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!
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, or even use Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for MakeCode, CircuitPython, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.