Piano Doctor

This app analyzes piano sounds. It detects the frequencies in the sound and estimates how far the sound's tone is away from the target tone. In doing so, this app measures a band of frequencies called the target frequency band, which is defined as the frequency range between -50 cents and +50 cents centered at the target frequency. Then, this app identifies the peak frequency where the sound has the biggest power in the band. The difference between the peak frequency in the target band and the target frequency is displayed in the gauge in the user interface. The frequency estimations are based on 131,072 point Fourier transforms and such long Fourier transforms provides very stable gauge readings.

While detecting the middle and higher tones of a piano can be done with little difficulty, detecting lower tones such as A0 (27.5Hz) raises a challenge. This is because most mobile devices' microphones cannot pick up such low tones. Measuring frequencies in the harmonic bands can help overcome this difficulty. Harmonic bands are defined as the frequency bands where the frequencies are integer multiples of the fundamental band.

This app measures frequencies in the 6 harmonic bands (from the second to the seventh) and selects the best one automatically if the "AUTO" button is selected in the harmonic selector. A separate gauge is provided to show the frequency offset calculated from the harmonic band. In the case where users want to see the frequency responses in a specific harmonic band, users can use the harmonic selector buttons located near the harmonic gauge.

In addition to the gauges showing the frequency offsets, this app provides two spectrum viewers. While gauges are showing only peak frequencies, spectrums viewers are showing frequency spreads in two-dimensional shapes. This helps identify which band's measurement is stabilized and ready. Users should pick up and read the gauge with a stabilized spectrum.

Pitch and octave buttons are provided to select target tones. The buttons allow users to select tones quickly and randomly. There are also seven preset buttons. These preset buttons are shortcuts for predefined target tones. The preset buttons can also generate their corresponding tone through the device's speaker if they are double-clicked. One of the purposes of the self-generated tones is to self-test this app. With the self-generated tone on and the speaker at the proper volume, at least one of the gauges should report the precise frequency that is being generated.