Frequently Asked Questions about the WebSDR project

Using the WebSDR (the client-side)

I don't hear any sound. How come?

Besides obvious problems like your computer's sound being muted, one possible cause is that your web browser does not support this. There are two entirely different techniques used to play the sound. Your computer has to support (at least) one of them for sound to work:

• Most used nowadays: HTML5 WebAudio, since this has been available in most web browsers since several years:
  • Firefox version 17 or later
  • Chrome version 10 or later
  • Safari version 6 or later
  • Opera version 15 or later
  • Edge (as supplied with Windows 10)
  Unfortunately, Internet Explorer does not yet support this technology.

• Essentially only for Internet Explorer, you can also use Java applets; for this, you need not only a webbrowser, but also version 1.4.2 or newer of Java working on your computer. Try http://javatester.org/version.html to find out if you have Java and which version it is. You may need to enable Java applets per site; see here. Some WebSDR sites have disabled Java support; those sites can only be used with HTML5 WebAudio.

So, which one do you use? WebSDR sites try to detect what your browser can do. If HTML5 WebAudio is available, it will be used; otherwise, Java will be tried.

I'm using Chrome and don't hear audio (on some sites)!

Since version 71, Chrome does not allow every website to start playing audio, in order to stop annoying advertisements. Chrome tries to guess whether you want audio or not, but doesn't always get it right. On some WebSDR sites, you'll get an "audio start" button, on some you don't.

If you don't get audio, try the following:

• At the top right, click the 4 vertical dots, and then Settings.
• At the bottom, click Advanced.
• Under "Privacy and security," click Site settings.
• Select "Sound"
• Select "Add" and enter "http://*"

(thanks to K9GL for these instructions)

Note that the above effectively disables Chrome's "autoplay" policy for all http sites.
Although stopping automatic sound from advertisements is a noble idea, I think Chrome's autoplay policy is fundamentally wrong. Instead of trying to guess what the user wants, the browser should simply ask the user whether he/she wants to allow the page to play sound (and remember that for later visits, of course).

I'm using Safari and don't hear audio (on some sites)!

It looks like Safari does something similar to what Chrome does as described above.
You can enable audio via Safari > Settings for this web site > Autoplay > Allow all Autoplay.
(thanks to M0IFA for these instructions)

Only some WebSDR sites work; on others I get no sound and no waterfall.

Please have a closer look at which sites work and which don't. Is it such that only those on "unusual" ports (i.e., those that have a number like :8901 in their URL) work, while those without a port number (those run on the default web server port 80) don't work? Or the other way around?

If only those on port 80 work, you're probably behind a firewall that only allows port 80 (and a few others). This is often the case in public WiFi networks and office networks.

If only those on ports other than 80 work, you're probably behind a web proxy server, which doesn't know how to handle the (non-standard) audio and waterfall data streams. The proxy server may also be in your internet service provider's network.

Can I use the WebSDR on a smartphone/tablet/etc.?

Yes, if there is a browser for your device which supports HTML5 WebAudio.

On Android devices, these include Firefox, Chrome and Opera (but not Opera-Mini); and even recent versions of Android's built-in browser should work.

On iPhones, iPads etc, you can use the built-in Safari browser, but only if you have iOS version 6 or later.

I'm using an iPhone/iPad and don't hear audio!

An issue with iPad (-like) devices is that they have two mute switches: one which affects music, youtube, etc., and one which affects system sounds (like mail notification bleeps).
Somehow, iOS treats the WebSDR audio as the latter. So please check that you haven't accidentally muted the system sounds. On some devices this is a software switch, on others it's a physical hardware switch.

How can I decode digital signals like PSK31, RTTY, etc.?

You need to feed the received audio from the WebSDR web page to a separate program that can decode these signals. This can be done as easily as by putting a microphone in front of the computer's loudspeaker, or by using software such as Virtual Audio Cable on Windows. See for example http://www.oz9aec.net/index.php/component/content/article/63-sdr/290-fun-with-websdr-and-fldigi and http://www.hamradioandvision.com/websdr-digital-modes/.

Can I decode DRM ("digital radio mondiale") signals?

No. These signals are 9 or 10 kHz wide, which is much more than the bandwidth streamed from the WebSDR server to the clients.
Decoding them serverside is not an option either, because of the CPU load for the server.
Furthermore, I think most DRM programmes can also be heard directly on the broadcaster's website, so there is not much reason for listening to them via a WebSDR receiver.

I get a warning that the WebSDR Java applets are "unsigned". What does that mean?

See here.

Can I specify the frequency and mode in the URL when linking to a WebSDR ?

Yes, using the ?tune= parameter, like http://websdr.ewi.utwente.nl:8901/?tune=198am to listen to 198 kHz in AM (BBC Radio 4).

Running a WebSDR server

Where can I download the server software?

Nowhere. However, I distribute it (without cost) via e-mail to people who are setting up a publicly accessible server (i.e., listed on websdr.org), and who have everything needed to set it up: a suitable SDR (see below), a computer running Linux, and a fast internet uplink. If you have all of these and are seriously interested in setting up a server, email me with the following details about your plans and equipment:

• what kind of SDR hardware (see below for what is supported)
• what kind of internet connection, computer, and Linux installation
• what frequency band(s)
• and what does that add to the current offering on websdr.org (see the next question) ?

Please also read the following questions, to check that your plans are feasible!

Does it make sense that I set up a WebSDR server?

Have a look at the current list of servers on http://websdr.org, and judge whether your server would add something to that, e.g. by being in an unusual location, having a special frequency range or a very good antenna site. Yet another 40 m WebSDR with a dipole in western Europe is probably not useful, but one on Antarctica would be quite nice...

