Author: Bill

When fire agencies began moving to trunked 700-800 MHz radio systems in larger numbers 5-7 years ago, I remember reading a study of similar systems used in

Australia which concluded that these systems have no business being used for emergency services, and that they should only be used, for example, for garbage pickup.

From Mobile Radio Technology (April 1, 2008):

On April 16, 2007, firefighter Kyle Wilson was part of a crew dispatched to fight a residential fire in Woodbridge, Va. He died in the line of duty.

A detailed report on the incident recently released by Prince William County Department of Fire and Rescue concluded that problems associated with the use of the county’s Motorola digital trunked radio system contributed to the tragedy. Issues reported by other firefighters during that incident, which was further complicated by strong winds, ranged from signal distortion and transmission failure to radios displaying “out of range” signals.

Fire safety advocates now are encouraging fire departments across the country to study the incident in hopes that future tragedies could be avoided. Prince William County’s fire department, through further tests, concluded that digital portable radios are “extremely vulnerable to poor environmental conditions and interference of digital noise from ambient sources, which negatively impact the ability of emergency personnel to effectively communicate.”

A handful of fire and police departments, fearing the loss of lives, have opted to continue using analog systems even when the rest of their county’s emergency personnel are using digital trunking systems.

The common complaint, which most affects fire departments, concerns the digital vocoder’s inability to differentiate between a voice transmission and background noise – whether a chain saw, sprayed water or personal alarm. Background noise renders the voice transmission distorted and often unintelligible. Another critical problem is that digital radios lose contact inside buildings. “In most cases, it is a very political and sensitive position to abandon expensive technology and go back to something that is old,” said Daryl Jones, owner and president of Telecommunications Engineering Associates, which manages public safety systems throughout the San Mateo area in California. “But many agencies are finding that complaints from line personnel, both in fire and police, are so significant.”

The Boise (Idaho) Fire Department spent about $1 million two years ago on mobile and portable radio equipment to join a cutting-edge countywide 700 MHz digital trunking system. While training users on the system, the fire department discovered problems with voice intelligibility when a firefighter’s low-air alarm went off. That led the department to investigate the issue further, and it found more instances where alarms interfered with the quality of voice transmissions. Today, the department and other fire departments in the county remain on analog VHF radios while the rest of the county operates on the 700 MHz digital trunking system.

“Right now our dispatch center wants to dump VHF,” said Paul Roberts, a captain with the Boise Fire Department, “[and] we are trying to look at alternatives to at least get on a system that will lessen the load on dispatchers having to patch all of this together. … But until there is a solution to the digital processing of speech when you have competing noises, we have to stay on analog.”

The problems associated with digital systems became known in 2006. Since then, the International Association of Fire Chiefs (IAFC) established a Digital Problem Working Group and appointed Chief Charles Werner of Charlottesville, Va., to serve as its chair. So far, the working group has explored the creation of a best practices solution to work around the problem until a long-term solution can be found. Prince William County’s findings have been forwarded to that group for inclusion in the process.

The National Institute of Standards and Technology (NIST) also is analyzing the problem, as are radio manufacturers such as M/A-COM and Motorola. They are expected to jointly release a formal analysis – in conjunction with the IAFC – that encompasses best practices to help departments to minimize the problems.

“We’re running through this scientifically and hope to distribute a wrap-up summary shortly,” Werner said.

Roberts, who chairs the IAFC testing group, says the testing – conducted by radio engineers – involves taking words that sound alike and requiring the listener to distinguish which word is being said over the background noise of chainsaws and hose sprays.

Motorola declined to comment, saying it was cooperating with the testing and awaiting the conclusions from NIST and the IAFC.

But Chris Lougee, vice president with LMR vendor Icom America, thinks older technology would help solve these background noise problems. “Everyone knew from the beginning that the P25 vocoder was a half-rate vocoder. As you speak into the microphone, you are converting human voice into a data stream that is reassembled at the end,” Lougee said. “TIA … is encouraging a move to a full-rate vocoder, which we are doing. It vastly improves the amount of audio and quality.”