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Date Reported : |
06/02/2004 |
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Agency/Organization : |
USFS |
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Event Date : |
03/31/2004 |
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Local Time : |
04:00 PM |
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Incident Name : |
COMMUNICATIONS |
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OR |
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Jurisdiction : |
USFS |
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Wildland |
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Other |
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Position Title : |
Electronics Technician |
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Task : |
Radio Communications |
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Management Level : |
1 |
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Resources Involved : |
Equipment, Radio |
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Contributing Factors : |
Communications; Equipment |
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Human Factors : |
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Other Factors : |
wideband into narrow/digital radio |
| Describe in detail what happened including the concern or potential issue, the environment (weather, terrain, fire behavior, etc), and the resulting safety/health issue. |
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Wide band analog going into a Daniels narrowband/digital repeater does cause problems. When we were at Daniels MT-4 Digital Radio training, I brought it up and the instructor gave us a demonstration that when deviation gets to be more than 4.2 KHz the audio shuts down. This is due to the way the Motorola chip processes voice, discussion brought up that Motorola digital radios start to do this at around 3.75 KHz.
Narrow band analog has a voice modulation of 2.5 KHz voice plus tone. Wideband analog radios are set for a modulation of 4.5 KHz +/- 500 Hz voice plus tone.
When I was messing with a MT-4 Daniels repeater here at the shop, it shut off audio at about 4.0 KHz with a 1KHz tone. Using a handheld radio I was able to get it to shut down, but it also has a really fast recovery time. It starts sounding metallic and distorted until the repeater audio shuts down. I could see on the O-scope where it was shutting down and coming back.
When people get excited they have a tendency to talk louder which would further aggravate the situation and no audio would be heard.
There is no distortion when deviation exceeds what the Motorola chip can handle because there is no audio. The older narrowband RPTRs did not have this problem due to not having the digital chip in it.
In reading some recent SAFENETs, this sounds like what the users are experiencing and not knowing what is going on. All they know is some communications get through sounding bad and some do not.
Here is some technical info a co-worker came up with.
My own voice modulating a King EPH5992A wide-band handheld does not reliably come through the Daniels MT-4 narrow-band repeater. When I speak loudly the audio is completely blocked. When I speak in a medium voice the audio blanks out multiple words. When I speak softly the audio comes through with just a little bit of distortion.
I injected a -90 dBm RF signal into the Daniels MT-4 narrow-band receiver. I modulated the signal with a 136.5 Hz tone to 0.75 kHz deviation in addition to the test tones.
With a 500 Hz test tone, the distortion was 17% when the deviation was 3.6 kHz. When the deviation was increased to 3.7 kHz, the receiver squelched. The receiver didn't unsquelch again until the deviation was reduced to 3.1 kHz, at which point the distortion was 6.5%.
With a 1000 Hz test tone the distortion was 7% when the deviation was 3.8 kHz. When the deviation was increased to 3.9 kHz, the receiver squelched. The receiver didn't unsquelch again until the deviation was reduced to 3.2 kHz, at which point the distortion was 6.5%.
With a 1500 Hz test tone the distortion was 7% when the deviation was 3.9 kHz. When the deviation was increased to 4.0 kHz, the receiver squelched. The receiver didn't unsquelch again until the deviation was reduced to 3.4 kHz, at which point the distortion was 6.5%. |
| Reporting Individual : please describe actions you took to correct or mitigate the unsafe/unheathful event. |
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Declare wideband analog radios not approved for Federal fire use.
Get the contract writers to amend that radios need to be narrow band analog capable. At least by January 2005, should have been for the 2003 season.
I know it is drastic, but there is a big problem in the way the Motorola chip in the Digital/Narrowband Analog radios handle more modulation than they are designed for. |
AGENCY CORRECTIVE ACTIONS |
| Reserved space for Agencies Supplemental corrective Actions. |
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Declare wideband analog radios not approved for Federal fire use.
Get the contract writers to amend that radios need to be narrow band analog capable. At least by January 2005, should have been for the 2003 season.
I know it is drastic, but there is a big problem in the way the Motorola chip in the Digital/Narrowband Analog radios handle more modulation than they are designed for. |
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