My SDR is an XYZ. Is it supported by the WebSDR server software?

Many early amateur SDRs are quadrature mixers which downconvert radio signals to the audio range and feed this to the computer's soundcard. This category includes the well-known Softrock kits, and very many similar schematics and designs on the internet. Also the FunCubeDongle and USB-Afedri work, since they emulate a soundcard. This category of SDRs is supported.
If the SDR has some kind of synthesizer or other configuration settings, the WebSDR software will not take care of this. You'll have to use other software to configure it.

Other SDRs typically use a fast A/D converter and digital hardware to filter part of the spectrum; they are typically connected to the PC via USB or ethernet. Unfortunately, there is no standardization among the interfaces for these SDRs, which makes it hard for me to support them. A generic interface for them is being added, but for now they cannot be used.

The (in)famous RTL-SDR dongles (cheap VHF/UHF SDRs) are now supported. Note that these SDRs have a rather small dynamic range, so should only be used in situations where there are no very strong signals. If you want to use these, please check carefully that they are not being overloaded, e.g. by comparing their reception (using normal SDR software) to e.g. a normal non-SDR receiver; in particular, pay attention to weak signals: they should not disappear into the noise.

Please tell me what hardware to buy.

See above for what hardware is supported. I expect you to be familiar with the principles and limitations of SDR, and having your SDR working with "normal" SDR software, before making it into a WebSDR.

My SDR can be tuned from 0 to 30 MHz (or from 25 to 1900 MHz, or whatever). Can I offer all of that tuning range to the users?

No. Such an SDR does not feed the entire 0-30 or 25-1900 MHz spectrum to your computer: that would be way too much data. Instead, a small part (at most a few MHz) are filtered out in external hardware, centered around some frequency that you can tune. With the WebSDR software, users can only tune around within that small part of the spectrum. You (as the operator of the site) choose the centerfrequency.

What Linux distribution should I use for the server?

That does not matter much, as long as it runs on Intel (compatible) processor, either 32 or 64 bits (or the ARM processor used in the Raspberry Pi, see below). Many WebSDR servers run on Ubuntu; I myself usually use Debian.

Can I run the server on the Raspberry Pi or another ARM processor?

Yes, the package now also contains a binary for the RPi. However, the RPi's computational power is limited, so it can't handle too much radio bandwidth. It seems 1 MHz is about the limit for the RPi3, while only the RPi4 seems to be just fast enough for the 2 MHz bandwidth of an RTL-SDR.

Do I need to install Apache, PHP, MySQL etc.?

No, the WebSDR program is self-contained, containing both the radio signal processing and a small webserver.

Can the WebSDR server software be run on Windows?

No, the server software only works with Linux. (But of course the client, i.e., the user side, can be any web browser, regardless of operating system.)

What soundcard should I use for the server, if using a soundcard-connected SDR?

Start with any that you happen to have and is supported by your Linux distribution. I can't advice you on more advanced sound cards. Don't overlook the motherboard's on-board card: modern motherboards often have surprisingly nice sound devices, with e.g. 4 channels at 192 kHz samplerate.

How much internet uplink bandwidth does a WebSDR server need?

Obviously, this depends on how many simultaneous users there are. Furthermore, it depends on how many waterfall displays each user has (i.e., how many bands), and the scroll speed of those waterfalls. With a single waterfall at "slow" speed, count on about 100 kbit/s per user.

Can I send the raw data from the SDR to another site where I have more internet uplink bandwidth, and run the server there?

Although it might be possible in theory, it typically doesn't help, because that raw data stream is much bigger than the data going to the users. For example, a 96 kHz wide SDR already needs about 3 Mbit/s for the raw stream (namely, 96000 (samples/sec) x 16 (bits/sample) x 2 (I and Q channel)).

What is the most recent version of the server software?

The latest distributed version is 11. If you have an earlier version, and have not been sent the 11 version, then most likely your WebSDR was not running publicly around the time I distributed the 11 version.

The WebSDR at the University of Twente (http://websdr.ewi.utwente.nl:8901)

Is the hardware being used at the 29 MHz wide WebSDR (running at the University of Twente) available for sale or duplication?

No. This is a rather experimental board; it's not a commercial product, and it is not well enough documented for easy duplication. Also the special WebSDR version running it is not ready for distribution.

What antenna is used there?

It's a variant of PA0RDT's MiniWhip design. See here, but there's lots of information on various MiniWhip designs on the internet. Also have a look at articles I wrote about this kind of antenna here and here. The latter has a paragraph at the end specifically about the setup used at the Twente WebSDR.

Where do all the station labels come from? Can I get a copy of them?

Most labels in the shortwave range come from the EiBi data, which you can download at http://www.eibispace.de/.
Most labels in the longwave and mediumwave range are maintained manually by myself, based on remarks people send me and my own observations. This list is not available, but that wouldn't be very useful as it has been tailored this location. In general, lots of good data on is available at https://www.mwlist.org.

I selected "FM" but don't hear any FM radio stations!

What is commonly called "FM radio stations" or "FM broadcast stations" are stations broadcasting using a technique known as Frequency Modulation (hence the name FM), in a frequency range of 88 to 108 MHz. The "FM" setting in the WebSDR just selects the Frequency Modulation method, but does not select the 88 to 108 MHz frequency range, because that is outside the range of the radio hardware used (for technical reasons). So that's why you can't find FM broadcast stations on the WebSDR.

Never asked questions (unfortunately)