WebSDR + Browser + FLDigi

Figure 1: My dog Hutch knows SDR now that he’s picked the bone a little bit

Posted November 20, 2015:

This page will detail some of my experiments with WebSDR, an HTML5 WebAudio-API enabled browser, and the FLDigi digital mode program.  The signal flow for decoding digital modes is like this:

WebSDR (a remote website providing SDR) -> Browser-with-HTML5 including WebAudio API -> sound card and/or FLDigi or WSJTx digital mode decoder.

The signal flow for listening to standard analog modes is like this:

WebSDR (a remote website providing SDR) -> Browser-with-HTML5 including WebAudio API -> sound card -> Headphones.

This is actually the easiest way to investigate and play with SDR. You don’t need any hardware! For digital modes, the first two components of the SDR chain are done for you by the website and the browser HTML5 / Javascript code. For standard modes, the site and browser do it all! Newer browsers have partial conformance to the HTML5 spec, and in particular the WebAudio API portion of that specification. Firefox (since v38, I think) has this feature. Chromium and Vivaldi also support the feature in recent versions. I think IE does not. If the browser supports the WebAudio API of HTML5, then you can cruise over to a WebSDR site, and use the interface on the page to select a signal (just exactly the same way you would do it in Linrad or Quisk, for a local SDR). If you route the audio from your browser (usually the default sound card is where browsers send the audio) to the FLDigi program, you can decode PSK31 and other digital data modes. If you don’t want to do digital modes, just plug a set of headphones into your sound card’s line-out jack, and listen to foreign broadcast or any other standard analog mode (CW, AM, FM, SSB). Just be sure to set the volume low (there’s a control on the web page) – or you could get more than a earful.

You can plumb Pulseaudio to connect the browser’s output to FLDigi, as the latter will show up in the recording tab of the Pulseaudio control panel with the name “FLdigi : Capture …”

For unknown reasons, the Pulseaudio control app is almost never installed by operating system’s basic installation process. You need to install it, with something like:

apt-get install pavucontrol   (This is for Debian or Devuan systems).

(To be continued)

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