The term vibrato refers to small, quasi-periodic variations in the pitch of a tone. On a violin, for example, vibrato is produced by wiggling the finger stopping the string on the fingerboard; a violin vibrato frequency can be very slow, or a bit faster than 6 Hz. A typical vibrato depth is on the order of 1 percent (a semitone is percent). In the singing voice, vibrato is produced by modulating the tension of the vocal folds. Vibrato is typically accompanied by tremolo, which is amplitude modulation at the same frequency as the vibrato which causes it. For example, in the violin, the frequency-modulations of the string vibrations are translated into amplitude modulations by the complex variations in the frequency response of the violin body.
To apply vibrato to a sound, it is necessary to apply a quasi-periodic frequency shift. This can be accomplish using a modulated delay line. This works because a time-varying delay line induces a simulated Doppler shift on the signal within it.