An anonymous reader sends a post by Finnish electronics hacker Oona Räisänen, who heard a mysterious digital signal in the audio accompanying a YouTube video of a police chase. The chase was being filmed by a helicopter. Räisänen wrote: "The signal sits alone on the left audio channel, so I can completely isolate it. Judging from the spectrogram, the modulation scheme seems to be BFSK, switching the carrier between 1200 and 2200 Hz. I demodulated it by filtering it with a lowpass and highpass sinc in SoX and comparing outputs. Now I had a bitstream at 1200 bps. ... The bitstream consists of packets of 47 bytes each, synchronized by start and stop bits and separated by repetitions of the byte 0x80. Most bits stay constant during the video, but three distinct groups of bytes contain varying data." She guessed that the data was location telemetry from the helicopter, so she analyzed it to extract coordinates. When she plotted them and compared the resulting curve to the route taken by the fleeing car in the video, it was a match.
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