Morse Decoder’s Lean and Sexy Search Algorithm

Often the Morse Code centered projects that we feature are to help you practice transmitting messages. This one takes a tack and builds an automatic decoder. We think [Nicola Cimmino's] project is well worth featuring simply based on his explanation of the Digital Signal Processing used on the signal coming in from the microphone. Well done. But he’s really just getting warmed up.

What makes this really stand out is a brilliant algorithm that allows conversion from Morse to ASCII using a lookup table of only 64 bytes. This provides enough room for A-Z and 0-9 without chance of collision but could be expanded to allow for more characters. Below is a concise description of how the algorithm works but make sure you take the time to read [Nicola's] project description in its entirety.

The algorithm can be decribed as follows. Have an index inside the lookup string inizialied to zero. Have an initial dash jump size of 64. At every received element (dot or dash) halve the initial dash jump and then increase by 1 the index inside the lookup string if a dot was received and by dash jump size if a dash was received. Repeat until a letter separator is reached, at that point the index inside the lookup string will point to the ASCII corresponding to the decoded morse.

Have you heard of this technique before? If so, tell us about it in the comments below. Before you jump all over this one, realize that Magic Morse uses a different technique.

Decoding News Helicopter Signals on YouTube

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A serendipitous YouTube video recommendation led [Oona] to a raw copy of a news helicopter car chase video. While watching the video she noticed an odd sound playing from her left speaker. That was all it took to put [Oona] on the hunt. Decoding mystery signals is a bit of an obsession for her. We last saw [Oona] decoding radio signals for bus stop displays. She isolated the left audio channel and sent it through baudline software, which helped her determine it was a binary frequency shift keyed (BFSK) signal. A bit more work with SoX, and she had a 1200 baud bit stream.

Opening up the decoded file in a hex editor revealed the data. Packets were 47 bytes each. Most of the data packets was static. However, thee groups of bytes continuously changed. [Oona] decoded these numbers as latitude and longitude, and plotted the resulting data on Google Earth. Plotting her data against the position of the car in the video revealed a match. [Oona] had a complete track of the news helicopter as it followed the car. The telemetry data is in 7-bit Bell 202 ASCII, and is most likely part of an Interruptible Foldback (IFB) system used by the helicopter news crew and the studio producers. Click past the break for the YouTube video that started this all.

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