Spectrum of FSK signal
With Frequency Shift Keying, the modulation (digital data) takes up bandwidth, so you can't just keep only the frequencies of the mark and space tones. A firm lower bound on how little bandwidth you can use is the distance between the mark and space frequencies, plus half the baud rate on either side. So for 1200 baud with frequencies of 1300 hertz and 2100 hertz, the absolute minimum bandwidth is (1300-(1200/2)) [700 hertz] to (2100+(1200/2)) [2700 hertz] which is a bandwidth of 2 kHz. People have tried to filter it tighter but if the reception still provides the correct data, it is only because of chance. Usually there is also some pulse shaping in the FSK signal before modulation to make the filter's job easier.