Distortionless frequency-shift keying: further results

In a previous letter, it was shown that digital frequency-shift signals could, in principle, be recovered without intersymbol interference, and a narrowband characteristic for binary f.s.k. was derived. The theory is now extended to cover both multilevel f.s.k. and wideband f.s.k. carried over lowpass channels. In all cases, channel characteristics can be found which yield zero i.s.i. in the signal after frequency demodulation. It is shown that, for a given information rate, the bandwidth required is minimised by use of 4-level f.s.k.