A device used with MAGNETIC TAPE in order to vary either the PITCH or TEMPO of recorded information but not both variables together. Normally with tape playback, a change of speed or tempo produces an equivalent change in pitch, but with this device the two variables may be treated independently.

The basic principle of the machine, first developed in Germany, depends on the relative direction and speed of the tape and that of multiple playback heads mounted on a rotating cylinder. The tape passes over the heads such that only one head is in contact with the tape at any one time, since the moment one head leaves the tape, a second head has rotated into position to continue contact.

If the rotating heads are moving in the same direction as the tape, the resulting pitch will be lower without affecting the tempo, whereas if the heads are rotating in the opposite direction of the tape, a higher pitch results. The principle is similar to that of the DOPPLER EFFECT which depends on the relative speed of sound source and listener.

The machine has a variety of uses, from simple adjustment of TUNING to the creation of new sounds with timbral or durational characteristics that are normally impossible. A variety of names apply to the device: the original German zeitdehner (literally, 'timestretcher'), as well as tempo regulator, pitch regulator and information rate changer, or simply rate changer. Compare: TAPE RECORDER.

Today, various digital techniques, such as granulation, that "window" and overlap short durations of sound perform operations similar to those of the tempophone such as time stretching.