>I tried it, and you're right, Don, the sound is stull understandable....
You
>can see the results of my 'efforts' at:
>http://www.bzzzzzz.com/beehive/keelanl/xortest/


Hows about XORing  with a pseudorandom function (selected by the key).  You
would need some way to synchronize the receiver ... shifting phase until
minimum average amplitude is achieved is a simple way to syncrhonize
assuming your encoded data has a fair amount of silence or quiet parts.

Here's an example of a pseudorandom generator (for 17Cxxx .. substitute
"STATUS,C" for "ALUSTA,C" if using 14 bit or 12 bit cores):

                        BCF             ALUSTA,C
                        RRCF            RXCH,F
                        BTFSC           RXCL,0
                        BSF             RXCH,7
                        RRCF            RXCL,F
                        MOVLW           0X31
                        ADDWF           RXCL,F
                        BTFSC           ALUSTA,C
                        INCF            RXCH,F
                        MOVLW           0X03
                        ADDWF           RXCH,F

RXCL and RXCH are your 16-bit pseudorandom number.  XOR RXCL with your 8-bit
data stream on both ends.  Make sure the pseudorandom generator is in the
same phase on both ends (seeded to the same starting value of RXCL and RXCH
for a given sample).  Wouldn't this sound much like static unless you match
the phase and pseudorandom generator function on both ends?

hope this works,

Ken