An introduction
fldigi is a program that incorporates many different digital modems and allows radio amateurs to transmit and decode audio messages using these different modes. While fldigi is basically alone in its field in the way of capability, it lacks some user friendliness and has many Ham Radio operator specific features that may not be relevant to all users.
In this guide we will not be discussing fldigi in depth, but will be walking through the usual decoding process as it pertains to online ARGs (alternate reality games) and other media where you have an audio file available instead of a radio.
In particular, there are many features of the UI that are not used and not covered in this guide (as above). For the sake of this guide, all settings and options will be left default unless otherwise noted.
Getting familiar with the UI
One thing most newcomers have difficulty with is the amount of options and information that fldigi presents on one screen, which can seem intimidating and unorganized. It's important to keep in mind that fldigi was made to put commonly used options at the fingertips of your typical Ham Radio operator, so many of these options you may not use, and you can expect the essentials to appear among the chaos.
In the above you will see a description of the options this guide will cover.
File management
In fldigi you can have files selected for input and output at any given time. If you select a file for input (or 'playback') it will be played over a spectrogram (by default, a waterfall) near the bottom to enable better analysis and decoding. If you select a file for output (or 'generate') then it will be used to save WAVE audio of any messages you transmit. After you are done with the file you should deselect the file - for output, this will finish saving the file.
To select a file, open the File menu and then the Audio menu inside of it and click the option corresponding with what you want to do (Playback plays a file for decoding), (TX generate opens a file for writing your transmitted messages). Keep in mind that WAV (WAVE) files are recommended for use in fldigi. MP3's may need to be converted to WAV.
Once you have selected an option and opened or saved a file, you will notice the option stays checked in this menu the next time you open it. This indicates that the file is still open for reading or writing. When you are finished playing or recording the file, select the same option again from this menu to close the file. (and save it if generating audio)
For decoding, short clips of the section of audio to decode with continuous looping enabled (when prompted) are recommended for your own ease.
Decoding a message
Now that you have loaded a file for playback, you should be able to see its signal displayed on the spectrogram as fldigi plays through the track. In waterfall mode the newest signals appear at the top and move downwards with strength indicated by color.
First before decoding you will need to identify the type of signal you have. In general this is not a trivial task and you may need to rely on outside information, listening to example audio in comparison, or trying several Op Modes to find one that fits. Additionally you may want to compare your signal with some known examples (courtesy SigIdWiki) of various types of encoding.
Then you can open the Op Mode menu and select the digital modem mode you want to use to try to decode the signal. Each mode comes in various bandwidths and spacing, which will in turn be reflected by your "VFO cursor" on the waterfall.
Next, find the signal you want to decode in the waterfall and position your mouse over it so that the new cursor position lines up with your signal. Generally the bars of the cursor should line up with the strongest parts of the signal lines that you can see if the cursor is separated. If it is a single bar, then it should be centered to contain the strongest part of the signal. The frequency of the signal is usually indicated by the middle of cursor. (Note: there may be exceptions to this when an actual LSB or USB signal is present as it may only appear on one side of the carrier frequency rather than centered)
Once you have the cursor in position, click on the waterfall to lock the receive/decoding VFO Cursor in position. After doing this, fldigi will attempt to decode any data at that frequency.
Using this information, you should notice that you can try different modes to see if the VFO Cursor lines up correctly and contains the full bandwidth of the signal, which can help to identify a signal, though there may be multiple matches.
Necessary adjustments and Denoising
After selecting a type of signal to decode and setting the frequency cursor, you may still not have any success getting a readable message because of a couple of settings.
First, you may need to set the Modulation appropriately. Usually this can be set to USB or LSB with some exceptions
Secondly, you will have to check if the signal uses reversed decoding (Rv, bottom right) Which you can check by attempting decode with it off and on and comparing results.
Additionally there are a couple of options you can use to help get a cleaner decoding result: Automatic Frequency Control, and Squelch.
If you are not sure of the exact frequency of the signal, AFC can perform some minor tuning adjustment to the VFO Cursor while decoding, but it may introduce inaccuracies and tuning drift.
Squelch on the other hand sets a signal strength threshold so that noise audio can be discarded when decoding, which can help clean up the output, and decreases the amount of garbage text that fldigi outputs while idle; However, if squelch is set too high, you may notice missing data. To set the squelch you can watch the green bar on the bottom right (the signal strength) as you decode, then you can move the slider beside it to the strength you want to require for decoding to occur. The higher it is set, the more restrictive the squelch filtering is.
Try it yourself
Challenge
Try decoding the following audio file.
Hints
It can be decoded with Modulation set to RTTY Reverse, USB, or LSB+Reverse.
Solution
I AM THE ZOL [...] FEAR ME