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DAB (Digital
Audio Broadcast)
What is DAB? (By World
DAB)
Digital Audio
Broadcasting, DAB is the most fundamental advance in
radio technology since the introduction of FM Stereo
radio. It gives listeners interference free reception or
high-quality sound, easy to use receivers, and an
unlimited potential for wider listening through many
additional stations, services and developments.
DAB can broadcast on
terrestrial networks for regional coverage, as well as
on Satellite for wider coverage. You are able to receive
the Digital Audio Broadcasting programs using solely a
tiny non-directional stub antenna. You can receive
CD-like quality radio programs even in the car without
any annoying interference and signal distortion. DAB
radio is designed for the multimedia age: DAB can carry
not only audio, but also text pictures, data and even
videos. In short, DAB fully complies with the tough
requirements of the Digital Age, which is emerging
before us.
Shown here is a simple overview of the DAB system
benefits:
Quick Facts
Over 300 million people
around the world can now receive up to 600 different DAB
services. Commercial DAB receivers have now been on the
market since summer 1998. There are now over 80
different DAB receivers commercially available.
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CD-like sound quality
With no more than a simple
whip antenna, DAB users can enjoy pure undistorted sound
quality.
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Easy program selection
Rather than searching
wavebands as present, users can select all available
stations or preferred formats from a simple text menu.
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Perfect reception
DAB eliminates
interference and the problem of multi-path while in a
car. It “blankets” wide geographical areas with an even,
uninterrupted signal. Once full services are up and
running, a driver will be able to cross an entire
country staying tuned to the same station with no signal
fade, without altering frequency.
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One receiver does it
all!
DAB is quite unique in
that both music and data services will be received using
the same receivers
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Program-associated data
DAB broadcasts can display
text information in far greater details than the RDS
system, such as program background facts a menu of
future broadcasts and complementary advertising
information. Receivers attached to a small screen will
display visual information as diverse as weather maps
and CD cover images.
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Information services
Services from sources
other than the broadcasting station are included within
the same channel for the user to access at will. These
include news headlines, detailed weather information or
even the latest stock prices.
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Targeted music or data
services
Because digital technology
allows a massive amount of different information,
specific information user groups can be targeted with
great accuracy because each receiver can be addressable.
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Wide choice of
receivers
It is possible to access
DAB services on a wide range of receiving equipment
including fixed, mobile and portable radio receivers
with displays or screens and even personal computers.
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DAB: A system designed
for terrestrial/satellite reception
DAB services will be
available on terrestrial and satellite networks, and the
same receiver could be used to provide radio program
and/or data services for national, regional, local and
international coverage.
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Lower transmission
costs for broadcasters
DAB allows broadcasters to
provide a wide range of material simultaneously on the
same frequency. This not only makes room for a vastly
increased number of program to increase user choice, but
also has important broadcast cost-cutting implications
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DAB
Coverage (by World DAB)
Click to view large version
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DMB (Digital
Multimedia Broadcast)
DMB and Digital radio
by Ian David (Worlddab newsletter october 2004)
Digital radio is the
transmission of packets of binary data over radio waves
so this data can be anything -- mono or stereo audio
information and even webpages or video. One technique
for doing this is called Digital Multimedia Broadcast (DMB),
which is based on the established Eureka 147 Digital
Audio Broadcast (DAB) standard with additional error
correction. The Asia Pacific region is driving the
development of this technology, especially Korea where
DMB broadcasts and receivers are expected to be
available for Christmas 2004. DMB is not a standard
specification from WorldDAB at the moment but is
currently under review.
There is another
technology that is being developed as an alternative way
to deliver multimedia to mobiles called DVB-H, or
Digital Video Broadcast – Handheld. This has evolved
from DVB-T, which was designed for use in mains powered
receivers. The new standard for DVB-H has the aim to be
less power hungry for battery powered receivers and
provide higher levels of protection against noise. This
power saving is achieved by time slicing so that the
receiver is only switched on in those time intervals
when viewing the channel of interest. These intervals
could be anything between a few milliseconds and a few
seconds. It therefore reduces power consumption by
being switched off for the rest of the time when
non-required data is being transmitted. There is
therefore a trade off between the data rate required for
the service and how much this can be packed into short
bursts to save the battery power of the receiver. A
similar technique has been used in DAB whereby the whole
frame is transmitted but the receiver only decodes the
audio channel of interest to save on battery power.
