From Manufacturer
The Korg VPT-1 Vocal Pitch Trainer is a tuner for your voice, so you may learn to sing in tune. It resembles a guitar tuner, yet it's designed to be held in your hand and show you, on its five-line musical display, whether or not you're in tune.
Red LEDs at the left of the display show you whether or not you're sharp or flat. The blue LED between them indicates that you're pitch is spot-on. Use the Octave Shift function to adjust the range of the tuner's detector in octave increments, so it can better work with your range, and display the note in the most readable part of the staff. Three adjustable tuning levels help you get better and better over time. Start with Easy and gradually move to Hard.
Generate a reference tone for you to match straight from the VPT-1 itself, or use the Sound Back feature when connected to headphones to help you improve your ability to stay in key by playing the reference tone that's closest to the pitch of your singing. This reference tone, together with the tuning guide LEDs, allows you to train yourself using three senses: sight, sound, and feel.