A Little DAB'll Do Ya -- Digital Radio an Almost
Invisible Reality in Canada
It was a pretty much unheralded birth. After a decade of
planning and testing, digital audio broadcasting (DAB) quietly slipped onto the Canadian
airwaves in the summer of 1999. Other than a few ads for the receivers themselves, there
was no hoopla to mark the launch. Now, radio stations are broadcasting digital signals in
Toronto, Montreal, Vancouver and Windsor, Ontario (strategically placed across the river
from Detroit), with the national capital the next target, but almost nobody knows about
them.
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
United States. 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. The little
brother of Musicam is the ubiquitous MP3.
To implement the establishment of digital radio in Canada,
a remarkable organization was set up in 1993. Digital Radio Research Inc. (DRRI) was
founded jointly by the Canadian Association of Broadcasters, representing the country's
private stations, and the government-run Canadian Broadcasting Corporation. Eventually all
the major private broadcasters would become members, a rare display of cooperation in a
sometimes fractious industry. As one hardware manufacturer, who has had many dealings with
DRRI, put it, "They don't even fight!"
This group, working closely with the federal regulators in
Ottawa, decided early on that the digital system would operate in parallel with the
regular AM and FM bands, at least for the time being, and so every existing station would
have a spot on the DAB dial from the start.
For that to work, 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 1.5MHz channels, called "pods", each of which
can carry five separate stations, occupying 300kHz 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 what DRRI calls
"value-added enhancements." These include 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 600 watts. When the system
is operating at its full capacity, that will rise to about 800.
The cities that now have digital radio account for about 35
percent of the country's population, and the CBC has said all along that it would try to
have 70 percent coverage within five years. It's been estimated that all stations could be
digital within seven years, and with all the major broadcasting companies involved with
DRRI, that's probably realistic.
What that means in the short term, however, nobody's
saying. The system will be extended to other cities, the expansion orchestrated by DRRI.
The focus of the radio companies, at least for now, is the
one audience they have exclusively: people in cars. In fact General Motors of Canada is an
enthusiastic supporter of the system, and will offer digital radios as an option in the
months ahead.
In North America, Canada is basically going it alone with
Eureka 147, although most overseas countries have adopted it in one form or another.
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,
and even the most optimistic promoters of DAB in the U.S. say that nothing will be
available for months or years. In the meantime, satellite-delivered digital audio from XM
and Sirius will soon be available to American listeners, which may make the whole question
of terrestrial digital radio moot in the U.S.
What happens in the U.S. will affect not only Canadians'
ability to do cross-border listening, but will undoubtedly have an impact on what sort of
listening equipment will be available. 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 an insurmountable
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.
Some of the stations included are suburban and, ironically,
they may not be audible in their home territories. Like the others, they will broadcast
from the downtown CN tower, 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 suburbia.
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.
In Europe, where DAB has been operational in several
countries for a number of years, all the major autosound manufacturers have digital radio
models, and that will ultimately be true here as well. For the past couple of years,
Pioneer seems to have the field to itself. Their unit is basically a "black box"
(although it's silver) that can be stashed under a seat or in the trunk, and connected to
the head unit by cable. With considerable foresight, Pioneer started making head units
that would accept the DAB box as far back as 1994, so owners of those units only need to
buy the DAB module and a special antenna.
For demonstrations of digital radio, both by their own
staff and by the various radio stations, Pioneer equipped a VW Beetle with a full digital
system. They named the car "DABug", and I had an opportunity to drive it for a
few days to check out the radio.
The first task was to switch off all the enhancements --
equalization, DSP ambience modes, etc. -- the Pioneer head unit provides, to achieve a
flat signal. (The bells and whistles can be fun, incidentally, but my purpose here was to
judge what the signal was like on its own.) Fortunately the Beetle is a very quiet car, so
with the windows closed and the air on, I had a reasonable chance to hear what was
actually going on.
The sound was impressive, as expected, with extremely wide
range and absolutely no noise. A single push of a button lets you switch between the
digital signal and its analog equivalent to check the difference. In a way it's not a fair
test because the digital signal is louder (it's not necessarily inherently louder, it's
just the way this unit was set up); when I switched from FM to digital, the signal seemed
to leap out at me, and that might well fool the unwary into thinking the improvement is
greater than it really is. When I took care to equalize the levels, I could still hear a
distinct improvement. I didn't bother to A-B the AM stations, but there the change would
be vast.
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
-- about 27 miles -- 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 for a few extra kilometers, and came in on the higher
points of land to the east of that.
While the signal is rock solid in its primary area
("bulletproof," one broadcaster calls it), 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 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. Most observers think it will be a long-term transition: "It will take as
long as it takes."
...Ian G. Masters
ian@mastersonaudio.com
|
