If you're using your AD for a voice transmission, you should use one of the compression techniques widley used nowadays in ISDN telephony like A-Law or u-Law 8-bit PCM coding. You can find a standard description on the internet, look for G.711 voice compression standard.

16-bit linear data is compressed to 8-bit A-Law without losing the voice dynamics. The signal is segmented to 8 ranges and coded according to the following transformation table:

Linear Input Compressed Code
-------------------------------
0000000wxyza 000wxyz
0000001wxyza 001wxyz
000001wxyzab 010wxyz
00001wxyzabc 011wxyz
0001wxyzabcd 100wxyz
001wxyzabcde 101wxyz
01wxyzabcdef 110wxyz
1wxyzabcdefg 111wxyz

First 3 bits define the segment range while 'wxyz' are the quantization bits of the signal.

This way, you can double the throughput rate from 115 to 230 kBaud.

I'm sure that you can find also the conversion routines on the internet.

Franc