Throughout history, one constant vexation for humans is how to speak with someone without anyone eavesdropping.

You may consider using a cipher or some other simple encryption, but what if the key is compromised? All your previous and future messages would be at risk. You could try cycling through different keys, but how do you communicate which keys to use? If you tell them the next key to use as part of the message it would help a little, but if one key is compromised then all future messages would be at risk.

Going beyond just hiding who your crush is, this can be a very high stakes game. Consider the torpedo.

Submarines and other naval craft aren’t very maneuverable. If a torpedo fires and flies straight, the entire ship has to be turned in order to aim which isn’t easy to do. It also means that the target has time to evade it if they see it coming. Early attempts to solve this involved programmable paths, although the paths were very basic and mostly solved the issue of maneuvering the firing ship, not the ability for the target to react.

Before acoustic and wake homing missiles, the better solution, invented by John Hays Hammond Jr., was to control the torpedo by radio. The Hammond Torpedo could be guided to its target from a long distance, allowing it to change course and match a target’s evasive actions.

But there are ways that a ship can defend itself from these weapons. When receiving a transmission there will always be some background noise. If you’ve ever had a broadcast TV, you’ve probably seen this in the “static” of an unknown channel or when picking up a channel from far away. In order to interpret the transmission, you have to do some signal processing to filter out the noise (typically a Fourier Transform). However, if the opposing ship knows the frequency you’re broadcasting at, it can broadcast signals at that same frequency. As the torpedo nears the ship, their signals will be louder than your signals and so during processing, your signals will be counted as background noise! With this technique, targets can hijack incoming missiles to steer them away.

We could try changing the frequency at regular intervals, but remember our little problem above? We can’t simply tell the torpedo what frequency we’re going to transmit on next. An enemy ship would only need to guess right once to still have complete control over the torpedo.

There’s a clever solution to this and it comes from the most unlikely places. But if you remember the title, you’ve probably already guessed it.

A player piano is a piano which plays itself. The earliest versions did this by a series of pneumatic valves which would rapidly release air to generate the hammer strike on a string. Which valves open is determined by air blowing through a sheet of paper. Holes in the paper represent notes, and the air blowing through is used to trigger the specific valves and strikes associated with that note.

(Image Source: http://www.pianola.com/ppworks.htm)

In her patent, H. K. Markey (better known by her maiden name Hedy Lamarr) describes how this can be used to secure radio transmissions. Instead of using the “notes” to strike a string, they are used to close or open switches, which modifies the frequency that the transmitter is using to send and the torpedo is using to receive. Instead of a steady air flow, suction (which is broken by holes in the control sheet) is used to lift the contacts and open circuits. By providing the same “sheet music” to both the transmitter and receiver, they will have the same sequence of frequencies to use.

(Image Source: https://patents.google.com/patent/US2292387A/en)

Both devices are also equipped with constant speed motors that are made to start turning at the same time. This is accomplished by using a closed circuit between the torpedo and the launcher while it’s stationary. As the torpedo is launched, the wires snap, opening the circuit and allowing the motors to proceed. With this technique, the transmitter and receiver use the same sequence of frequencies with near perfect time synchronization.

Moreover, because the frequency modulation is shared ahead of time, the target can’t intercept it! Even if they guess the frequency correctly once, they would only have control for a fraction of a second before the frequencies change again.

Frequency hopping is so successful that, like many war time technologies, it found its way into consumer devices.

Bluetooth is a prime example of this. Bluetooth uses frequency hopping to avoid interference in a band of frequencies that is used by other household devices, like WiFi and microwaves. When you “pair” devices, they are exchanging information about what sequence to use, so that they will be communicating on the right frequencies at the right time. Information that is sent over a crowded frequency can be retried later on a different frequency, when no interfering signals will be present. A secondary benefit is additional security due to the sequence being unknown to outsiders.

Though the implementation changes from pneumatic valves to digital tuning, the concept remains the same. A sequence can be used to produce changes in a configuration. That configuration can be used to play music or avoid interference. It can even prevent eavesdroppers from hearing who your crush is. It’s limits are set by the creativity of the person using it.

Idea Project’s inspiration, Idea Channel, would show that classic philosophy can be applied to understand the cultural things we interact with everyday, but this is hardly the only discipline this applies to. In technical and mechanical systems, ideas and inventions are reused a lot, which leads to things like a player piano being the basis for Bluetooth. This reuse is hidden from us because we don’t always try to understand the technology we use. But there’s a lot of interesting things hiding under the surface, if you’re willing to start thinking about it.

As we say at the end of all these posts, Idea Project is an open community of volunteers looking to produce content in the wake of Idea Channel’s ending. If you want to contribute to either this blog or our YouTube channel, join our Discord. We’d love to have you!

Works Cited:

“Bluetooth radio interface, modulation, & channels”. Electronics Notes. Online: https://www.electronics-notes.com/articles/connectivity/bluetooth/radio-interface-modulation-channels.php

Levins, Sandry. “Hedy Lamarr, Serious Inventor Trapped in a Hollywood Image”. Wednesday’s Women. Jan 3, 2018. Online: https://wednesdayswomen.com/hedy-lamarr-serious-inventor-trapped-in-a-hollywood-image/

MacNeil, Jessica. “The father of radio control is born, April 13, 1888”. EDN Network. April 13, 2019. Online: https://www.edn.com/Pdf/ViewPdf?contentItemId=4439177

Markey, H. K. et al. United States Patent No. US2292387A. 1941. Online: https://patents.google.com/patent/US2292387A/en

“Player Piano”. Wikipedia. May 1, 2019. Online: https://en.wikipedia.org/w/index.php?title=Player_piano&action=history