Receive Analog Video Radio Signals From Scratch

If you’ve been on the RTL-SDR forums lately you may have seen that a lot of work has been going into the DragonOS software. This is a software-defined radio group that has seen a lot of effort put into a purpose-built Debian-based Linux distribution that can do a lot of SDR out of the box. The latest and most exciting project coming from them involves a method for using the software to receive and demodulate analog video.

[Aaron]’s video (linked below) demonstrates using a particular piece of software called SigDigger to analyze an incoming analog video stream from a drone using a HackRF. (Of course any incoming analog signal could be used, it doesn’t need to be a drone.) The software shows the various active frequency ranges, allows a user to narrow in on one and then start demodulating it. While it has to be dialed in just right to get anything that doesn’t look like snow, [Aaron] is able to get recognizable results in just a few minutes.

Getting something like this to work completely in software is an impressive feat, especially considering that all of the software used here is free. Granted, this wouldn’t be as easy for a digital signal like most TV stations broadcast, but there’s still a lot of fun to be had. In case you missed the release of DragonOS, we covered it a few weeks ago and it’s only gotten better since then, with this project just as one example.

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This Frequency Generator Knows How To Get Down

What kind of clever things could you do with a signal that had a period of 2 hours? Or 20? Any ideas? No seriously, tell us. Because [Joseph Eoff] has come up with a way to produce incredibly low frequency signals that stretch out for hours, and we’d love to figure out what we can do with it.

To be fair, it’s not like [Joseph] has any ideas either. He thought it would be an interesting project, and figures now that he has the technology, maybe some application will come to him. They say that if you’ve got a hammer everything looks like a nail, so maybe the next project he sends our way will be a sinusoidal fish feeder.

[Joseph] says doing the software side of things with Pure Data wasn’t a problem, but getting it out of the computer proved to be tricky. It turns out that your average computer sound card isn’t equipped to handle frequencies down into the millihertz range (big surprise), so they need to be coaxed out with some extra hardware. Using a simple circuit not unlike an AM demodulator, he’s able to extract the low-frequency signal from a 16 kHz carrier.

So if you ever find yourself in need of a handful of hertz, now you’ve got the tool to generate them. At least it’s more practical than how they used to generate low frequency signals back in the 1900s.

FM Signal Detection The Pulse-Counting Way

Compared to the simple diode needed to demodulate AM radio signals, the detector circuits used for FM are slightly more complicated. Wrapping your head around phase detectors, ratio detectors, discriminators, and quadrature detectors can be quite an exercise. There’s another demodulation method that’s not so common, but thankfully it’s also pretty easy to understand: the pulse counting detector.

As [Allan (W2AEW)] notes in the video below, pulse counting is a bit of a misnomer. Pulse counting works by generating a narrow, fixed-width square wave pulse at a set point in the received FM signal’s waveform, usually at the zero-crossing point. Since the frequency of the modulated carrier changes, the duty cycle of the resulting pulse train varies. That means there will be a fixed number of pulses, but by taking the average voltage of the pulse train, we can tease out the original audio frequency signal.

Simple in theory is often more complicated in practice, and [W2AEW] goes into some detail about those complications, such as needing to use a down-converter to make the peak-to-peak frequency deviation in the pulse train more easily detectable. As is his style, he walks us through a test circuit to prove that the theory works in practice. A simple two-transistor circuit generates the pulses at the zero-crossing point, a low-pass filter cleans up the signal, and a cheap audio amplifier reproduces the original audio. It’s a crude circuit to be sure, relying on the stray capacitance of the breadboard to work, but it proves the point and serves as a jumping-off point for further experiments – perhaps using an Arduino to count the pulses?

We always enjoy [W2AEW]’s videos and learn a lot from them. Not long ago we featured another of his videos talking about the mysteries of RF modulation; SSB, anyone?

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