The newest of digital modes uses "Phase Shift Keying" to signal mark and space values. Instead of changing the frequency like RTTY, DSP is used to change the phase by 180 degrees when a switch from mark to space is needed. This is the same as instantly skipping half of the simple sine wave cycle of the single pure tone (called "a coherent signal"). The ear detects each transistion but then perceives it as the same tone as before (which it is). DSP analysis by the program instantly detects this "Phase Shift". The baud rate is 31 and it can use the full Ascii set like Pactor. The signals themselves are often found on 14069 KHz. The last time I heard such a sound, I was underwater and an outboard was passing by! Some more references are on: http://www.megalink.net/~n1rct/psk/pskin.html
