Be heard with your BA
Published: 01 September, 2008
Could your BA equipment interfere with your portable digital radio? According to the International Association of Fire Chiefs (IAFC), it could do – and in the US work is continuing to isolate the problem and find technical ways to resolve it. Fire services take note.
The IAFC formed a Digital Project Working Group following reports of communication problems in 2006-2007 involving digital two-way portable radios in close proximity to fire ground noise.
Firefighters were reporting unintelligible audio communications while using digital radios in close proximity to low-pressure alarms of their self contained breathing apparatus. Other fireground noises such as power tools, diesel engines and personal alert safety system devices were also reported to affect voice intelligibility.
After months of testing by the US National Institute of Standards and Technology and the National Telecommunications and Information Administration, the report Intelligibility of Selected Radio Systems in the Presence of Fireground Noise: Test Plan and Results, was published at the end of June this year.
Results indicated that in some environments analogue radios performed better than the digital radios tested, and the report recommended that this information should be considered whenever a US agency was preparing to purchase and deploy a new communications system.
Worryingly, the problem is not specific to any one radio manufacturer in the US – it is an industry wide issue, and it includes all equipment on the fireground.
The Digital Project Working Group (DPWG) of the IAFC has suggested a number of best practices to maximise user safety and efficiency.
One such recommendation is that fire services get actively involved in the design and development of their communication system from the beginning.
Additionally, standards and guidelines should be developed for scenario-based user training using communications equipment – no such standards exist at present.
The tests
During testing by the Institute for Telecommunications Sciences (ITS), three systems (two vocoder technologies – Project 25 Full Rate and P25 Enhanced Full Rate – and one analogue, 25kHz FM) were assessed for intelligibility in different noise conditions.
P25 digital radios were developed by the Association of Public Safety Communications Officials and the Telecommunications Industry Association to standardise digital radio and promote interoperability among digital land mobile radio systems.
Scenarios tested included:
- mask, with no background noise
- mask, two PASS alarms
- in-mask low-air alarm
- mask, rotary saw cutting metal garage door.
The tests showed some alarming results, including that there are significant radio communication challenges regarding SCBA and other equipment. “In the case of a mask with no additional background noise at the transmission site, there was a notable degradation in intelligibility for the digital systems”, said the report.
Some SCBA low-air alarms and PASS device alarms also caused “significant intelligibility degradation” during testing.
The 25kHz analogue system – significantly – was either statistically similar or better than digital, in all scenarios tested.
The 25kHz analogue spectrum, however, is being phased out in the US by the Federal Communications Commission, who has mandated that agencies move to 12.5kHz channels by 1st January 2013. The move is a result of a “narrowbanding” drive by the FCC, with the aim of increasing the number of channels (and users) on public safety frequencies in the VHF and UHF bands, by forcing fire services to stop using their 25kHz channels and move to new 12.5 KHz channels.
From this year, US agencies have not been able to purchase any type of new portable or mobile radio capable of wideband.
The report acknowledges that unintelligibility is a critical issue for firefighter safety because firefighters routinely work in noisy environments.
“Any improvements that recognise and can improve the public-safety working environment and increase communications intelligibility should be considered a high priority. This may include both reduction in noise emissions on the fireground as well as modifications to reduce the noise impact on the communications systems.”
The report acknowledges that the US fire service has had no significant participation in the process established to incorporate user requirements into the standards development. “The fire service must be an active participant and provide feedback on these public safety communications issues, including narrowbanding, P25, vocoder development and similar issues.”
The vocoder used in P25 is thought to lie at the root of the current intelligibility problems. The vocoder is essentially a filter that codes speech for transmission, which is then resynthesised at the receiving end. When background noise is stronger than the voice, the voice can be overridden and audio transmissions can become unintelligible. The vocoder was selected in 1992 after a number of tests – known as the Improved Multi-Band Excitation – and it was standardized by TIA as TIA-102.BABA.
Throughput data rate of the vocoder is limited to regulations set forth by the FCC due to narrowbanding and other spectrum related issues and restrictions. Changing the vocoder data rate therefore cannot be done without regulatory changes, and it would also impact available radio spectrum.
Although a new enhanced vocoder has been developed and tested, it is not an easy retrofit due to space requirements and design changes that would be needed.
