What could be wrong if you cannot decode an RTTY or other FSK signal even though it is apparently tuned in properly?

Decoding RTTY (Radio Teletype) or other Frequency Shift Keying (FSK) signals requires your receiving software or hardware to perfectly match the transmitter's parameters. If the baud rate (speed of transmission) is mismatched, the decoder will only produce garbled text. Furthermore, FSK relies on two specific tones called 'mark' and 'space.' If you are listening on the wrong sideband (e.g., USB instead of LSB for traditional RTTY), these tones will be inverted. Many software programs also have a 'reverse' toggle that can flip the mark and space frequencies even if the sideband is correct. Because any of these issues will completely prevent a signal from being decoded despite being perfectly centered on the waterfall display, all of the choices are correct troubleshooting steps.