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It’s 1887. The only electrical components you have access to are electromagnets. And you want to build a digital to analog converter. What do you do?

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This device is somewhat of a revolution in telegraph technology from the 1870s. It uses a fixed-length 5-bit binary encoding, the kind of encoding that we would expect to see in silicon-based electronics equipment. Baudot created a receiver that resembles a mechanical computer to perform the decoding and printing. Looked at in modern terms, it uses a 5-bit latching transfer register and a second 5-bit decode register for actually printing the received codes as human readable characters instead of squiggles or dots on paper. The encoding used by the device is named after its inventor Emile Baudot. I’ll run through the entire device, although it is the receiver where things get interesting.

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Here is one of the early patents for electroacoustic multiplexing of telegraph signals. (Note that this is not the same as electroharmonic multiplexing, which used multiple frequencies as carrier waves to achieve multiplexing.) I like this one because it really shows the acoustic component of the system. Here is a picture of a quadruplex system.

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This is an early attempt at an isochronous distributor-based multiplexing telegraph from 1873. It demonstrates the problematic nature of using Morse code with this kind of multiplexing system. First, let’s have a look at the top view of the device.

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If you hear the word Morse Code, you probably imagine people tapping on a key and listening to the code through headphones or perhaps on old-style buzzer. However, this was not how Morse originally conceived his telegraph. Here is the first patent Morse filed for his invention (filed in the 1830s and granted in the 1840s).

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The Wheatstone needle telegraph operated based on 5 needles that could tilt to the left or right. Owing to the simplicity of the device, there’s not much detail to show. However, the design is interesting in how it differs from how we would build a similar device using modern ideas. First, here is a picture of an actual telegraph machine (from Wikipedia):

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The letter-printing telegraph patented by Royal Earl House in the 1830s was one of the first devices to operate under virtually the same principles as modern computer terminals. They had a keyboard for typing in letters, which were then transmitted down a wire. They also had a printer for printing out the signal they received from the wire as letters on paper. (So theoretically, you should be able to hook one of these up to a modern Linux box as a /dev/tty device.) However, electronics in the 1800s were not particularly advanced, and it turns out that the biggest improvements that could be made to the device was the addition of steam-power. Read the rest of this entry »

This page shows the detail of the hydraulic regulator in the letter-printing telegraph described here.

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This is the receiving station for the letter-printing telegraph described here. This is a lot more complex than the transmitter, so we’ll break it into stages. This first stage is responsible for converting the electrical signal back into a mechanical signal. Here is the pic from the patent.

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This is an example of one of the first letter-printing telegraphic devices where the sending operator press alphabetically labeled keys and the receiver unit printed the letters on a strip of paper. The patent was filed sometime in the late 1830s and granted in 1846. At the time, electric components were pretty much limited to electromagnets and batteries. Thus, most of the operation of the apparatus is achieved through clock-work mechanical means. I’ll go through the patent starting from the sending station, but the magic all happened at the receiver.

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