Pitch trainer - Harmonize is a music game app developed by Toshihiko Arai. It is designed to help users learn perfect and relative pitch, with a specific focus on double-stop notes in just intonation. This app is suitable for anyone, from those who want to improve their karaoke skills to members of brass bands and orchestras.

The app features the playing of two sounds simultaneously, with the decision based on just intonation. For example, when matching the major third "do" and "mi," the interval in equal temperament is 400 cents. However, in just intonation, "mi" has a frequency ratio of 5/4 to "do," resulting in an interval of approximatelycents. The app provides a piano sound as a guide, but users are encouraged to match the tone by feel rather than relying solely on the piano sound.

One interesting aspect of the app is that users need to operate it in a way that they lose the beat. At a certain position, the sound suddenly becomes clear, indicating the just interval tone. It's important to note that the chords' reference tone in the app is A=442 Hz.

Pitch trainer - Harmonize is a free app available on the Android platform. It offers an enjoyable way to learn and improve pitch recognition skills, making it a valuable tool for musicians and music enthusiasts alike.