Originally RTTY was strictly FSK. The frequency was actually shifted +/- 170 Hertz. This was by either shifting the VFO or switching between two crystals. At the far end a discriminator recovered the FSK modulation. When RTTY operation shifted to computers or more modern terminal devices, and equivalent method developed. It was easy for the computer to generate AFSK tones. When those tones are inserted into an SSB modulator, you get the equivalent of FSK due to the fact that the SSB signal consists of only the one sideband which at that point is either one of the two ASFK frequencies. At the receiving end the same equivalence applies. if you tune a true FSK signal with an SSB receiver, the local oscillator beats with the two FSK frequencies which then are recovered from the receiver as a tone. With AFSK the same reciprocity exists.
R Andrew O'Brien wrote: >OK, I think I get it but.. > > > >> With FSK, there is no setting up of the drive levels from the sound card >> since you are basically switching the frequencies from inside the rig. >> >> >> > > >tell me more about the above. How does the switching of frequencies >generate tones heard at the other end. > > >
