Tag: olivia mfsk

Using an SDR and VAC to Decode Data Modes

Web SDR receivers allow you to remotely monitor the radio spectrum, and phone (voice) is quite easy you just click and listen. You can also use SDR receivers to decode date modes, be that CW, PSK, RTTY, HF-DL or Olivia.

The principle is the same, you need to route the audio out of the SDR, and into your data modes decoder. This requires a virtual audio cable, you will find quite a few both paid and free version on the web. It is “virtual” because you need to connect the audio from one port on your computer to another with software.

Here you will find some free virtual audio cables (untested) or you can try the most popular (but not free) version virtual audio cable program.

So you have now downloaded your VAC, next find an online SDR like this one, or others are available at sdr.hu.

Start up the web SDR and select the frequency and mode that you want to decode. Here you can see we have centered on an Olivia signal, 16/500 on 5 MHz (5366.5)

Now you need to configure the audio IN and OUT. In my case, it looks like this. Note the WAVE IN set to my sound card, and WAVE OUT set to Virtual Audio Cable

You now need to decide what program do you want to use to decode the data. Fldigi is a good free choice, I used DM780 from Ham Radio Delux but you may also want to check out Sorcerer.

HRD also have an old free version, it is worth considering and can be downloaded here.

You will need to tell the data decode program to use the virtual audio cable as the “input” sometimes called “microphone in”

Here is my sound card setting in DM780. Note the INPUT device is shown as “Line 1 Virtual audio cable”

So we now have VAC running, and our software looking for audio on VAC. Now we need to select the mode to be decoded, here is Olivia 16/500

The signal should now decode, the text should be appearing in the box, and you can see the data signal in the window.

Remember you can decode virtually any mode with the correct software.

Working Hellschreiber from a cold war bunker

Today I was able to operate the special event call sign GB0SNB from the Kelvedon Hatch Secret Nuclear Bunker. Hellschreiber was my mode of choice, its fun and easy to identify on the air  and also quite rare. Operating was completed from the original home office radio room deep inside the bunker.



Here you can see the door to the “Home Office Radio Room” and a picture of the mast when it was full of microwave links connecting the site to a number of other locations around the country. The radio room is located 3 levels below ground.


The radio room is located directly below the radio mast, and a tunnel connects you to the surface. Feeder cables run towards the surface and connect to the doublet antenna. Here you can see the radio room and both modern equipment (used for GB0SNB activations) and period equipment.



The laptop is running Ham Radio Deluxe and using the digital mode HELL. The Icom 7100 was running 50w RF into the clubs doublet antenna located 26m on the mast. The doublet antenna is 40m per leg. A cup of tea is required to keep the operator warm, its not often above 12c in the shack.

I managed to work several station from Belgium, Netherlands, Germany, England and Sweden on 7 MHz and on 5.3665 MHz I used Olivia MFSK. The bunker is also a museum so often busy with members of the public most days of the week.

The mast is also home for GB7KH the Kelvedon Hatch Dstar repeater.