Quick theory to application question.
In Frequency shift keying we usually have the output of a sequence generator into a VCO, then this leads to a high frequency for the mark bit and a lower frequency for the space. What would I have to do to set it up so that the high frequency is the space bit and the low frequency is the mark.
This is referred to as FSK polarity, and the exact configuration varies for different implementations. The labelling of which is a 0 and which is a 1 is arbitrary, so you can decide how you interpret the data.
In your specific example, if you wanted to be sending a high frequency for a 0 and a low frequency for a 1 (without changing any of the transmission hardware), you would just need to invert your data stream before you send it to the transmitter, then on the receiver end when it is demodulated, invert it back again to recover the original data.
Of course, the easier way to do this would be modifying your Tx/Rx scheme to swap them round, but the exact method of this would be dependent on the exact scheme in use.
