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Encoding data on a pea whistle

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Pea Whistle Steganography from Ooona Räisänen.

Would anyone notice if a referee’s whistle transmitted a secret data burst?

I do really follow the game. But every time the pea whistle sounds to start the jam I can’t help but think of the possibility of embedding data in the frequency fluctuation. I’m sure it’s alternating between two distinct frequencies. Is it really that binary? How random is the fluctuation? Could it be synthesized to contain data, and could that be read back?

I found a staggeringly detailed Wikipedia article about the physics of whistles – but not a single word there about the effects of adding a pea inside, which is obviously the cause of the frequency modulation.

To investigate this I bought a metallic pea whistle, the Acme Thunderer 60.5, pictured here. Recording its sound wasn’t straightforward as the laptop microphone couldn’t record the sound without clipping. The sound is incredibly loud indeed – I borrowed a sound pressure meter and it showed a peak level of 106.3 dB(A) at a distance of 70 cm, which translates to 103 dB at the standard 1 m distance. (For some reason I suddenly didn’t want to make another measurement to get the distance right.)

Later I found a microphone that was happy about the decibels and got this spectrogram of a 500-millisecond whistle.

The whistle seems to contain a sliding beginning phase, a long steady phase with frequency shifts, and a short sliding end phase. The “tail” after the end slide is just a room reverb and I’m not going to need it just yet. A slight amplitude modulation can be seen in the oscillogram. There’s also noise on somewhat narrow bands around the harmonics.

The FM content is most clearly visible in the second and third harmonics. And seems like it could very well fit FSK data!

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