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By JIM ROWE
The Future of
Radio
Broadcasting?
There is no doubt that DRM digital radio would provide the best way of
extending radio broadcasting over the whole of Australia – and further. Here’s
how it works.
D
RM or Digital Radio Mondiale
was developed and is promoted
by the DRM Consortium, an international not-for-profit consortium
which has over 100 member organisations in 39 countries.
Many of the members are broadcasters, but there are also many transmitter
and receiver manufacturers as well as
broadcasting standards bodies.
The aim of the Consortium is to support and spread a digital broadcasting system suitable for use in all of
the frequency bands up to VHF band
III. You can find more about the DRM
Consortium at www.drm.org
By the way, “mondiale” simply
means “world wide” in both French
and Italian.
There are two main variants of
DRM. First there is DRM30, intended
specifically for use on the traditional
low, medium and high-frequency
(shortwave) bands below 30MHz and
on the existing AM broadcasting channels within them. The other variant is
DRM+,which uses frequencies from
47-108MHz – these include the old
analog TV channels 1 and 2 as well
as the FM broadcast band.
They can also carry digital data
services along with the audio signals,
such as station names, time, date and
program information.
DRM30, DRM+ and DAB+
So where does DAB+ fit into this
proposed DRM-based digital radio future? After all, we’ve now had digital
radio broadcasting in Australia for the
last eight years or so using the DAB+
system.
But because DAB+ works in VHF
Band III (174–240MHz), it has a relatively short range and as a result is really only suitable for the larger cities
and their suburbs.
Although DRM30 looks set to become the world standard for digital
radio broadcasting below 30MHz,
DRM+ might well end up competing
with DAB+ in the VHF band.
This is quite possible, because
DRM+ is being promoted as a replacement for analog FM broadcasting in
the 88–108MHz band.
Receivers able to receive both DAB+
and DRM+ – as well as DRM30 , analog
AM and FM – are starting to appear.
But what’s the difference between
DRM and DAB+ anyway? In fact, there
are many technical similarities and not
many differences.
All are digital audio broadcasting
systems which use OFDM – the technique of modulating digital information on an array of closely-spaced RF
subcarriers, instead of a single main
carrier.
This is exactly the same kind of
modulation used in DVB-T television,
wireless LANs (IEEE 802.11a, g and
n) and ADSL broadband over copper
telephone lines.
DRM has now been updated to xHEAAC which is backward-compatible
to HE AAC V2. The xHE AAC can
produce excellent speech quality at
a much lower bit rate. DAB+ is yet to
upgrade. HE AAC is used for sound
in MP4 or MPEG4 video. These compression systems reduce the amount of
data required for transmission so that
it will fit in the channel bandwidth
Vive la différence!
The differences between the two
Fig.1: the main difference between DRM30 and DRM+, apart from frequency, is the
transmission frame length – 400ms for DRM30 vs 100ms for DRM+.
siliconchip.com.au
September 2017 63
systems are rather more subtle.
DAB+ uses 1,536 subcarriers transmitted in parallel, each with a bandwidth of 1kHz and spaced apart by the
same figure. This gives a DAB+ subcarrier channel with a total bandwidth
of 1.537MHz and can convey between
15 and 26 different high quality digital
audio signals as well as their accompanying data.
The program data is assembled into
blocks, labeled and each program is
sent sequentially until all have been
sent and then the sequence is repeated
continuously. The individual signals
are separated again in the receiver.
In contrast with this DAB+ multiplexing system, DRM30 has been designed specifically for use in the ‘AM’
bands below 30MHz.
Since Australian AM radio stations
have an RF bandwidth of 18kHz, this
can also be used. For HF broadcasting
the bandwidth could be 5, 10 or 20kHz
depending on frequency availability.
The greater the bandwidth, the higher
the reliability or better quality.
DRM30, DRM+ and DAB+ can all
transmit stereo sound but HF DRM30
can give continent-wide stereo coverage.
Modes, bandwidth and
QAM options
To achieve the desired level of performance on the bands below 30MHz,
DRM30 broadcasters use four different
encoding ‘modes’ designated A, B, C
and D, while DRM+ broadcasters use
a fifth encoding mode designated (you
guessed it!) E.
Each of these modes is designed to
achieve the best performance in a different broadcasting application, as you
can see in Table 1.
You’ll also note from this table that
the main service channel or MSC (ie,
the digital audio channel itself) of both
DRM30 and DRM+ signals is generally
The idea behind this is that 64-QAM
can encode 64 points in its amplitude/
phase or “I/Q constellation”, allowing the subcarriers to carry five bits
of information in each digital sample
or ‘symbol’ – and hence a higher total bit rate.
