DTMF alerting with portable radios

[isf83]

Gentlemen,

I've had success using DTMF encoding via a zetron encoder for fire station alerting. It's a faster technology than two tone encoding, and we can hide some extra information in the DTMF string that the decoding equipment can use to perform various relay activated tasks.

Here's my problem.

In my current system installation, I'm using DTMF with wildcards to alert battalion chiefs with portable radios in the field. The repeater site is decent and covers the area well; however, I seem to have hit or miss success with alerting those portable units. I'm guessing that their decoding unit isn't getting sufficient rx signal when indoors (which is when these chiefs are using these for alert mode). The previous installation was two tone encoding via Minitor V pager.

I don't want to go back to two tone encoding just because of one small piece of the pie in this system that's not working. Any suggestions? We're talking VHF here.

Thanks

[Bill_G]

Yep. Been there, done that with dtmf systems. Under low signal conditions, dtmf can fail easily. Some automated systems like Speedcall employed ack to ensure full strings were delivered and decoded correctly. Call alert doesn't have automated resend. Some of the new mobiles and portables have ack tone upon decode, but there is no corresponding automation in the consoles to retry X times until timeout or acknowledgement. I haven't played with a Zetron dtmf encoder. So, I don't know what it's capabilities are. But, if you can slow down the digit speed - ie lengthen the tone periods - you should have better success. Alternately, you may have to blindly send them twice alerting some units two times hoping to reach those that failed to decode on the first attempt.