Digital Signals Decoding Interfaces

Last Updated on: 01/27/2002

All of the Applications featured on these pages require either the 2-Level FSK interface or the 4-Level FSK interface.  You must build this interface, or buy it from some of sources listed at the bottom of the page.  NOTE that these interfaces DO NOT provide computer control for your scanner, but rather they interface the output from your scanner so that the computer and application can decode the digital signals. 

These interfaces connect to the discriminator output on you scanner (and ONLY the discriminator output)  This is usually a connection inside of your scanner that must be made with a soldering iron.

The Discriminator

The Discriminator output from a scanner is the raw, unfiltered signal that a scanner produces before it is sent to the audio stage for output through the speaker -- also called the "baseband audio."  This discriminator source is required for all of the applications featured by this page. Most scanners must have a simple modification made to them to output the discriminator audio output to the interfaces outlined on this page.

The 2-Level Interface

The 2-Level Interface (also called the Hamcom interface) is required for most of the applications that are available for decoding digital signals today.  See the applications page for further information on which interface is required for each application.  There are numerous different version of this interface, but they all accomplish the same task. 

The most popular version of the hamcom interface.   See this picture of how to build this interface using a Radio Shack experimenters board (thanks to Mitch Savage KJ5NL

Here is a schematic of the 2-Level interface designed around the TL082 Op-Amp.   This version of the interface works very well!  (This is the version that I use, and it works wonderfully)

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Here is another schematic of the 2-Level interface based around the MAX232 chipset.  This interface is very reliable and is reported to work VERY well with Trunker and Etrunker.

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The 4-Level Interface

The 4-Level Interface is a new generation of interfaces that are required for the faster digital modes.  For decoding Flex paging and RD-LAP MDT modes, this interface will be required.  The only production application today that uses the 4-Level interface today is Pocflex.  This interface will however have further uses when APCO-25 decoding becomes possible.

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Here is a schematic of the 2-level and 4-level interface combined into one interface.

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Another version of the 4-level interface with power supply can be found below:

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Also see this link for PCB Board art work for the 4-level FSK Decoder:

Experiences and information from folks who have gotten the interface working, their testimonials (Tips, Tricks, FAQs, etc)

If you are interested in buying the interface instead of building it, here are links to some vendors: