Frequency modulation, or FM, is a variation in the frequency of a signal being generated, according to the instantaneous value of a modulating waveform. The signal whose frequency is being modulated is usually referred to as the "carrier" (a term borrowed from radio). When the modulation is of subsonic frequency, the result is a slow or rapid variation in the pitch of the carrier signal which is referred to as vibrato. (Performers often use vibrato to make a sustained sound more interesting to the ear; this is a practice that long precedes the advent of synthesizers.When used judiciously the effect is engaging, but over-use can make it cloying to the ear.) When the modulation is of audio frequency, the result is a composite waveform containing a complex frequency spectrum mathematically related to the frequencies of the individual harmonics contained within the carrier and the modulation.As it happens, frequency modulation is capable of mimicking the harmonic content of a wide variety of acoustic instruments, ranging from piano to trumpet to sitar. The Yamaha DX-7 synth of the 1980s exploited this characteristic to great effect.FM has a reputation for being a difficult method of synthesis to program sounds in; it is not always easy to know how to approach getting a particular type of sound or predict in advance the results of a particular patch. Compare with amplitude modulation.