A pitchshifter is an electronic device for changing the frequency of an audio signal without changing the timing characteristics.

It is common knowledge for example that you can take a given audio sample and play it at double speed, and the frequencies will all be twice as high, but the duration of the sample will also be half the original.

A pitch shifter allows the frequency of the signal to be altered while keeping the duration of the sample constant.

How is this possible? I don't exactly know, but I know it's possible because I have one of these devices. The one I have is called a Digitech Whammy Pedal and is generally used by guitar players for a variety of pitch shifting effects.

I've played my stereo through this thing, just to see how well it works. It works best with small changes in pitch. Large changes tend to introduce quite a lot of audible and not necessarily pleasant distortion into the signal, especially an arbitrary (non-guitar) signal such as that from a stereo. Changes of exactly an octave up or down are pretty good though.

Some of the settings on the Whammy Pedal:

Harmony:           Pedal up              Pedal Down:
Original signal +  down 1 octave         up 1 octave
                   down 5th              down 4th
                   down 4th              down 3rd
                     up 5th                up 7th
                     up 5th                up 6th
                     up 4th                up 5th
                     up 3rd                up 4th
                     up b3rd               up 3rd
                     up 2nd                up 3rd

Whammy:
                  original signal        down 2 octaves
                  original signal        down 1 octave
                  original signal        down 2nd
                  original signal        up 2 octaves
                  origianl signal        up 1 octave

Detune: ("shallow" or "deep")

The unit has a "bypass" switch, a high impedance phono-plug input for guitar, and 'wet" (with effect) and "dry" (without effect) outputs.

The power supply required is 9V, 780mA AC (NOT DC)

How it works

A pitchshifting algorithm chops up a sound into granules. It then plays each granule back several times, not necessarily integer, at a higher speed. This way, the pitch changes while the overall duration is maintained.

For example, let's examine a simple sine wave with a frequency of 440 Hz. The period of this wave is 1/440th of a second. In an ideal case we can get a granule that is exactly one period long, or any integer number of periods for that matter. Since this is as ideal as we're going to get, let's take any multiple of 1/440 seconds. Now, the pitchshifter takes grains of that length and plays each one back twice at double the speed. This results in a one octave pitchshift.

Unfortunately, pitchshifting like this is is only possible if you are doing sinewaves with equal periods. When pitchshifting a complex waveform (like a guitar part) it is impossible to choose a grain size of which the edges will coincide with the ends of all periods. In that case, pitchshifting will produce unwanted artifacts in the frequency domain that change the timbre of the sound.