A Digital Future
for AM
broadcasting?
With the recent launch of the first public Digital Radio Mondiale services,
LAWRIE HALLETT MIBS explains the technologies involved.
which has its roots in the 19th century is clearly no easy
ention digital radio and most people outside
task. Not only that, but any new standard needs to offer
our industry will think you are referring to
sufficient advantages to persuade broadcasters and
Digital Audio Broadcasting (DAB), the
listeners alike to invest in new transmitters and
technology intended to replace analogue FM
receivers – and it needs to be cheap... sorry, cost
transmissions. This system – developed in the 1980s
effective!
and known formally as Eureka 147 DAB – has taken a
So just what is on offer from DRM? The table below
long while to get as far as it has, but is being used for
lists the benefits to broadcasters, regulators and
broadcasts in many parts of the world. The number of
listeners:
listeners is still fairly modest in comparison to FM,
mainly due to the scarcity and relatively
high price of suitable receivers, but the
Feature
Listener
Broadcaster
Regulator
numbers are now growing steadily.
Improved audio quality
Yes
Yes
—
There are, however, many different
Improved reception
Yes
Yes
Yes
flavours of digital radio – satellite
Automatic tuning
Yes
Yes
—
delivered, Internet, and so on – and
‘Simulcasting’ modes
Yes
Yes
Yes
one of the most recent incarnations,
Text and data features
Yes
Yes
—
and perhaps that with the greatest
Use of existing frequencies
Yes
Yes
Yes
potential, is Digital Radio Mondiale or
Single Frequency Network
Yes
Yes
Yes
DRM. Sketched out at informal
Use of existing equipment
No
Yes
—
meetings in the latter half of 1996, the
DRM consortium was formally
launched in 1998. Consisting of various broadcasters,
In addition to the above, the success of an AM
broadcast equipment manufacturers, network
replacement technology would also provide
operators, academics, and research centres, its declared
tremendous opportunities for the broadcast
aim was:
manufacturing industry in terms of receiver and
“To formulate a digital AM system design, which
transmission equipment sales, which on a world-wide
could serve as a single, tested, non-proprietary,
basis would be considerable.
evolutionary world standard, which would be market
driven and consumer oriented” And… “To facilitate
Technology
the spread of AM digital technology around the
The technology behind DRM is arguably more
world.” 1
traditional than the approach taken by Eureka 147 DAB.
Whereas DAB uses a multiplex to achieve a degree of
The reason for this declaration was that AM
spectral efficiency (particularly for multi-transmitter
broadcasters, particularly those operating in the
networks), DRM continues to achieve the excellent
international Short-Wave bands, recognised that with
spectral occupancy performance of AM, using just 9kHz
advances in the audio quality of other mediums the
bandwidth transmissions spaced in 10kHz increments
days of AM broadcasting were surely numbered unless
through the spectrum, to provide a monophonic signal
something was done.
of near FM quality. In comparison, DAB needs a
whopping 1.536MHz per multiplex, typically for just
Dwindling AM Audiences
eight stereo programme feeds!
“The sound quality of AM is much worse than that of
Part of the reason for this difference is the fact that
FM and audiences are voting with the off button: AM
DRM is a more recent technology which takes
audiences are dwindling.” 2
advantage of the improved (that is, reduced) data rates
With more than two billion AM radio receivers in
now available with coding schemes such as MPEG-4.
use around the world, replacing a global standard
M
22 LINE UP Nov/Dec 2003
The DAB system is stuck with the far less efficient (by
current standards) MPEG-2 Layer II coding scheme,
which was ‘state of the art’ when its technical standard
(ETSI EN 300 401) was being finalised in the early
1990s.
