How To Set Up And Use A Decoder

While you're listening to the signals, you may come across some data or Morse Code. If you do not know Morse Code yourself, or want to decode the data, you'll need what is called a decoder. Note that the information in this section should not be used to decode encrypted data or listen to private cell phone converations as this may be illegal in some areas. Please see the disclaimer link at the bottom of this page.

A decoder will take sound that is from either the device internal sound or sound card on a computer, or sometimes even a recording of the signal and decode it into something that you can understand (such as text from Morse Code, for example). There are quite a few out there and there are also many types of transmission modes that can be decoded. In general, using a computer is far easier to decode with than on a mobile device. Therefore these tutorials will not cover decoding on a mobile device.

Which Mode To Use

When decoding a signal, you will need to Identfy the Signal Type. Once you know what you are dealing with, you need to pick the correct mode in the decoder.

Decoder Software and Setup

To start off, please see Capturing Sound and Recording to set up your computer to capture sound from the sound card. This is how the signals get into the decoder program. Then you would start up the webSDR, and after that the decoder program.


In Linux, you can use a program called fldigito decode signals via your sound card. Fldigi should be in most distribution repositories. You can install it by typing the following in a terminal window or by searching for it in your package manager.

$ sudo apt-get install fldigi qsstv pavucontrol

This will also install other packages that are needed to run the program.


  1. Be sure you have your mixer set up properly. See Capturing Sound and Recording for more information.

  2. Next visit the webSDR radio in your web browser.

  3. Run the fldigi program. You can do this by typing fldigi in your menu search box or go to the Hamradio menu and choose flidig (Amateur Radio Digital Modem).

  4. You need to set up Fldigi first in order for it to work. When the Fldigi configuration wizard appears on startup, close it without going through the wizard.

  5. From the Configure menu, click the Sound Card tab. Go to the Audio tab, then Devices tab. Click on the check box next to Pulseaudio. Do not fill in the box next to it. Click save first and then close.

  6. Run the Audio Mixer (pavucontrol) program. Note that Fldigi must be running before you run the audio mixer for this step.

  7. Go to the Recording tab. In the Show area at the bottom, click the button and select All Streams.

  8. You'll see something saying capture from and numbers in parenthesis or fldigi: capture from. Click on the box to the right and select Monitor of Built-In Audio Analog Stereo.

    You can now close the volume control. You only need to do this once and this saves the information so next time you run Fldigi, it should remember you want to get audio from the sound card.

Decoding A Signal

Now go in the webSDR and fine tune into the signal to decode.

If you are tuning in SSTV signals, you'll need to use qsstv. The best way to do this is to record the SSTV signal to an audio file (such as .wav) and then decode it in qsstv. Please see Decoding SSTV from a file using Linux by m0nkc for instructions on how decode sstv files from an audio file in qsstv. Note that I had tried to play the file in VLC but it would not link to the virtual cable. I had success using the command line option to play the sound file so that qsstv could decode it.

For other types of signals, you can use fldigi. In Fldigi you'll also see a waterfall which represents the area that the cursor is located in the webSDR. Place the cursor in Fldigi into the area you want to decode.

Then select the band mode by clicking on the box in the upper right under the frequency box (the box with large numbers is the frequency box). In this drop down, select the same band that you are listening to in the webSDR.

In the Op Mode menu, select the mode to use to decode. For example, to decode Morse Code, use CW. You will then see the program proceed to decode the signal. You can right click in the area where the decoding is taking place and choose save as to save the decoded text.

Other Operating Systems

Since I do not know how to set up other operating systems, you'll need to look into this on your own. The following links may help: