What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?

When an AFSK signal's input audio level is excessively high, the transmitter's amplifier stages are driven out of their linear operating region. This non-linear operation causes the audio frequencies to mix with each other inappropriately, generating unwanted sum and difference frequencies. This phenomenon is evaluated as Intermodulation Distortion (IMD). Options like Signal-to-noise ratio or Baud error rate are incorrect because they evaluate the overall quality of the received data link and decode success, rather than measuring the specific physical distortion being generated by the overdriven transmitter.