A vocal harmonizer pedal is a type of pitch shifting device that combines an original pitch with its shifted pitch to create two or more notes in harmony. It is a method of pitch shifting that alters the original pitch to produce different pitches at various wavelengths. Often, the device is called a pitch shifter; a device or sound effects unit that raises or lowers the pitch, by a preset interval, gotten from an audio signal.
Vocal harmonizing involves pitch shifting, and there are diverse methods used. One of the simplest methods used is to increase pitch and reduce duration, or increase duration and reduce pitch. The approach here is to keep replaying the waveform of the sound at different speeds than the speed it was recorded. An old way to do this is to use a reel-to-reel tape recorder by changing the drive shaft diameter of the capstan or using a different motor entirely.
Now let’s get back to the characteristics like the speeds of the motor can be controlled using an electronic servo system circuit. A straightforward is by using ‘vari-speed’ capstan motors that allow speeds to change and be achieved more simply. If you use a vinyl record turntable, you can place your finger on it to slow it down with friction, or you can spin it to give it more speed.
How to judge which one is most suitable for you
Technology has improved over the years, and you are most likely to use modern methods and devices, there are plenty of decent choices, decently take a look at this great resource Best Vocal Harmonizer Pedals | Review 2018 - CGuide.