Clearing the air
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.”
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