I'm developing an underwater modem which uses acoustic signal for transmission and reception .for that i need to have a FSK modulator.since CPFSk is not practically possible in underwater ,i decided to use VCO.I have to modulate data from a file sourc(output type byte) using 10khz carrier.i don't know how to do this.when i connected a file source -uchar to float- vco -scopesink and observed output i couldn't see what i was expecting.can't i input the vco with a file source?please help me.