Receiving Digital Modes
with a webSDR


   This article describes how to receive amateur radio digital modes such as PSK31 using a webSDR instead of your own radio. A webSDR is a software defined receiver that can be used remotely via the Internet. You can connect to a webSDR with a Web browser, tune the radio to receive the signals you want, and use a digital modem program on your computer to decode the text.

   WebSDR servers are open for free use by anyone with a broadband Internet connection. The user interface and the accessibility of webSDRs for blind and vision-impaired hams are described in a separate article.

   This article gives details about how to channel webSDR audio from your browser into a digital modem program for decoding.

PSK31 Reception Using webSDR

   Figure 1 shows an fldigi modem receiving a PSK31 signal via the webSDR station at the University of Eindhoven, Netherlands. It was transmitted from the Ukraine on the 20 meter band and received by a software defined receiver in the University. The webSDR server there streamed the audio over the Internet to fldigi running on my computer in the USA.

Figure 1. Fldigi receiving a webSDR PSK31 signal.



Capturing Streaming Audio from a webSDR

   One way to route webSDR audio from a browser to a modem program is to use an audio cable between the Speaker/Headphone jack and an audio input jack on a PC. Please note that without an external attenuator in the circuit, this might damage the sound card.

   You can also use a microphone to pick up the audio from a speaker.  This is a quick and easy way to test your setup, and hearing the audio helps blind or vision-impaired hams tune the webSDR. A disadvantage of using a microphone is pickup of extraneous noise.

   As an alternative to using a cable or microphone, software extensions can route the audio electronically. Here are some options:

Audio Routing in Mac OS X

Soundflower is a free, open source Mac system extension that enables you to channel audio from one application to another. After installation, Soundflower appears as an audio device in the Sound output menu of OS X [System Preferences > Sound > Output Tab > Soundflower (2 ch)]. Select it for the output, and also select Soundflower as the input source for your digital modem program.

   The SoundflowerBed application for simultaneously listening to the audio on Soundflower channels is included with the download [/applications/Soundflower].

Audio Routing in Linux

   The Configuration chapter in the Fldigi Users Manual includes a page about sound card configuration. Here are three ways to connect the audio output from a WebSDR to fldigi running on a PC with a single audio device (sound card or chip):

PulseAudio Sound Server.PulseAudio is a consumer-grade sound server that is part of many Linux desktop distributions, including Ubuntu. The easiest method of routing WebSDR audio to fldigi is to use the PulseAudio Volume Control graphical user interface. It is available in the Ubuntu Software Center, or you can install and open it from the terminal:

     sudo apt-get install pavucontrol


To configure this simple setup:

1.  Launch Mozilla Firefox, visit the webSDR site, and select a receiver. In the Playback tab of the pavucontrol panel, select “Show All Streams” at the bottom of the window. The sliders under "CubeUtils: AudioCallbackDriver" control the audio level.

2.  Launch fldigi. In the Soundcard tab of the fldigi Configure menu select "PulseAudio," save, and then close.

3.  In the Recording tab of the pavucontrol panel select “Show All Streams” at the bottom. In the “Fldigi: capture from” area of the window click on the pull-down menu box (upper right) and select “Monitor of Built-in Analog Stereo.”

   Settings in the pavucontrol panel persist after rebooting. To minimize the CPU load, close the pauvucontrol panel after selecting the settings.

   With this method you can not mute the audio. If that is a concern, consider adding a switch in the earphone/speaker cable. Also, WebSDR must be the only source of sound data, otherwise fldigi will receive a mixture of audio.

PulseAudio with Expanded Routing Options. In an iCW forum post, Jacek Holeczek shows how to expand the audio-routing options in PulseAudio. The installation process makes use of the Linux command line and a provided shell script. By this method you can mute audio and run multiple applications that simultaneously output audio. It is easier to configure than the JACK based setup described below.

   The attachment to his post includes clear instructions for routing audio to fldigi.  His method worked well in my tests using Ubuntu 14.04.3 LTS.

JACK sound server.  The JACK Audio Connection Kit, is a professional-grade Linux sound server. When used with a low latency Linux kernel, it is the best choice for applications that require low audio latency. A tutorial by AB9IL describes how to stream WebSDR audio to fldigi on Linux systems. The setup procedure is more complicated than those described above.

Audio Routing in Windows

    The Configuration chapter of the Fldigi User Manual includes a page about sound card configuration.

    Many Windows-compable sound cards (including sound chips on motherboards) have a loopback feature that routes audio back to the audio input port of the sound card. If that feature is enabled, "Stereo Mix" will appear as an audio device in the Recording Tab of the Windows Sound Control Panel. Select it as the default recording device, and also select "Stereo Mix" as the Input in the configuration menu of the digital modem program. With no change to the default Properties for Stereo Mix, audio is also routed to the PC Speaker/Headphone jack for monitoring.

   If the sound card hardware supports Stereo Mix, but it is not listed as an audio device in the Windows Sound Control Panel, you may need to enable it or download a newer version of the sound card software. These issues are discussed in several threads on the forum. Search for "stereo mix answers sound-mixer-in-playback-or-output" (no quotes).

   If Stereo Mix is not supported by the sound card hardware, you can use a third-party program to route audio from a webSDR to your digital modem program:

Virtual Audio Cable by Eugene Muzychenko is an advanced program with a complicated user manual, but with the default settings it works well with audio from webSDRs. The free trial version injects spoken reminders to purchase the software.

VB-Cable is developed and distributed by Vincent Burel of VB-Audio. It works with Windows XP, Vista, and Windows-7, 8, & 10. A donation is requested, after which the software driver is upgraded to offer the option of routing two or more different audio streams simultaneously.

Text-to-Speech Interference

   With some of the audio routing methods described above, the audio from digital mode text-to-speech applications interferes with digital mode reception using webSDR. PulseAudio with Enhanced Routing (and possibly the JACK sound server) does not have this problem. With the other routing methods, the signal fed into the modem program is a mixture of webSDR audio and speech audio. The horizontal streaks on the cocoaModem waterfall in Figure 2 are due to interference from the Mac OS X Voice. Figure 3 shows the same problem on a Windows system where JAWS Voice interference appears on an fldigi waterfall.

Figure 2. Mac OS X Voice interference on cocoaModem waterfall.



 Figure 3. JAWS Voice interference on fldigi waterfall.




WebSDR and Java

   On the WebSDR site, PA3FWM, explains that until July, 2014, the software required a Java-enabled browser on your computer. In January, 2014, a Java security update from Oracle made the WebSDR program more difficult to use. To access a WebSDR radio, it became necessary to enter the URL in a list of acceptable Java sites.

   In July, 2014, PA3FWM updated the WebSDR software so that Java

is no longer required. The software uses HTML5 WebAudio instead. He points out that the updated software does not work with Internet Explorer and older versions of other browsers.


   WebSDRs can be used to receive digital mode signals on Mac OS X, Windows, and Linux-based systems. A software extension, a microphone, or a cable plus an attenuator can route the audio from a browser to a modem program.

   Digital mode text-to-speech applications interfere with digital mode reception using a webSDR.


   Thanks to Chen, W7AY, author of cocoaModem, for answering my questions. Also, thanks to Jacek Holeczek for help with the Audio Routing in Linux section.

Author Information

   Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. This Web site has no ads or conflicts of interest.

Email:  HamRadioAndVision "at" gmail "dot" com.

rev. 8/13/2016

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