Just like the analogue technology it seeks to replace,
DRM operates on the principle of single, stand-alone
transmitters. Unlike DAB where all services in a given
multiplex cover exactly the same geographical area,
DRM coverage can be tailored on a per-service basis, by
altering individual transmitter locations, power and/or
antenna pattern (just as at present). However, unlike
traditional AM signals, DRM can be also be operated as a
Single Frequency Network (SFN) which offers a
significant advantage. For example, BBC Radio 5
(currently using 693 & 909kHz) would only need one
of these frequencies to cover the whole of the UK,
using multiple transmitters configured as an SFN.
The principles behind DRM have been developed
from communications technologies used for many years
on the international short-wave bands. Although
designed to operate within existing 9 or 10kHz AM
broadcast band plans, DRM can also be used with
smaller bandwidths (4.5 or 5.0kHz) and includes
provision for multiple channel operations (requiring,
for example 18 or 20kHz bandwidths). Such ‘widerband’ modes could be used to carry stereo signals, full
CD-quality audio, enhanced data features, and so forth.
The precise modulation scheme employed can be
varied according to the quality of the material being
broadcast, or the transmission path being used (local,
international, or multi-hop sky-wave). The DRM
receivers automatically recognise and resolve the mode
being used, and also have the ability – similar to RDS
on FM radio – to automatically switch to alternative
station frequencies to find the best reception.
More difficult signal paths (such as international
Short-Wave broadcasting) naturally require additional
error-protection to ensure reliable reception. However,
given that the bandwidth available on any channel is
fixed, any increase in error protection data leaves less
room for programme material. Even with DRM, the
cake can’t be had and eaten at the same time!
The DRM signal does not multiplex different
programme streams in the same way that DAB does,
but it does multiplex three separate transmission signal
elements: the Main Service Channel (MSC); the Fast
Access Channel (FSC); and the Service Description
Channel (SDC). The particular functions of these are
given in the table opposite:
Programme sound is encoded using MPEG-4 with
the optional Advanced Audio Coding (AAC) and
Spectral Band Replication (SBR) extensions. Used
together, these two techniques manage to squeeze
near-FM quality audio out of less than 10kHz of RF
spectrum. AAC is generally accepted as being at least
twice as efficient as MPEG-2 Layer III (otherwise known
as MP3) in its use of data capacity to achieve a given
subjective audio quality. However, at low bit rates most
transform coding systems – such as AAC – perform
better if the audio bandwidth on which they are
operating is reduced. This is because the limited
number of data bits available can be used most
efficiently if the number of frequency analysis bands is
reduced. Therefore, a given number of bits per second
can either be used to reproduce the full audio
frequency range with poor accuracy, or can describe a
more limited frequency range with good accuracy. This
second approach tends to achieve the best results and
so the AAC codec is arranged to concentrate on
providing a limited audio frequency (typically up to
7kHz) with good quality.
It is the SBR element that delivers the higher
frequencies necessary to provide the perception of near
FM quality. The cunning principle behind this system is
that instead of transmitting the actual audio data to
describe the higher audio frequency components, the
system examines the audio signal at the encoder and
dynamically describes the ‘spectral shape’ of the audio
signal being encoded, using a low bit-rate (~2kb/s) data
stream. The SBR decoder in the receiver uses this data,
combined with the harmonic structure of the AAC
encoded audio, to intelligently synthesise an extended
frequency range.
The resultant audio has good subjective quality in
the most critical audio range (500 - 6,000Hz) but also
has an audio bandwidth that extends up to 15kHz – and
yet all this can be achieved using a data rate of between
just 22 and 25kb/s 3.
Robust Transmissions
For more robust transmission modes, or for speech only
material, alternative audio coding methods are also
available. These include the MPEG-4 Code-book Excited
Linear Prediction (CELP) algorithm (which can also be
Main Service
Channel
64-QAM* or 16-QAM.
(16-QAM employed
when enhanced signal
robustness is required)
This contains the data for all the
programme services in the DRM
signal (audio, text data etc). The
gross bit-rate of the MSC is
defined by the DRM channel
bandwidth in use and by the
transmission mode
(error correction level used, etc).
Fast Access
Channel
Always 4-QAM.
(for fast resolution).
