Linear FM

As we mentioned in the glossary entry on FM, frequency modulation is a common synthesis technique where the pitch of the carrier oscillator is varied very quickly – usually at audio rates – by the modulator oscillator. It would seem to make sense to use an exponential control voltage for this, as the same level of modulation – say, +/– 1 volt – would produce the same offset in pitch, regardless of the tuning of the carrier oscillator as you played it up and down the keyboard. And many oscillators have exponential FM inputs. The problem is, this makes it trickier to tune, as adding frequency modulation can change the apparent fundamental pitch of the carrier oscillator. Therefore, many prefer using a linear FM input on oscillators. The Yamaha DX7 synth and its descendants use linear FM.