Car owners guide to:

Car owners guide to: Car Audio > DAB Digital Radio

As consumers we've taken digital audio technology for granted. The music quality we get from today's CD and DVD players has become the expected standard, but until recently listening to radio in the car fell a long way short of what we enjoy from CDs.

Although much improved over the last two decades, background noise and interference have continued to spoil our radio listening, especially in a moving vehicle. FM, medium wave (MW) and long wave (LW) radio transmissions can be affected by a variety of obstacles to good reception.

One is quite literally obstacles - buildings or other structures that get in the way and cause a problem known as "multipath interference".

Because of the line-of-sight nature of FM transmission, the signal can be partially blocked. The radio signal is also reflected from buildings, creating multiple versions of the same signal that all arrive at the car antenna but slightly shifted in terms of time and phase. This results in signal cancellations and additions that cause an effect similar to the kind of "ghosting" you may have seen when using a portable aerial on a television.

With a fixed antenna in the home the problem can be minimised by selecting a position where a good strong signal can be received. But a car that's constantly moving presents a real problem, and only thanks to advanced receiver technology are today's car radios able to deliver such good results on stereo FM.

With MW and LW bands multipath interference is less of an issue because, in simple terms, these transmissions are less directional. But they suffer hugely from another limitation 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 large enough and so the original signal has to be trimmed around the edges so that it passes through the pipe. In real terms this means the high and low frequencies of the music are lost, leaving a relatively low-fi sound that's okay 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 atmospheric conditions. This results in varying reception quality at night and sometimes because of unusual weather patterns.
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 not perfect. Some background noise is unavoidable, 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.

DAB - the difference is tangible

Digital Audio Broadcasting (DAB) has changed all this. Because the signal sent to the radio is in a digital data form rather than an analogue signal, it is far less susceptible to the various forms of interference that plague FM and MW/LW. Infact DAB is able to use the multipath effect in a positive way as a reflector, actively combining the direct and reflected signals. Instead of causing signal cancellation, data signals received fractionally delayed can be corrected and added to the main signal.

The digital transmission method also means we can forget our earlier analogy of the pipe. Available space on the airwaves, or 'bandwidth', is no longer an issue - infact digital broadcasting frees up space for both radio and TV and this is a big reason why the broadcasters and government are encouraging us towards digital TV and radio.

It shouldn't need too much encouragement once people see and hear the difference, especially now that prices are coming down. And as well as CD-quality sound, DAB can also offer a whole host of additional information data features, more advanced than the current RDS system.

In-Car Digital Radio - things to consider

DAB brings the radio into the digital age at last, but there are one or two things to consider with DAB in-car.

UK coverage in terms of the transmitters is now (as of March 2006) very good across nearly the whole of England and the densely populated Scottish corridor from central west to the north east beyond Aberdeen, but the north of Scotland and the Border area is still poorly served. Much of west and central Wales is poorly covered but the south of the country is well served. Northern Ireland has BBC Digital Radio coverage but little or no commercial digital radio stations broadcasting.

UK coverage is now good and will continue to grow - speak to your MMSA dealer for up to date information about the digital radio stations available in your area. They will also be able to advise you about reception conditions, because even with DAB there can be issues where the terrain is hilly or where transmission levels are not high enough.

Digital radio transmission, rather like FM, is directional in its nature and so there can be pockets where reception is affected. In such situations the receiver will automatically switch to an available FM station. Also, DAB broadcasters will set the power output level of their transmitters to give good results without wasting power. In some areas the compromise has proven not to be ideal for in-car receivers, but broadcasters continually monitor user feedback to avoid this.

The one and only real drawback is that digital radio reception demands a particular antenna - you cannot successfully use the one on the car that's designed to receive FM/AM.

Fortunately there are a variety of options. A roof-mounted aerial is best and there are types that combine DAB/FM/AM reception, so that the vehicle's factory-fitted roof aerial can be replaced, avoiding the need to drill a new mounting hole. However, this is usually the most costly option because of the installation required (removal and replacement of the roof lining etc).

External glass-mount and internal screen-mounted aerials are common alternatives. The glass-mount is generally better because it uses a short external mast - the signal is transferred through the glass by capacitive coupling (no holes are drilled through the glass!). The internal screen aerial is usually the least expensive option but also the least effective - depending on reception conditions in your area it may be fine or you may be advised against using a screen aerial. If your vehicle has a reflective windscreen you almost certainly will need to use a glass-mount.