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Wavelengths
Clearing the air
By Glenn Bischoff
May 2, 2008
In the April print edition of MRT, contributing writer Lynnette Luna
reported on some of the problems fire departments across the country are
having with digital radios. A common complaint concerned the inability
of the digital vocoder to distinguish between a voice transmission and
background noise typically found on the fireground, such as sprayed
water, chain saws and personal alarms.
Apparently, this article caused quite a stir, primarily due to comments
attributed to ICOM America Vice President Chris Lougee, who not only
said that use of a full-rate vocoder would solve the problem, but that
TIA is encouraging such use. I heard from a few people on this,
including Charles Werner, chief of the Charlottesville (Va.) Fire
Department and a member of MRT's editorial advisory council, who said
Lougee is off base, because a full-rate vocoder would be too spectrally
inefficient.
“People have this expectation now that a full-rate vocoder is possible
… and that we’re going to check it out, which is not the case,” he
said.
But when I talked to Lougee a couple of weeks ago, he stood by his
comments, and told me that TIA’s vocoder task force currently is
studying whether enhanced full-rate and enhanced half-rate vocoders
would improve the quality of the digital signal in a high-noise
environment.
I spoke this week with Craig Jorgensen, APCO P25 Project Director, in an
attempt to clear up the confusion. Jorgensen said it already has been
decided that all new P25 Phase I radios (operating in 12.5 kHz channels
and utilizing FDMA technology) will use the enhanced full-rate vocoder
and that new Phase II radios (operating in 6.25 kHz channels using
2-slot TDMA) will use the enhanced half-rate vocoder.
According to Jorgensen, most manufacturers of Phase I radios “will be
able to achieve that objective by the end of 2008.” (Indeed, Lougee
told me it is ICOM’s intention to introduce an enhanced full-rate
vocoder into its Phase I radios this year.)
Standards work on both enhanced vocoders already is “earnestly
underway,” Jorgensen said. But no decisions will be made until after
the International Association of Fire Chiefs finishes its fireground
noise study, Jorgensen said. “We want to clearly identify the problems
… so we’re not dealing with folklore information or hearsay.”
Jorgensen cautioned that it might not be possible to completely
eliminate the possibility of noise swamping the vocoder. “But we can
make improvements, if we have some data … so we can attack it from a
scientific approach.”
Mike Heavener, owner of MT Communications, a two-way radio dealer in the
Washington, D.C., area, offered a possible solution. He suggested
replacing the standard speaker microphone—which resides outside a
firefighter’s breathing mask, making it more prone to the effects of
noise—with a throat microphone, which he said has been around since
World War II.
But while a throat microphone might be effective in some situations, it
too has its limitations, according to Jorgensen.
“When a firefighter’s low-air alarm goes off, it’s a mechanical
reed, and when that reed begins to vibrate, it’s also going to vibrate
in the throat and bone, so it actually will enhance the problem.”
Jorgensen said that placing the microphone inside the mask is another
possible solution. “But you’d have to figure out how to do that
without running wires. There’s probably a new technology that’s
going to have to be inserted into the mask to make it work.”
The good news for firefighters is that Jorgensen and his colleagues are
committed to finding a solution. Speaking of the IAFC report that is
expected to be delivered this month, Jorgensen said, “Quite honestly,
we don’t expect that we’re going to be happy with what we see, and
we expect we’re going to have to implement changes to ensure that
firefighters—in particular—are protected.”
E-mail me at glenn.bischoff@penton.com.
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