While many say that the signal is FM, it really is not. It is frequency shift keying where, when using positive shift, the center frequency is increased by about 5 killocycles for a data pulse "one" and at the center frequency for a "zero". If the transmitter is on negative shift, the data pulse is about 5 killocycles below center frequency for the "one".
The difference between dual conversion and single conversion is that, for dual conversion, the receiver crystal is about 11 megacycles different from the transmitted frequency while for single conversion, the receiver crystal is 455 Killocycles different than the transmitted frequency.