[liberationtech] Efficient digital one-way communication

[flane at pgm.com]

There are many such musical-sounding MFSK systems in use, mostly on HF 
SSB, that have been doing this since the 1950s. See
http://www.nonstopsystems.com/radio/radio-sounds.html for samples. FM 
and SSB are very different modes, but the design constraints for the 
signal are similar. You'll notice that most of the old HF modes turn the 
tones on and off slowly, this is really important to keep the bandwidth 
within the channel.
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