Digital Radio a
[Hidden] Reality
It was a pretty much-unheralded birth. After a decade of
planning and testing, digital audio broadcasting (DAB) quietly slipped onto the Toronto
airwaves a year and a half ago. Other than a few ads for the receivers themselves, there
was no hoopla to mark the launch, and in the intervening months, most of the stations that
are broadcasting digital signals have kept the news pretty much to themselves. Although I
haven't checked, the situation is probably similar in Canada's other digital cities:
Montreal, Vancouver, Ottawa and Windsor, Ontario.
In the 1980s, it became obvious that the trend in all media
was to become digital, and radio broadcasters around the world began to work on what they
hoped would be an international standard for DAB. What resulted was a system called Eureka
147, developed mainly in Europe but with substantial contributions from Canada and the
U.S. In 1990, the first North American demonstrations of the standard were conducted in
four cities across Canada.
One major Canadian contribution to the acceptance of the
standard was a series of comparative listening tests among various possible data reduction
systems, which took place in 1992 in Ottawa. The aim of the system is to produce
CD-quality audio over the air, but bandwidth limitations dictated that some form of
digital compression be employed, so it was important that the one chosen be transparent. A
series of experienced listeners, including me, spent several days listening for the
tiniest of flaws. At least one system appeared to have no artifacts, and that one -- the
form of MPEG audio known as Musicam -- is the system used in Eureka 147.
In Canada, as in most countries, a new part of the radio
spectrum had to be allocated to DAB, and the very high-frequency "L Band" (1452
to 1492 MHz) was chosen. This is divided into 23 1.5 MHz channels, called
"pods", each of which can carry five separate stations, occupying 300 kHz of
space.
That bandwidth is adequate partly because Musicam uses a
6:1 compression ratio, and because the presence of a number of stations on a single pod
means that a signal that momentarily places a heavy demand on the bit rate can usually
borrow some from another station.
One advantage of digital transmission is that there can be
several separate versions of the same signal on the same frequency, either intentionally
or inadvertently, and the receiver will lock onto the strongest and ignore the others.
That means multipath-free reception in the city and also permits the use of supplementary
transmitters to reach areas where the main signal fades. Broadcasters can use such
transmitters, if necessary to duplicate their analog radiation pattern, without further
licensing.
Down the road, a combination of satellite and terrestrial
delivery is possible, as is a continuous cell-phone-like network along the major highways,
which would allow uninterrupted listening as you travel, the receiver jumping seamlessly
from transmitter to transmitter.
There's also enormous potential for things like displaying
song titles and other information about the programming, continuous traffic reports and
other material. With the proper interface, it could even be used for color graphics that
could be displayed on a computer.
One environmental benefit of digital radio is energy
conservation. While conventional analog stations put out tens of thousands of watts of
power, the digital transmitters at the moment deliver just over 800 watts.
The cities that now have digital radio account for about 40
percent of the country's population, and it's been estimated that all stations could be
digital within seven years, and with all the major broadcasting companies involved, that's
probably realistic. The inclusion of Windsor is because of its proximity to Detroit, which
gives it an opportunity to showcase the technology to a large chunk of the American
population, as well as the car companies.
The automobile industry is important because the focus of
the radio companies, at least for now, is the one audience they have exclusively: people
in cars.
More important, perhaps, is to expose the system to part of
a country that has become odd-man-out when it comes to digital radio. Originally
supportive of Eureka 147, American broadcasters soon dropped it in favour of an
"in-band, on-channel" (IBOC) system that would let them piggyback a digital
version on their existing analog signal. The trouble is, no workable system has emerged
although the major contenders in developing one have merged into a single company and will
undoubtedly come up with one standard.
Even the European version of DAB is slightly different from
Canadian Eureka 147: it uses a different part of the radio spectrum, so European radios
can't simply be imported and used here. That's not a big problem, however, as
modifications are fairly simple.
In Toronto, some two dozen stations are on the air
digitally, including the four CBC outlets and all the major commercial stations. Because
several stations would share each pod, there had to be a high degree of cooperation among
competitors, but in fact that already existed; the FM transmission has been done jointly
for years.
The roster includes a few suburban stations. Ironically for
them, they may not be audible in their home territories. Like the others, they will
broadcast from the CN tower in mid-Toronto, and signals from there don't reach as far out
as FM. For now, the range is line-of-sight from the tower, which means Toronto itself and
only the closer bits of the surrounding area.
And, while the transmission is fully digital and capable of
CD-quality sound, the actual signals provided by the stations themselves vary somewhat.
The ideal would be to tap the signal before any processing -- equalization, compression
and so forth -- which is still necessary for the analog version of the signal, but that's
not possible in every case, so some stations are "straight" and others
processed. There is still the odd bit of analog production equipment in use as well.
The most dramatic improvement is with AM stations, whose
digital versions are indistinguishable from their FM counterparts, especially in the case
of stereo-AM outlets. Some AM talk stations have retained their mono format, but this
gives them a much greater amount of bandwidth to be used for non-audio information.
For demonstrations of digital radio, both by their own
staff and by the various radio stations, Pioneer Electronics equipped a new VW Beetle with
a full digital system. They've named the car "DABug", and I had an opportunity
to drive it for a few days to check out the radio.
The sound was impressive, as expected, with extremely wide
range and absolutely no noise. A single push of a button let you switch between the
digital signal and its analog equivalent to check the difference.
As I drove around the city, there was no hint of multipath,
fading or any other nastiness. I was particularly interested to see what kind of range the
signal had as I live in what is considered, in radio terms, a fringe area (43 kilometers
north east of the CN tower, to be precise). As it happened, the signal quit about half a
kilometer from my door. Farther south, roughly on a line with the tower, the signal was
constant to about the 50-kilometer mark, and came in on the higher points of land beyond
that.
While the signal is rock solid in its primary area, and
silent beyond it, it does do some strange things in the transitional zone. Just before it
mutes, the sound takes on a really strange gurgling sound; you can still hear the music,
but it's underlaid with this distortion. After it quits, it tends to flash back on
momentarily as you crest small rises in the road or, perhaps, as the antenna picks up
stray reflections. None of that can be held against the system, of course, but it is
interesting to find out what happens when you push it beyond its limits.
Will DAB ever supplant AM and FM completely? Unlike digital
television in the U.S., there's no plan in Canada to reclaim the older bands and reassign
their frequencies on a specific timetable. Going in, the idea was that the analog signals
would be maintained for as long as anyone wanted to listen to them, which might be forever
or might not. One spokesman thinks it will be a long-term transition: "It will take
as long as it takes."
...Ian G. Masters
ian@mastersonaudio.com
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