This contains service selection
information and ‘housekeeping’
data to allow a receiver to
correctly decode the signals
carried in the MSC (details of
transmission mode audio coding).
A receiver resolves this channel
before it tackles the MSC or SDC.
Service Description
Channel
16-QAM or 4-QAM.
(4-QAM employed when
enhanced signal
robustness is required)
This contains service information,
such as details of alternative
frequencies, frequency schedules,
and audio metadata information.
(*QAM: Quadrature Amplitude
Modulation)
used with SBR for high quality speech), and the MPEG-4
Harmonic Vector Excitation Coding (HVXC) algorithm
for very low bit-rate speech broadcasts. This latter
coding method is particularly suitable for use under
tough transmission conditions – such as during poor
atmospheric conditions, or in the presence of jamming.
It is also possible for the DRM signal to carry audio
signals with more than one type of coding, multiplexed
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23
An R&D
receiver rack
used during the
initial testing of
DRM.
within the same transmission. For example, providing
sufficient data capacity is available, a full bandwidth
AAC audio service could be carried alongside a low bit
rate HVXC speech signal, possibly carrying a speechonly news or information service. 4
Having emphasised the point that DRM technology
is rather different to that of DAB, I’m now going to
confuse the issue by pointing out that one of the
underlying principles used in these two systems is, in
fact, the same! Specifically, DRM and DAB both use a
type of transmission system known as Coded
Orthogonal Frequency Division Multiplex (COFDM).
This is a transmission scheme which divides the
encoded audio, control signals and associated data
across a large number of closely spaced carriers. This
technique is robust against interference and multipath
distortion yet still keeps the transmitted energy within a
single 9 or 10kHz broadcast channel in the case of
DRM.
A major contributing factor to the robustness of
COFDM signals is that the programme and control data
information is not only spread out across multiple
carriers over a spread of frequencies, but also
interleaved over the time domain as well. This means
that the data rate on any one carrier is very low and the
interleaving provides useful error protection. The effect
of multipath interference on an analogue signal (AM or
FM) is the result of the receiver picking up multiple
versions of the same signal – one directly from the
transmitter and others from reflections off buildings or
rough terrain. Even with radio signals travelling at the
speed of light, reflected signals arrive at the receiver
after the direct path signal and, since an analogue
receiver has no way of distinguishing the direct signal
from its reflections, it resolves all of them – resulting in
multipath distortion.
By comparison, COFDM signals make a virtue out of
multipath. With so many carriers the data rate on each
individual carrier is relatively low, and thus slightly
delayed reflected signals tend to combine
constructively, rather than destructively. During the
data cell transitions periods there will naturally be some
destructive cancellation, but during the central portion
of each cell – the so-called ‘guard-interval’ – direct and
reflected signals combine constructively so the data is
24 LINE UP Nov/Dec 2003
actually enhanced and stable – and thus multipath
distortion becomes irrelevant.
Having said that, signals which arrive too late (with
delays greater than the guard-interval) will obviously
have an adverse effect upon the received signal
strength. The time between the arrival at a receiver of
the direct signal and the last reflection is defined as the
‘delay-spread’, and the guard-interval is arranged to
ensure that most, if not all, signals arrive within it. For
local Medium-Wave DRM broadcasting, the guardinterval can be as short as 2.66ms, which corresponds
to the least robust mode of DRM. For international skywave Short-Wave reception, which requires much
greater robustness, the guard-interval can be as long as
7.33ms. 5 Incidentally, this guard-interval idea is also
what makes it possible to operate Single Frequency
Networks. If the transmitters are spaced across the
country to ensure that strong signals from adjacent
transmitters arrive within the guard-interval of the local
transmitter, their signals will enhance the reception
rather than interfere with it.
Testing, Testing
The practical over-air testing of the DRM system was
completed long ago and regular long-term test
broadcasts began in January 2002. Following these trials
the required international standards were eventually
agreed and the public launch of the system took place
in Geneva on June 16th, this year to coincide with
WRC2003 (the World Radio Conference of the
International Telecommunications Union).
