Once touted as the “Savior of
Shortwave,” Digital Radio Mondiale has not lived up to its hype. Proposed in
1988, with early field-testing in 2000, inaugural broadcasting in 2001 and its
official rollout in 2003, DRM has had a lackluster career over the last decade.
With the allure of FM-quality audio and
fade-free operation, it had appeared that DRM might revive the shortwave
community. Unfortunately, it has been overcome by other events, some technical
and some social. The main weakness has been alternate sources of information
and entertainment, fueled by the very technology that gave DRM hope.
Additionally, in areas of the world
without ubiquitous social media, DRM has yet to realize receivers at a moderate
cost with adequate battery life. The very processing technology that allows
improved operation using the more complex DRM waveform costs more and consumes
more power than the standard AM receiver. A quick look at standalone DRM
receivers over the past decade shows almost a dozen companies entering the
market, only to retreat when the promise didn’t materialize.
A DEMAND AND A
If you mention “shortwave,” the average
person pictures a wiry-looking “ham radio operator” in a basement or attic. But
in fact, most of the less-developed world listens to shortwave. Outside the
U.S., shortwave has been, and probably will continue to be, a serious contender
for disseminating entertainment and information.
The rise of the Internet has influenced
many broadcasters to cease their shortwave transmissions in favor of
broadcasting over the World Wide Web. When BBC World Service discontinued
service to Europe, North America, Australasia and the Caribbean, it generated
many protests. The shifting of resources from shortwave to Internet and television
by the Broadcasting Board of Governors, which oversees U.S. international
broadcasting, further reduced broadcasting hours in the English language. With
recent budget slashings of 70 to 80 percent, resulting in announcements of
closing large stations such as the Radio Netherlands Bonaire and Radio Canada
Sackville sites, increased pressure has been placed on shortwave to perform.
Although most of the prominent broadcasters continue to scale back their analog
shortwave transmissions or completely terminate them, shortwave is still common
and active in developing regions, such as parts of Africa and South America.
Examining both the location of DRM
stations and target areas, Fig.
1 shows that most DRM shortwave stations and target areas are in Europe.
Until an inexpensive, battery-conscious receiver is available, continents such
as South America and Africa won’t be viable target areas.
Fig. 1: Number and target areas
of DRM shortwave stations.
Adil Mina, VP of business development
for Continental Electronics, chairs the DRM USA Group. He has written on its
website that “the receiver that all of us are looking for is still the small
receiver, the inexpensive receiver that will have a good battery life. That’s
what most people are looking for. It’s the one that should be like your
BlackBerry, your telephone, that can sit for two days, three days, without you
having to go back and charge it.”
IN PURSUIT OF
The main requirement of DRM development
was to ensure that far greater audio quality could be achieved whilst keeping
the transmissions in a form to operate alongside existing AM transmissions.
This meant having the ability for the transmissions to occupy a variety of
different bandwidths dependent upon the location and frequencies in use.
There are two main elements to the DRM waveform: audio
coding and RF modulation. Along came several leaps in technology to compress CD
audio into a manageable size. Improved computer technology also provided the
necessary processing speed to adopt a complex waveform. However, with higher
processing speed comes increased cost and battery consumption.
DRM’s audio compression system employs
two main techniques. The first is called Advanced Audio Coding. The brain does
not perceive all the sounds that are heard by the ear. A strong sound on one
frequency, for instance, will mask out others close in frequency that may be
weaker. AAC analyzes the audio spectrum in sections and only encodes those
sounds that will be perceived. However AAC on its own does not provide
sufficient compression of the data to enable the transmissions to be contained
within narrow shortwave bandwidths.
To provide the additional data
compression, a scheme known as Spectral Band Replication is employed. This
analyzes the sounds in the highest octave, which are normally from sounds such
as percussion instruments of those that are harmonically related to other
sounds lower in frequency. SBR analyzes them and sends data to the receiver
that will enable them to be reconstituted later.
The DRM transmitted signal uses a form
of modulation known as Coded Orthogonal Frequency Division Multiplexing, as
seen in Fig. 2. It is
resilient to many common forms of interference and fading. Its main drawback
has been that it requires a significant level of signal processing to extract
the data from the carriers and reassemble it in the correct fashion. Signal
processing ICs are now sufficiently powerful and are at a reasonable cost to
make the use of this form of modulation viable.
Fig. 2: The COFDM Spectrum of a DRM
COFDM uses a large number of
closely-spaced carriers that are modulated at a low rate data. Each carrier is
modulated with Quadrature Amplitude Modulation using a selectable error coding.
Normally, closely-spaced signals would be expected to interfere with each
other, but by making the signals orthogonal to each another, there is no mutual
interference. This is achieved by having the carrier spacing equal to the
reciprocal of the symbol period. The data to be transmitted is split across all
the carriers. By using error correction techniques, if some of the carriers are
lost due to multi-path fading effects, then the data can be reconstructed.
COFDM has gained a significant presence
in the wireless market place. It is now popular with wideband digital
communication, digital television and audio broadcasting, DSL broadband
Internet access, wireless networks and 4G mobile communications. The
combination of high data capacity, high spectral efficiency and resilience to
interference as a result of multi-path effects makes it ideal for the high data
applications that are becoming common in today’s communications scene.
