For a long time I have thought it would be cool to build some sort of a modem. And finally I had the need for something that could send low-speed wireless data from my GPS to a computer.
There are many ways to do this, but I wanted to make something that would use audio tones so that the data could be sent over any audio link, recorded as audio, etc. This would give the most flexibility in the transmission medium.
Ham radio operators have used RTTY (Radio TeleType) for years on the HF bands to send low-speed text data over the air. This technology came from original teletype machines, and the encoding is Baudot, not ASCII. Basically, it’s 5 bit data with a start bit and either one or two stop bits, sent at 45bps. Using standard frequencies of 2125Hz for 1 and 2295Hz for 0 (170Hz shift) means that it will work with any equipment set up to receive RTTY. Conventionally RTTY is sent by changing the carrier frequency by 170Hz. Receiving stations adjust the tuning until they hear the two tones at the correct frequency. That is called FSK. (Frequency Shift Keying) With the advent of FM transmitters, etc. it was useful to be able to simple send the tones, so AFSK (Audio Frequency Shift Keying) was created. All it means is that the tones are generated and sent as audio.
There is a great free program called MMTTY (check Google) which uses the PC sound card and some DSP code in software to detect RTTY tones and display the data on the screen. It works very well even with noisy or weak signals. This is the program that I use on the receiving end. It can transmit as well, if you have a Windows PC at both ends of your link. My project is designed for transmitting ends that require a minimal amount of equipment. It’s quite possibly one the simplest data modulators that can be made.
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