national radio and a second for national commercial radio. Four others would carry BBC and independent local services, leaving the final one unallocated. By 2011 there were five operational multiplexes of which one was for BBC channels and another for national commercial stations. The other three carried regional stations broadcasting local news, sport, weather, travel information and music of different genres. Each multiplex occupies a mere 1.5MHz of spectrum space, which sounds impressive although proposed variants of DAB (see later) are even better in this respect.

The precise number of stations which each multiplex can carry depends on several variables. In simple terms, the desired audio quality has a bearing on how much data (the bit rate) is transmitted and hence how much space is left in the multiplex for other broadcasters. For example, a classical-music station would be expected to require more space than a sports commentary. Pressure on space within its multiplex has caused the BBC steadily to reduce the bit rate of many of its stations. The consequent negative effects on audio quality have attracted strong criticism from some quarters. Many tests and carefully controlled double-blind comparisons have shown that a high-grade FM system sounds better (more natural and less fatiguing) than a high-grade DAB equivalent. This is the case even if the latter has a high bit rate such as that used for BBC Radio 3. However, it is reasonable to argue that in a moving vehicle the abolition of multipath and reduction in overall noise greatly outweigh the reduction in quality.

Because DAB has the capacity to carry large quantities of data, detailed real-time traffic information can easily be provided and integrated with vehicle navigation systems. In principle this allows routes to be optimised to minimise journey times. Other data services (known generically as programme associated data or PAD) offer considerable potential for domestic receivers, if not perhaps quite so much for mobile reception. Limited traffic information services are currently available via DAB in the UK under the auspices of the Highways Agency but it is fair to say that they have not been universally well received. Certainly there is no DAB equivalent to the RDS TA/TP system and it currently appears unlikely that there will ever be, despite the fact that the specification explicitly makes provision for it.

DAB Reception: UK Coverage & Antenna Requirements

As with the majority of digital communications systems, a shortcoming of DAB is that it either produces consistently good results or horrible noises. From the point of view of the motorist, nothing is more irritating and disconcerting than to be driving along and listening to an excellent-quality broadcast which suddenly turns into the infamous "boiling mud" characteristic of a DAB receiver running out of error correction. At worst, the audio can suddenly disappear completely. Since it will be a long time before DAB transmissions cover all areas, and indeed there will always be pockets of poor reception, it follows that fast reversion to a corresponding analogue broadcast is ideally required in a mobile DAB receiver. This in turn requires good RDS performance. Sadly, no DAB receiver yet trialled performs this function well and indeed most do not offer it at all.

To be fair, DAB coverage in the UK is generally good but there are some areas of poor or non-existent reception. The UK government is keen to switch off FM broadcasting and re-use the spectrum but has given a commitment that this will not occur until the BBC has ensured that DAB coverage is at least equivalent to that of FM. In practice a digital radio switchover seems to be several years away, especially given that DAB radios are still relatively uncommon in vehicles.

As always in radio, good reception starts with the antenna. DAB is transmitted on different frequencies from FM (approximately 218-225MHz for UK multiplexes) and in principle it therefore requires a differently sized antenna. In practice, existing car antennas may work reasonably well provided they are passive, i.e. do not have some form of FM/AM tuning network or amplifier stage associated with them. Unfortunately many modern vehicles have active or amplified antennas, especially those which make use of the front or rear screen heating elements, and these will need specialist attention to make them capable of DAB reception.

Glass-mount antennas are a possible alternative but they generally do not work well and should be avoided wherever possible. Amongst other things, a VHF antenna of the form used in vehicles should ideally be mounted over what is known as a ground plane - in a perfect world it would be sited at the centre of the vehicle's roof - and a glass-mount antenna by definition cannot be. The best approach is undoubtedly to have a dedicated DAB antenna fitted to the vehicle, either as a replacement for the existing AM/FM antenna or supplementary to it.

Report on DAB and the future of digital radio commissioned by the MMSA from Crew Green Consulting Ltd. ©2011 Mobile Media Specialist Association Ltd. No reproduction allowed, in part or in full, without written consent.