Turning a Walkie Talkie into a Radio Modem

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Remember those modem things that made screatchy noises like mutant mice? Shockingly, some people will not. Getting over this outrage, -- before the advent of phone data and broadband, these were the devices that let you play Command and Conquer over the phone line with your friend, download an MP3, or maybe several if you left it on all night…

Remember those modem things that made screatchy noises like mutant mice? Shockingly, some people will not. Getting over this outrage, -- before the advent of phone data and broadband, these were the devices that let you play Command and Conquer over the phone line with your friend, download an MP3, or maybe several if you left it on all night…

Recently, our technical director ordered a new Baofeng Transceiver for himself. After finding fun things to listen too, and bizarre transmissions of people counting out strange numbers, we started hearing odd noises coming from his ‘ordered’ and ‘efficient’ ‘work area’.

It turns out that his latest invention is a primitive form of modem. This modem used two audio tones, denoting 0 and 1, which represented a message (plain text initially). This text was encoded as bytes (Modulated) and sent warbling out of an Apple iMac’s audio socket, to be sent through the air, as audio which was detected by a microphone receiver and decoded (demodulated). Slowly, the famous words did appear.

o World! Hello World! E

At a stunning speed of approximately 8 bits per second.

People in the office were told to keep quiet, and the air-con was switched off, as this thing called ‘noise’ upset the signal and made erroneous characters appear in place the intended letters.

What is a Modem?

Everybody knows the answer to this. The box thing that lets you access Facebook and use Bit Torrent (for legitimate purposes). It used to make odd noises and take its time about things; these days, it is quiet and very fast. It might have Virgin written on the side of it, but how do they actually work? A modem is an abbreviation of Modulator-Demodulator. These devices encode digital information into an analog carrier signal (in the initial example here, that analog signal was a sound wave moving across the room). The modem tasked to receive the data, demodulates this analog signal and converts the data back to digital information.

The history of the modem is a large topic. What has made this research project interesting is how quickly Nat (the Technical Director) has been enhancing the original idea and introducing new features which follow a similar trajectory to the evolutionary path of original modems.

The next step was to change the analog channel from standard audio to a radio frequency. This was where the Baofeng came into play. The digital data was converted using Javascript. The resulting audio sent out of the iMac’s headphone jack and into the Baofeng which transmitted it. Another Baofeng and a Macbook Pro at the other end reversed the process and the message was displayed to the screen as it was received.

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Go faster dammit!

The initial system was not particularly quick managing a blistering speed of about 8bits per second. This data was sent in a linear sequence, one piece of data followed after the other in a neat orderly queue. The next upgrade was to research increasing this and the next iteration is considerably more complex.

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The image here shows 8 dedicated frequencies, each one of which can hold a byte of data. In the new transmission, a clock is used to send a time stamp. The receiver continually checks the this stamp and if a new incremented stamp appears then it knows that new data has arrived. This allows the transmitter to control the rate at which the data is sent. The audio for this ranges from creepy fairy tale weird to something that Aphex Twin or Ryoji Ikeda would could up with. There is even a basic error checking algorithm in place to keep the time stamp correct.

Could this be THE-FUTURE?

An idea being bounced around the lab is to buy a transmission license and host our website via radio transmission. Something along the lines of a Videotex text system like Prestel or Minitel but via radio. Practical? Perhaps not, amazing? Most definitely.

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