Power consumption is vital for a battery power unit and,
at the moment, DMB draws less power than DVB-H.
Whilst having the same method of Forward Error
Correction as DVB-H, DMB additionally uses time
interleaving to solve problems of the harsh conditions
found typically in mobile environments such as impulsive
noise (e.g. car ignition noise or hair dryers). It is
also particularly effective when using one antenna; the
extra time interleaving works by spreading the errors in
time over 16 logical frames (384mS), so that the
receiver can correct the errors. This additional
technique used in DMB makes reception geographically
possible in more areas and ensures that the video
is error free more often. DVB-H, on the other hand,
uses Multi Protocol Encapsulation with FEC to address
this issue and can only cope with errors in one time
slice. However, errors in transmission usually occur
not as single errors but as burst errors
over several time slices so this technique is not as
effective when compared to DMB.
Another important feature
that DAB has and DVB-T/H lacks is Unequal Error
Protection (UEP); this can be set for each individual
program or audio channel and protects the bits of higher
importance within the channel. This is very important
for mobile reception, as the reception conditions are
variable. The loss of a single bit can corrupt a key
video frame and cause a breakdown of video stream
continuity causing disruption for the user.
Many
countries already have the hardware in place to
broadcast DAB radio with an extensive coverage area so
there is little additional cost to roll out DMB.
However, in places like the UK where DAB audio is very
successful, more spectrums will need to be made
available, as the Band III frequencies are already
pretty full.
Fortunately, L-band provides a second set of DAB
frequencies. Using L-band has the added benefit that
the antenna length on the receiver needs only to be a
few centimeters, unlike the lengthy telescopic antennas
currently used in Band III, making it ideal to integrate
into small mobile devices. The drawback is that a
higher density of L-band transmitters is required
than for Band III, which increases the infrastructure
capital costs.
However, due to the extra sensitivity of the DMB
receivers, the required planning RF field strength is
considerably less than DVB-H. This makes DMB an
attractive proposition when considering transmitter
power requirements.
So DVB-H transmitters need more power than DMB
transmitters and the capital costs of a DVB-H
transmitter are also greater. That said, it may be
possible to save some of the capital costs of
building a DVB-H transmitter network by sharing existing
DVB-T infrastructure, which is why the DVB-H system is
generating interest within the industry. Again due to
the extra equipment required even on a current DVB
network i.e. video/audio encoders, DVB-H encapsulators
and in most cases modulators, it remains unclear as to
whether this has any advantage over a DMB network where
only video/audio encoders are necessary to add to a DAB
network to deliver streaming video.
In summary, DAB was designed from the outset for mobile
reception where as DVB-T was designed for non-mobile
reception. DVB-H is based on DVB-T with adaptations to
try and make it more robust for reception in challenging
mobile conditions and to reduce its power consumption –
making it more DAB like. With the additional forward
error correction techniques of DMB, this DAB-based
technology provides a more robust solution and without
time interleaving it is hard to see how DVB-H can
compete with DMB in the mobile environment. Also DMB
draws less power compared to DVB-H but, with better
algorithms, this gap could narrow over time.
However, the real difference is that while one or other
or both of the DAB frequencies are available for use in
almost every country -- apart from the US and Japan --
the frequencies for DVB-H are already in use in some
European countries, such as Germany, for analogue
television. It will be several years until these
frequencies are freed up by the move from analogue to
digital television. During this time DMB has the
opportunity to establish itself as the pan-European
solution to the delivery of video on the move assisted
by the Asia Pacific powerhouse that has already
committed to this route in its region. With two major
regions using the same standard, DMB has a good chance
to become the technology for delivering video and data
on the move around the world.
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