This is a radio – but also a modem, in a literal sense of the word. You may be familiar with AM and FM. Have your tried using them backwards?
In this patch, an audio signal is modulated using SSB, AM, and FM then frequency shifted to about 1/4 the nyquist frequency. Then the process is reversed revealing the original audio.
notes
Have improved and simplified FM and AM detection. There is no frequency shift prior to detection. This eliminates some phase distortion.
Pd Version. (runs at 44.1 kHz, because I was too lazy to figure out how to do audio file playback which detects the SR of the audio file.
FM detection: The DC offset and makeup gain levels are sensitive to carrier frequency and SR. Will need to automate these corrections to prevent audio output problems.
download
max-projects: https://github.com/tkzic/max-projects
folder: demodulation/max
- modem3.maxpat
- obama.aiff
folder: demodulation/pd
- modem-pd.pd
- freqshift-abs~.pd
- obama.aiff