However, the 64 points in a 64QAM constellation are inevitably
closer together in both amplitude and
This GR-216
DRM30
receiver has
been evaluated
by Tecsun Radios
(Aust) and they have confirmed
that it receives DRM transmissions from
New Zealand in Australia. This receiver also handles AM and FM reception.
modulated onto the RF subcarriers using the quadrature amplitude modulation (QAM) system. DRM30 broadcasters have the option of choosing either
64-QAM or 16-QAM coding, while
DRM+ broadcasters can use either 16QAM or 4-QAM.
phase, making it more susceptible to
data corruption, due to noise and interference.
In contrast, 16-QAM has only 16
points in its amplitude/phase constellation, so the individual points are further apart – making it more suitable for
noisy conditions, even though it can
encode only 4 bits of information in
each digital symbol (and hence a lower
overall bit rate).
The 4-QAM option available for
DRM+ takes this trade-off even further, allowing it to encode only two
bits per digital symbol and hence a
lower overall bit rate again. But that’s
not really too much of a problem
when DRM+ signals are encoded into
a 100kHz wide channel, as you can
see from Fig.1.
DRM’s three data channels
Table 1: the choice of frequencies, modes and coding options depends to a large
extent on the coverage distance desired.
64 Silicon Chip
The next thing to understand about
DRM is that each DRM broadcasting
signal consists of three basic data
channels.
There’s the Main Service Channel
(MSC), which generally carries the encoded digital audio data; then there’s
the Fast Access Channel (FAC), which
carries a set of data parameters allowing
siliconchip.com.au
Table 2: Australia is significantly lagging behind when it comes to DRM broadcasts
– this table shows Radio New Zealand’s DRM schedule to the South Pacific.
the receiving decoder to quickly confirm things like the modulation system
being used in the DRM signals.
Finally there’s the Service Description Channel (SDC), which carries
advance information like audio and
data coding parameters, program service labels, the current time and date,
and so on.
Fig.1 should give you a basic idea
of the way these three data channels
are grouped into the data stream transmitted in DRM30 and DRM+ digital
broadcasting.
The DRM30 modes group the data
into 1200ms-long “super frames” consisting of three frames 400ms long,
while DRM+ groups the data into
400ms-long super frames each consisting of four frames 100ms long.
In both cases the SDC data is transmitted across all subcarriers for a pe-
riod of two symbols at the start of each
super frame.
For the rest of each super frame, the
FAC data is transmitted using a specific
sub-group of subcarriers during each
transmission frame, while the coded
audio data in the MSC channel is
transmitted using all of the remaining
subcarriers, in parallel with the FAC
data for all of rest of the super frame.
DRM status world wide
While we haven’t heard much about
DRM as yet in Australia, it’s now well
established in the UK, many of the European countries, Canada, India and
Russia – plus in New Zealand.
Radio Australia did transmit DRM30
on shortwave to Papua New Guinea
from Brandon (Qld) but that ended in
March 2015.
Radio New Zealand International
This “Avion” AV-DR-1401DRM Digital Radio sells on Amazon in India for about
AU$330. Touted as India’s first DRM, it will also receive AM and FM broadcasts.
siliconchip.com.au
broadcasts DRM30 on shortwave for
about 20 hours per day, mainly to the
Pacific Islands.
Receivers capable of receiving
DRM30 are still in fairly short supply in Australia, and a lot of the DRM
reception to date seems to have been
using PC-based SDRs (software defined radios) – see our articles in the
November 2013 issue of SILICON CHIP
(www.siliconchip.com.au/Article/
5456 and www.siliconchip.com.au/
Article/5459).
However some of the European manufacturers like Morphy Richards have
been producing DRM30 receivers, and
Indian firm Avion Electronics (India)
lauched its AV-DR-1401 radio recently.
Chinese firm Gospell Digital Technology has also announced its GR-216
DRM receiver.
Other DRM receivers you’ll find on
the web are the Himalaya DRM2009,
the Technisat Multiradio and the Uniwave Di-Wave 100.
Why DRM30 for Australia?
DRM30 digital broadcasting is particularly suitable for Australia, because of its much larger range. For example a DRM30 broadcast transmitter
operating in the ‘AM’ band will have
a range virtually identical to that of
our existing analog AM broadcasters.
And a 250kW HF DRM30 transmitter
located in the geographical centre of
Australia (Kulgera, NT) could cover
just about all of the continent and surrounding waters.
A much lower power DRM30 transmitter located in the geographical centre of Tasmania (Liena) could similarly
cover the whole of that state.
So adopting DRM30 would be the
best way to ensure that ALL Australians received good broadcast radio
reception – even those living in or
moving through remote areas.
And this brings up another point:
DRM30 operating at HF provides much
better reception in moving vehicles
than either FM or DAB+ – which operate in the VHF spectrum.
Best of all, though, is that existing
AM and shortwave transmitters could
in most cases be converted for DRM30
broadcasting at very low cost.
The question really is this: why is
Australia dragging its heels and letting just about all the rest of the world
move into the digital radio future with
DRM30 – when we could join them
with very little outlay?
SC
September 2017 65
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