Mystery signal from a helicopter
Last night, YouTube suggested a video for me. It was a raw clip from a news helicopter filming a police chase in Kansas City, Missouri. I quickly noticed a weird interference in the audio, especially the left channel, and thought it must be caused by the chopper's engine. I turned up the volume and realized it's not interference at all, but a mysterious digital signal! And off we went again.
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.