Well-known international broadcasters including
the BBC World Service; Deutsche Welle; Radio Canada
International; Radio France Internationale; Radio
Netherlands; Radio Vaticana; Swedish Radio
International; the Voice of America, and the Voice of
Russia all broadcast programming via DRM, and their
transmissions targeted Europe, North America, the
Middle East, Australia, and New Zealand. In addition,
national broadcasters in Europe – Deutschland Radio
and Radio France – as well as some local stations,
employed DRM for more localised Medium-Wave
transmissions. Although no specific DRM spectrum
allocations have been made as yet, the regulations
suggest that existing AM allocations in ITU Regions 1 &
3 (Europe, Africa and Australasia) can be used provided
that average power levels are 7dB down on those
allowed for analogue AM transmissions on the
particular frequencies concerned.
For the anoraks amongst you, PC software and
details of necessary receiver modifications are now
available from a dedicated DRM web-site
(www.drmrx.org) so you can check out the system for
yourselves. Another web-site (www.drm.org) has more
general details of the system as a whole, including a
selection of audio samples as downloadable files.
Like DAB before it, DRM has arrived on the air
before compatible receivers are readily available.
However, the consortium has worked hard to liaise
with receiver manufacturers and it hopes that suitable
designs will be available in volume by late 2004 or early
2005. Furthermore, since the official launch the DRM
consortium and the WorldDAB Forum (which promotes
the more established DAB technology) announced an
alliance which will see the two organisations
collaborating over the introduction of multi-standard
digital radios. These will be able to resolve both types of
digital signal, which should encourage the take-up of
digital radio broadcasting as a whole as well as helping
to minimise listener confusion!
At present, UK based DRM broadcasts are being
transmitted from VT Merlin Communications’ sites at
Rampisham in Dorset, and Orfordness on the East Coast
of Suffolk. Two transmitters provide Short-Wave facilities
at Rampisham, while Orfordness has been equipped
with a brand new 200kW DRM-equipped Nautel
transmitter for Medium-Wave broadcasts on 1296kHz.
DRM technology also offers the possibility of using some
HF allocations for local broadcasting to supplement
existing MF and LF allocations, and tests on 26MHz
frequencies, using radiated powers of around 100 Watts,
have provided good reception over distances of 50
kilometres or so. So there is certainly the potential, at
least, to provide cheap local radio services on such
frequencies in the foreseeable future.
References:
1. DRM Consortium web-site (April 2002): Brief History.
www.drm.org/consortium/globhistory.htm
2. Lindsay Cornell, BBC Radio and Music (Canberra, Australia 3-4
May 2001).
Paper presented at the Australian Broadcasting Authority’s ‘Radio,
Television and the New Media’ Conference: Digital Radio: The
BBC Perspective.
www.aba.gov.au/abanews/conf/2001/pdfrtf/Cornell.rtf
3. DRM Consortium web-site (April 2002): What is SBR?
www.drm.org/newsevents/faqs/faq-049.htm
4. DRM Consortium web-site (April 2002): Q2 - What type of
audio coding does the DRM system use?
www.drm.org/newsevents/globfaqs.htm
Thanks to
Peter Jackson
at VT Merlin
Communications,
and the BBC, for the
picture of the DRM
rack receiver.
5. DRM Consortium web-site (April 2002): Q1 — How does the
DRM Work? What is the Guard Interval?
www.drm.org/newsevents/globfaqs.htm
6. Digital Radio – New Opportunities in Broadcasting.
A report compiled by Simon Spanswick which gives a useful
overview of the various ‘flavours’ of digital radio now available:
www.wmrc.com/businessbriefing/pdf/broadcast2002
/reference/18.pdf
Useful Links:
Radio Netherlands International – Details of DRM including
transmission schedules of various major broadcasters:
www.rnw.nl/realradio/html/drm.html
www.rnw.nl/realradio/html/drm_latest.html
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