Two-Tone Pager Decoding Using Multimon

Here is a handy Linux based project that I recently worked on for a friend who happens to be a volunteer fire fighter.

A lot of rural volunteer fire departments still rely on the Motorola two-tone sequential paging system and analog Motorola Minitor pagers for dispatching their crews to a fire scene. The standard "Motorola Quick Call 2" paging protocol consists of playing two separate audio tones, the "A" and "B" tone. The "A" tone is played first for one second, then the "B" tone for three seconds. Both of these tones are transmitted on the fire dispatch frequency (VHF usually) which the pager is tuned to. Inside the older Minitor pagers, a mechanical reed is used to filter and decode each of the proper tones. While this may sound primitive, it is actually very reliable. A modern tweak to this type of paging system would be for the fire dispatch page to also be sent to your computer or cellular phone via text or email message. That is what this project will attempt to cover, with the pager tone decoding being done in software instead of having to tie up an additional pager.

For the tone decoding software, I used a slightly modified version of Thomas Sailer's multimon Linux radio transmission decoder.

{Update 2013}
Someone forked the original Multimon.  The original version was badly in need of updating for compatibility with modern Linux installs.  The fork is called multimonNG.


http://dekar.wc3edit.net/2012/05/24/multimonng/

In addition to showing how to modify the source code to match the tone sequences you want to monitor, there is a patch to enable a "quiet output" option to the DTMF decoding and also flushes stdout for better reliability when used in this application. A potential Perl script to trigger an external commands such as start recording or send a text message/email to ones phone is included.

You can read more about the specifics here.

Amateur Paging

Back in the late 80s through the 90's, two-tone paging was pretty popular. Many repeater controllers were setup to send these. Hams caring around the popular Minitor pagers on their belts could be summoned via a touch-tone invoked setup on the repeater. You don't see much of that anymore for whatever reason. The only people you noticed still being voice paged are usually volunteer fire / EMS guys.

The idea of digital paging always interested me, but it never seem to have gotten much attention in the ham radio world. Perhaps now that digital voice formats are emerging it's time to touch briefly on the idea again.

Back in the 90's Clive Cooper, G8UNK wrote a software POCSAG digital paging encoder. Kantronic's also came out with the KPC-9612 TNC cable of encoding POCSAG. (If you are wondering what POCSAG is, it's a well known standard for radiopaging)

Paging is kind of a one way deal, much like those beaconing the APRS locations. If this sounds interesting, here is a link for more information:

http://www.qsl.net/kb9mwr/projects/pager/plan.html