PSK31 Description

You can see also how much spectra they use. These images have been obtained with EvmSpec.
You can hear these modes here: Note that these files will be correctly decoded if feeded on the DSP board or SoundBlaster, that's why they are so lengthy.Note the preamble at the start and end of the transmission. If you only want to know how they sound, you can get the ADPCM compressed version (I haven't tested if they decode correctly). A basic approach to understand and design a PSK31 modem is here, by Peter G3PLX. See also articles section