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 JAVA-capable browser and 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. N7YG's Web site [PSK Stuff > PSK Connections] gives some details, including the fact that the Line In jack on some sound devices is designed to accept higher audio levels than the Mic In jack.
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 enable 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
A tutorial by AB9IL describes how to stream webSDR audio to fldigi using the JACK Audio Connection Kit on a Linux system. The article also includes a general description of digimode reception with a webSDR.
Audio Routing in Windows
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 answers.microsoft.com 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. A donation is requested, after which the software driver is upgraded to offer the option of routing two or more different audio streams simultaneously.
With the audio routing methods described above, the audio from digital mode text-to-speech applications interferes with digital mode reception using webSDR. 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.
Java Security Problems
WebSDR requires a browser with a Java Web plug-in, which is an add-on that allows Java code execution in the browser. At this time there are ongoing discoveries of malicious Java applets embedded in Web pages. Many of these are remote code execution exploits. Many security experts advise that the risk from using Java in a browser is too high, even if you apply updates when they are released. For example, in January, 2013, the U.S. Department of Homeland Security advised browser users to disable or uninstall Java browser plug-ins.
The small number of servers and users makes WebSDR a low profile target for hackers. PA3FWM also notes (in reply to my emailed question) that WebSDR "does not build on 'standard' webserver software such as Apache. Thus, 'standard' exploits don't work, and a criminal would have to specifically find a weakness in my software to hack the server (use a different path, e.g. through other software that happens to run on the same computer)."
One approach is to enable Java in a second browser that you only use to visit the WebSDR page. For all other Web access use a different browser with Java disabled. This general approach (if Java is required) is recommended by security expert Steve Gibson in his 2/27/2013 Security Now Webcast (Episode #393, Question #2).
Another approach is to enable the "Click to Play" option in Google Chrome, Mozilla Firefox, Opera, and Safari. This procedure is described by Brian Krebs in a KrebsOnSecurity blog posting.
In the same article he points out that "Oracle's Java installer re-enables the plug-in when the program is updated."
Apple recently updated the Safari browser so you can enable Java on a site-by-site basis via a "white-list."
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.
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.