VA Hospital P25

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realgeo

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Good morning, everyone!

Once upon a time, there was a thread in this forum relating to unknown P25 traffic on 408.0000 in Omaha.

After listening for the past few days, it's clear that this is being used by VA Hospital Security Police. In fact, they have a phone patch connected to the system where people who have received tickets can contact the Police to arrange payment, court dates, etc.

If it's one thing that P25 really doesn't seem to pass well, it's DTMF and single-frequency tones. When the phone patch comes up, it emits a long ~1KHz. tone, whose sound improves over its 10-second interval. However, at the end of the call, the Officer taking the call uses DTMF tones to knock the phone patch down and from the sound of those tones, it's a wonder that they can be used to control anything!

73's
 

obijohn

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Nov 5, 2006
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Greetings George...I have not had an opportunity to play with DTMF over P25 yet. I suspect that I need to do this in the near future. Anyway, I have been led to believe that the vocoder doesn't like DTMF.

The fact that it is working in a commercial application intrigues me. I suspect that the subscriber units send only short DTMF bursts, therefore the DSP doesn't "average" them out and cancel them. The old "Bell Labs" standard only required about 150 ms. bursts to work, but with modern PBX switches who knows if they are held to the old standard?

Your "10 second" observation is worth further testing. Thanks for the update! I will start listening to the VA at work.
 

NeFire242

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Mar 30, 2006
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Nebraska
Yeah take two cellphones and try it, or even an analog vs digital radio and try listening to say fire tones, etc. Its horrible. Thats why they choose not to drop tones over digital.
 
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