Over on his blog "ele y ciencia" has written up two very useful blog posts - one on how to decode AFSK signals from scratch and the other on how to reverse engineer any unknown digital signal. The blog is written entirely in Spanish, but Google translate does a decent enough job at getting the message across (in Chrome right click anywhere on the page and select Translate to English or use the Google translate webpage).
The first post is about decoding an AFSK protocol and explains that you need to record the signal with an RTL-SDR or other SDR, apply a low pass filter to obtain the signal envelope and then apply thresholding with the known baud rate to obtain the demodulated digital signal. The tutorial is high level and just explains the process, but doesn't show how to do it in any software. Later on in the post he goes on to show how he reverse engineered a train-land radiotelephone system and a TCM3105 modem chip which utilizes a FSK system.
In the second post he shows how to decode any unknown digital signal using just an RTL-SDR and Audacity. He starts off with finding and recording an unknown digital signal with an RTL-SDR and then reverse engineers it in a sort of manual fashion without using any tools like Universal Radio Hacker. The post goes through the full details and steps that he took, and in the end he gets data out of the signal discovering that it is data from a Fleet Management System used in his country for monitoring data such as speed and engine data from commercial vehicles like trucks and buses.
The two posts are very detailed and could be an excellent reference for those interested in reverse engineering some unknown digital signals in your area.
In our previous post we featured a video by OH2FTG which showed an RTL-SDR transmitting at 1270 MHz. Now OH2FTG has written in to give us some more information about the RTL-SDR transmitter. He has done a short writeup explaining how it’s done on his website. It turns out that the RTL-SDR is actually capable of transmitting a FSK morse beacon using it’s leaky oscillator.
In the video, code written by another ham OH2EAT is used. OH2EAT’s code essentially changes the frequency on the transmitting RTL-SDR at up to 300 times a second using a modified driver. This is used to create a Frequency Shift Keyed (FSK) transmission.
The modulating transmitter code is not yet available as it is not yet ready for release. In the future OH2FTG hopes to build an amplifier to boost the signal output for further experiments.
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