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Physics for all



How does auto-tune work? When it changes the speed is the pitch changed and vice versa?

Similar to a spell-checker, or photo-editing software, the audio program Auto-Tune can correct a singer's bad notes and wavering pitch. This is important, since only very few singers are perfect. Sometimes, the pitch of a singer's vocal slightly misses the exact note he/she is trying to hit. Auto tunes can also be used to distort the human voice when pitch is raised or lowered significantly.

So, how does it work?
The first thing to realize is that sound waves have very similar properties to waves in sea; that is, sound is a repeated perturbation in a medium, which can be air, water, walls, etc. (It is not identical to waves in the ocean, since sound waves are pressure waves - they are longitudinal, rather than transverse, but this doesn't matter to the rest of the explanation). The difference between the different pitches in sound waves is the length of each perturbation, or their frequency, which is measured in cycles/seconds, known as Hz. Humans, for example, can hear frequencies between 20 Hz (very low, "deep" sounds) to ~20,000 Hz, while other animals have different hearing range (many animals can hear at frequencies much higher than 20,000 Hz).

The pitch of a note depends on the frequency of the sound wave produced. For example, the "A" above the middle "C" is usually defined at 440 Hz, the B above it is ~493.8 Hz, etc. In fact, there is a precise mathematical formula for the frequency of each note: each successive pitch in a piano note, for example, is derived by multiplying or dividing the previous one by the twelfth root of 2, (2{1/12} ~ 1.05946..). This corresponds to exactly semitone (which is the difference in notes between two adjacent keys in a piano key).

Returning now to auto-tune, if a singer produces a voice that is out of tune - say attempting to sing "A" but it comes out at 445 Hz rather than at 440Hz, a computer can be used to correct the frequency back down to where it was intended to be, ensuring that the recording sounds in tune.

Technically, that requires a bit of manipulation, since one cannot directly change the frequency of a wave, because the frequency is related to its length (or speed): if one tries to change the frequency directly, the sound will "speed up" - just what happens when speeding-up takes the effect of a chipmunk-like sound. This, however, can be done digitally.

In digital sounds (which is what you have in your CD, for example), every frequency is represented by a discrete number. These numbers can be altered: first, one can alter the duration of the sound (without altering its frequency), and then alter its frequency. Thereby, the altered sound can be made to both maintain the original duration, but at the desired frequency (=tune). Overall, this requires some advanced math (mainly, Fourier transform), but can easily be done with modern computers.

For further information, see http://www.physics.org/article-questions.asp?id=75

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