Tubular Bells and VLF interference

Tubular BellsRemember that album from Mike Oldfield? A user on YouTube found out that this album contains a hidden and unintentional CW message.

This was caused by a powerful VLF station located next to the recording studio, which interfered with the recording equipment. Because the signal is very weak nobody ever noticed it — until now.

The video below shows you how to receive VLF signals with nothing else than an antenna plugged into the microphone input of your sound card and SDRSharp to make it visible and audible.

The decoding of the the Tubular Bells signal is shown at 9:54.

(via Reddit)

Out Of Phase

During one of my antenna experiments I took 2 Baofeng UV-5R radios with me. One has firmware version BFB231, the other BFB251. While listening to some signals I noticed a sudden decrease in volume when both radios were in close proximity to each other.

Interesting. The audio coming out of both radios was not in phase, which caused them to cancel each other out almost completely. A strange experience, but easy to explain. Every change in software can cause a DSP or SDR design to process the signals either slower or faster. In this particular case the combination of both UV-5Rs acted as a noise canceling system, similar to the image below.


You can stop saying “Testing 1-2-3” now.

I’ve been playing around with glSDR, an Android app which enables you to connect to Web SDR servers around the world. The app is free and can be downloaded from Google’s PlayStore. Any doubts about range, QSB or TX audio? Now you can listen to your own station and to yourself (which, BTW, can be very revealing at times.)

Some videos float around the Internet, none of them very professional. I just picked one.

SDR, a quirky but promising technology

I always find it striking that HAM radio operators talk about SDR as if this technology is the holy grail of the amateur world. When the Baofeng UV-3R came out, operators from all over the world started to wave with their credit cards. They just had to own one. “It’s Software Defined Radio in a package the size of a cigarette pack! There’s a DSP inside, too!” The Baofeng UV-5R triggered similar reactions.

Fact is that SDR and DSP inside these HT’s aren’t that impressive. For now, the only advantage is price. And, as we all know, there ain’t such thing as a free lunch. Something has to give. Let’s take a look at the hardware side of things first.

Receiver architecture
Most receivers use a variable-frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. However, in some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the analog-to-digital converter (after amplification).

Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt, nanowatt-power radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example, if spurious signals are present (which is typical), these compete with the desired signals within the amplifier’s dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio’s flexibility, and flexibility is the whole point of software defined radio.

Disappointing results in the real world
For the reasons described above, the receivers of Baofeng hand helds have a rather poor reputation. How come? They’re digital, right? Yes, they are. What many people tend to forget is that the word ‘digital‘ in itself means nothing when it comes to signal quality. I can produce digital audio files of very poor quality. No sweat. Digital TV sometimes delivers an image quality inferior to analog, and the Dutch DVB-T system is a good example. Compression is so high that same-color parts of the image fade away in a circular pattern.

Weird sounds
Both the UV-3R and UV-5R can sound ‘weird’ when the DSP kicks in. Sometimes this translates into a hissing noise, sometimes into muffled RX audio. It is as if the digital hardware inside can’t make up its mind. Put an analog HT next to it and you know how it can/should sound. Yes, this aging analog technology can serve as an example, because it had the time to mature to the point of perfection.

In theory however digital systems are capable of producing far better results than their analog counterparts. It’s all a matter of time and money. And even when all problems are solved, there’s a human factor to deal with. Perfect digital signals don’t always sound pleasant. When Siemens developed and introduced the first ever VoIP phone system for use within companies, they had to introduce ‘comfort noise’. The signal was so clean that users thought that the system was dead, and hung up their phone. Just a little bit of artificial noise on the line was sufficient to solve the problem.

Shaved Legs
The same proved true for images. I’ve seen digital images coming from a DSLR which were so perfect that it started to bother me. Digital is sometimes like shaved legs on a man – very smooth and clean, but there is something acutely disconcerting about it.

OK, enough about the hardware, let’s go to the software side of things.

“A program is good when it’s bug free. Which is impossible.”
This quote was used at a press conference I once attended. SDR is a system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded computing devices.

While faults or problems in a conventional design can often be solved with a soldering iron in one hand and decent test equipment in the other, with SDR you’re depending on the qualities (or lack thereof) of the programmer. This is especially true for ham radio equipment such as the Baofeng hand helds. If you are a good programmer, you might be able to fix things yourself. Unfortunately these Chinese hand helds do not allow that. As far as we know, you can’t change even one bit. You can’t update the firmware either. If such a device is littered with bugs, the only option is to send it back to the manufacturer, or throw it away if the warranty expired. For many operators this is a real pain in the butt.

Will SDR and DSP concur the world? Yes, I believe so. In the long run I expect this technology to become better than analog. At higher price levels there are enough examples of well implemented SDR-based systems. At this point in time however I keep cherishing my analog hand helds and rigs.