http://www.svn.net/mycal/digital/encode.htm
Frequency Shift keying (FSK) as important. Related:
RTTY decoder multimon AX25msxtape fskmodem.c in Asterisk source code soundmodem drivers in kernel and soundmodem package discussion of soundoops and speaker twiddling on kernel mailing list
use:
cat /dev/dsp | ./devdisplay [width] [height]http://1010.co.uk/devdisplay.c
gcc -o devdisplay devdisplay.c -lSDL