I've not tried this, and I have no idea how hard it would be to
implement in a PIC, but a 'new' mode called PSK31 is being used
in the Radio Amateur world. Its very narrow band and, if reports are
to be believed, has very good range / power ratio. Here's a link to a
description of the mode. there is also an egroups.com discussion
available.
http://www.packetradio.com/psk31.htm

Mike W  G8NXD

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