Data Slicers
Trunking Systems
Mobile Data Terminals
FLEX Paging
GNU Radio Hardware

This page provides a brief introduction and links to several radio signal decoding projects.


Almost all hobby data monitoring projects begin with a data slicer. This is really nothing more than a zero crossing detector, and can be done either in software or in hardware.

Many signals of interest are two-level FSK (frequency shift keying), and a simple op-amp circuit or Hamcomm-type interface will do just fine.

Sample sounds of many digital signals as well as a schematic for a 2-level interface can be found here.

A good diagnostic aid to check a the output of a two-level sliver can be found here.

More complex systems may use four-level FSK. Texas 2-Way sells their DDM-4P 4-level FSK decoder, although not everyone is happy with it, as you can read here.


Many public safety and service agencies are converting their radios to trunking systems. Although Uniden is marketing their TrunkTracker, a number of enterprising computer enthusiasts are using specialized software to decode the trunking messages transmitted between mobile handsets and base stations.

In early 1997 a document was posted to Usenet describing, in detail, how one such system works and provided the source code to a computer program that would decode Motorola-type trunking systems.

Since then a number of people have worked to improve the program, adding features and reliability. There is now also program to decode GTE's EDAC system.

The latest decoding programs are available here.

Note that there are a number of trunk-tracking scanners on the market. You can get more information about them by checking my Signal Harbor web site.


Many police departments use mobile data terminals to request and receive text information while on patrol. One common system is based on the Motorola MDT-4800 protocol, which you can read about here.


Wireless equipment producer Ericsson developed a terrestrial network protocol called Mobitex. In yet another Usenet posting, an in-depth description of the system and some source code for a program to decode unencrypted Mobitex transmissions is available here.

Currently, Cingular Wireless operates a Mobitex network in the United States.


The common POCSAG paging standard is being supplanted by a new, four-level signaling protocol called FLEX, marketed and licensed by Motorola.

An interesting FLEX decoding project used to be here, but has disappeared. Some additional information might be found here, although even he appears to have disappeared.


The GNUradio folks are using a Microtune 49X7 cable modem tuner (part of Microtune's 4937 DI5 RF tuner module series) with a 5.75 MHz IF and Measurement Computing PCI DAS4020/12 12-bit 20 MSPS A/D card to do some interesting software defined radio (SDR) work.

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Links updated May 1, 2003