"A leslie makes a steady pitch actually alternatingly lower and higher, a typical example of bipolar modulation. In contrast adding some vibrato to a guitar string by bending the string by a finger will only shift the pitch up, so this is a typical example of unipolar modulation. So, the basic question is does the modulation have to change some reference signal in only one direction or into two directions symmetrical around a reference signal, e.g. a pitch played by a key on the keyboard."

I'm discovering 'relative modulation' and 'wave aberration' in relation to the Doppler effect.