Hi Andi,

I think that building the modem in software you are making the job more dificult as it should be.

TCM3105 is out of production since at least4 or 5 years. You can use MAX614 it is cheap and small.

I have no source code that implements 1200 bd modem in software for the 8051 but some time ago I got in the net a full project to make it with a PIC :-P , source code, schematics, etc

If you are interested, I can mail it to you.

Best regards.

Fran Dominguez