Fldigi, the Fast Light Digital modem application, is a free and open–source program which allows an ordinary computer’s sound card to be used as a simple two–way data modem. The software is mostly used by amateur radio operators who connect the microphone and headphone connections of an amateur radio SSB transceiver or an FM two way radio to the computer’s headphone and microphone connections, respectively.

This interconnection creates a “soundcard defined radio” whose available bandwidth is limited by the soundcard’s samplerate and the external radio’s bandwidth.

Such communications are normally done on the shortwave amateur radio bands in modes such as PSK31, MFSK, RTTY, Olivia, and CW (morse code). Increasingly, the software is also being used for data on VHF and UHF frequencies.

Using this software, it is possible for amateur radio operators to communicate worldwide while using only a few watts of RF power.

Fldigi software is also used for amateur radio emergency communications when other communication systems fail due to natural disaster or power outage. Transfer of files, emails, and FEMA ICS forms are possible using inexpensive radio hardware.
  — Wikipedia

The SATERN International Digital Net makes use of the FLDIGI NBEMS software suite from W1HKJ, Dave Freese. NBEMS is an acronym for Narrow Band Emergency Message System. It consists of a suite of applications that work together to provide considerable capability to the amateur operator.

Fldigi is a computer program intended for amateur radio digital modes operation using a personal computer. Fldigi operates (as does most similar software) in conjunction with a conventional HF SSB radio transceiver, and uses the PC sound card as the main means of input from the radio, and output to the radio. These are audio-frequency signals. The software also controls the radio by means of another connection, typically a serial port.

Fldigi is multi-mode, which means that it is able to operate many popular digital modes without switching programs, so you only have one program to learn. Fldigi includes all the popular modes, such as OLIVIA, THOR, DominoEX, MFSK16, PSK31, and RTTY.

Unusually, Fldigi is available for multiple computer operating systems; FreeBSD, Linux, OS X™ and Windows™.

The main window of the FLDIGI application is shown here. However, the NBEMS suite consists of not only FLDIGI providing the digital modems, but FLMSG (the text message creation tool), and FLAMP (the message broadcast application). Additiopnal applications are available from the same source HERE which will provide rig control, Net Logging, and extensions to the NBEMS suite from 3rd parties.


The Advanced NBEMS Software Suite - Part 1

The acronym NBEMS stands for Narrow Band Emergency Message Software. A quick exploration of the tools available in the suite of programs will quickly justify this moniker.

Let’s take each piece one at a time and see how it fits into the overall SATERN International Digital Net operating strategy (i.e. "Get the message through").


When we send digital messages, the over-riding focus is to get the message to the receiver error-free as sent. To do that requires constant error checking and perhaps even data resending.

FLMSG running on Linux Mint 18.2 with a Mac OS X theme

FLMSG is a text message formatting program that takes a plain text message and wraps it in formatting and error checking text tags. For instance, to designate a message is compressed with binary 64-bit compression, a tag may be inserted that looks similar to; [b64 80 13] . It would then represent a text compression of 64-bit compressed text formatted into 13 blocks of 80 characters each. To signal the end of this compression, the [end b64] tag could be used.

This example is an over-simplified demonstration of how FLMSG is able to send formatted text using nothing but the 256 charater ASCII or UTF-8 character set instead of binary data.

FLMSG contains forms that take data to be formatted into recognizable well-established forms such as ICS-213 or the ARRL Radiogram, Red Cross forms, and many others as well as completely custom forms of a simple nature.

These forms when filled out and sent with FLDIGI as the modem, are easily verifiable. If you need to broadcast the form to multiple locations, the FLAMP application provides a compression/block-send method that self-verifies. Sending error-free peer-to-peer (P2P) is accomplished with FLARQ in the fully syncronous Automatic Repeat Request mode - albeit with some inefficiencies inherent in this mode.

The image shown is of the ICS-213 formatted message entry form. When the form is used to enter the data for a ICS-213 formatted message, the output will look entirely different:

            AD5XJ 20172210132911
            :inc:36 Incident Name (e.g. Hurricane Maria)
            :to:22 Person or organization
            :p1:34 Position of Individual or in group
            :fm:26 Person sending the message
            :p2:26 Position of person sending
            :d1:10 2017-10-22
            :t1:6 13:28Z
            :sb:22 Subject of the message
            :s1:5 AD5XJ
            :p3:23 SATERN Digital Net Mgr.
            This is a messsage in ICS-213 format.

This formatted text has the plain text from the entry form, with formatting text injected into it. When this message is received error-free the receiver station need only open it with FLMSG, view the document with HTML, and either print it as is, export to a PDF and/or email over WinLINK or Internet as an attachment to another document.

HTML View of the message above

This is an extremely small ICS-213 message of only a few words. More elaborate documents may contain hundreds of words and thousands of characters. This is of concern to FLMSG/FLDIGI users on HF because of the length of transmit time when using some of the slower (albeit more robust) digital modes like OLIVIA.

Use of non-error correctiong modes like PSK-32 are highly discouraged due to the unreliability and high error rate in high-noise environments. On VHF and UHF noise is rarely a consideration and the error-correcting versions of PSK may be used in very high speed sub-modes with success.

[Author Note: PSK modes do not play well with compression. Use of compressed text should be avoided when sending in any PSK mode.

The premise, "use the highest speed mode for reliable communication" is well served when considering conditions on each communication channel.





Up to date instruction documentation from the developers of FLDIGI for the most current versions are provided in this series of links:

Beginners’ Guide to FLDIGI
FLDIGI Online User Manual
FLAMP User Manual (PDF download)
FLMSG User Manual (PDF download)


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