Over-the-Air Acoustic Modem for Text Transmission
This MATLAB modem sends short text messages between two laptops through their built-in speakers and microphones. The project follows the whole path from ASCII characters to an audible waveform and back, including the practical problem of finding a message whose arrival time is unknown.
The transmitter converts characters to eight-bit ASCII, adds a 32-bit alternating preamble, and turns the frame into bipolar pulses on a 3 kHz carrier. The receiver demodulates the recording and applies a 101-tap FIR low-pass filter before isolating the packet. It tests candidate alignments against the known preamble and printable-ASCII plausibility, then uses each pulse segment’s mean to recover bits and reconstruct the text.
The figures show the received packet and recovered baseband from the two-laptop trial. That run recovered HELLO WORLD with zero errors across 120 frame bits, including 88 payload bits. I was the primary developer, working with Max Pohl on implementation, testing and analysis.
The zero-error result is one documented observation, not a reliability guarantee. Distance, microphone gain, room acoustics and background noise were not characterized systematically.
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