> can you use FSK with a zero-crossing-detector on the receiver side 
> [measuring pulse width to determine frequency]

Yes. I think that corresponds fairly well to class 2 that I listed.

> 2.    TX generates dual tone FM or mark space modulated frequency or 
> some other tone generation system. receiver processor detects this 
> directly. Hard on receiving processor. Noise immunity poor. Possibly 
> cheaper than 1.


        RM 

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist