What's All This DAB Radio Stuff Anyway?
DAB digital radio is the new medium for sound broadcasting. Now
well established in many countries around the world, new DAB
radios are hitting the market. While these new radios are still
a little more expensive than those that receive the older
analogue AM and FM transmissions, they offer the significant
improvements that come with DAB digital radio.
Advantages Possibly the most important advantage for the
listener is the near CD audio quality that DAB can bring. Hiss
and fading are now a thing of the past. Not only is the quality
better but the more stations can be accommodated within the
available space and this means that there is a much greater
degree of choice.
Another important facility provided by DAB digital radio is its
data capability. Data is transmitted alongside the main audio
and this can be displayed on a screen that many DAB sets have
built in. This may be the title of a song being broadcast, a
plot summary for a play, up to the minute sports results, or
whatever the broadcaster feels is appropriate. In fact some
stations scroll news headlines across the screen, or provide
other useful free information including Electronic Programme
Guides.
Not only does the system bring many advantages for the listener,
but it also helps the broadcaster. Lower power levels are
required for the transmitters. This saves considerably on the
cost of the electricity. This is particularly important for
large national networks like the BBC and others. With
transmitters often using 100 kW and more, the costs for running
these transmitters are very high. Also by reducing the power
consumed, it helps the environment.
Another advantage is that DAB uses what is termed a Single
Frequency Network. This means that all transmitters in the same
network use the same frequency without interfering with each
other. Currently a national network using VHF FM, for example,
has to use a number of frequencies so that the transmissions do
not interfere. DAB Digital Radio does not need this and the
space needed for the broadcasts is much less. This also brings
another advantage for the mobile listener because there is no
need to retune the receiver onto another frequency when moving
out of the coverage area of one transmitter into the next.
Technology DAB Digital Radio works by converting the sounds into
a digital format. Once in this format they can be manipulated
using digital signal processing techniques to compress and
manipulate the signal. Furthermore when the signal is
transmitted error correction techniques can be used to ensure
that once the data representing the sounds is transmitted, it is
received correctly at the receiver.
The sound compression uses techniques that conform to the MPEG
standards. Also the radio transmission uses a form of signal
modulation known as Coded Orthogonal Frequency Division
Multiplex or COFDM. Here the data representing the sounds and
the other data to be transmitted is spread across a large number
of close spaced radio signals. By spreading the data over a wide
frequency in this way the whole transmission is less susceptible
to noise and other forms of interference including fading and
multi-path interference.
Multipath interference results when signals are reflected from
objects such as hills and buildings and several signals from the
transmitter arrive at the receiver, all at slightly different
times because they have travelled over slightly different paths
and have taken slightly different times to travel. For VHF FM
this is a particular problem and often results in the signal
becoming distorted. DAB digital radio is able to combine all the
signals, and make the overall reception more robust.
Naturally the technology to enable all this to happen makes the
radio receivers more complicated. However the improvements in
integrated circuit technology in recent years have enabled the
required capabilities to be built into a radio for a reasonable
cost. It is also expected to fall further as DAB becomes better
established.
Summary DAB Digital Radio is a great improvement over the
analogue transmissions that are broadcast. Offering the
possibility of better quality audio, accompanying data, push
button tuning and no requirement to re-tune when moving from one
service area to the next, it is certainly provides a
considerable improvement over the older analogue broadcasts. By
offering all these facilities, it sets DAB digital radio in a
position to take over as the new form of broadcasting for the
21st Century.