This signalling and detection technique
incorporates kinematic filtering and signal multiplexing, aptly named
“kineplex.” In the early 1950s Collins Radio foresaw the need to transmit data
over relatively narrow channels. The 16-tone Tactical Digital Information Link,
TADIL-A, is used by the U.S. Navy to share radar tracking data, which makes a
classic buzz sound like an alligator happily making little ’gators. It’s one of
the more distinctive sounds on shortwave. The receiving end of the link used a
bank of 15 extremely-sensitive electromechanical resonators, housed in an
equipment cabinet six feet high by three feet wide by two feet deep.
Receivers have come a long way since
DRM receivers evolved from front-end
down-converters feeding personal computers. Later, digital signal processor
manufacturers started producing chip sets containing both the DSP and the RF
digitizer. Standalone receivers used these chip sets or modules without the PC.
Fig. 4:The DRM software radio uses an RF down-converter
ahead of the sound card in a standard PC.
Initially, many firms tried to “seed”
the shortwave market by offering “add-on” accessories to use the down-converter
of existing higher-end receivers and adding a complex processor at the IF, as
shown in Fig. 4. The
bandwidth of a DRM signal varies from 9 kHz to 20 kHz, and the number of
carriers used in the COFDM-modulation is relatively small (a maximum of 460 at
the highest bandwidth vs. lowest carrier spacing options). These features
motivated a real-time software implementation of a DRM-receiver on a conventional
personal computer using the sound card as the input and output device. A long-,
medium- and shortwave front end with an intermediate frequency (IF) between 5
kHz and 15 kHz is used to receive the DRM signal. This addressed the
technically-adept, but didn’t apply to the villager starving for entertainment.
|Fig. 5:The Coding Technologies Digital World Traveller was a convenient shortwave
accessory for the traveler with a PC on the go.
One of the more-interesting DRM modules
was the Digital World Traveller, Fig. 5, by Coding Technologies introduced in 2004. This handy
little module was connected to the USB port of a PC or Notebook. Priced at
$260, the device came with software capable of receiving DRM, FM and AM radio
programs without any additional power supply or battery.
Fig. 3:The evolution of DRM ‘standalone’ shortwave
portable receivers has left many artifacts behind.
Around 2007, a few manufacturers started
selling standalone receivers (Himalaya Electronics, Technisat, Morphy Richards,
Starwaves, UniWave, Sarapulsky Radiozavod). Most of the receivers were based
upon the discontinued Radioscape RS500 module. The standalone models relied on
household electricity and thus were not portable, as seen in Fig. 3. We do see a steady
price reduction headed for the magic $100 goal (in production quantities, and
not including taxes, V.A.T. or shipping).
DSPs can be found in most of our
day-to-day consumer devices, including mobile handsets, digital cameras, navigation
devices, TVs, DVD players and game consoles. They are ubiquitous in multimedia,
telecommunications and networking applications. These products use a variety of
hardware approaches to implement DSP, ranging from the use of off-the-shelf
microprocessors to field-programmable gate arrays (FPGAs) to custom integrated
Programmable “DSP processors,” a class
of microprocessors optimized for DSP, are a popular solution for several
reasons. In comparison to fixed-function solutions, they have the advantage of
potentially being reprogrammed in the field, allowing product upgrades or
fixes. They are often more cost-effective (and less risky) than custom
hardware, particularly for low-volume applications, where the development cost
of custom ICs may be prohibitive.
The lowest complexity processors are
“hard-wired” or dedicated processors such as FPGAs, lacking the flexibility to
be reprogrammed should the DRM specification change. This was the nemesis for
Digital Audio Broadcasting (DAB, Eureka-47), where dedicated chip sets
specifically developed for DAB were termed worthless when the standard was
updated, leaving receiver manufacturers reluctant to enter an immature arena.
Cutting-edge technology can allow a DSP to have lower
power than the equivalent ASIC, due to the ASIC using older technology. Also,
the DSP can usually be controlled to minimize clock speeds when the processing
load allows.Alternatively, the power in an ASIC is minimized by
ensuring that the signal-processing operations are dimensioned correctly,
particularly in terms of the bit widths being processed.
FUTURE OF DRM?
The success of DRM is dependent on a
combination of satisfying a need and technology arriving with the ability to
fit that need. In order to provide better quality, a more complex carrier
signal and additional processing was required. The need was to provide a better
service, but the very enabling technologies also provided alternate forms of
entertainment and information. Within the last decade’s window of opportunity DRM
took one step forward and other mass media took several steps forward.
In a developed country, the speed of
development favors the Internet because of rapid acceptance of service. The
hardware is inexpensive, the bandwidth is expanding and the social media
networks have been providing the individualized services that people demand.
Given a limited budget for a household in an undeveloped country, they will try
to maximize their “bang for the buck.”
With the advent of the Internet and SmartPhones and social
media networks, all bets are off. We have recently seen the value of social
media networks in social uprisings in the Middle East and Africa. People can
get the same shortwave information over social media with the uncensored
spontaneity of amateurs.
Without a viable (cost and
battery-conscious) receiver, DRM has a hazy future.
Ernie Franke is a broadcast consultant in St.
Petersburg, Fla. He earned a master’s degree in electrical engineering and has
been the chief engineer at several broadcast stations.