Choir conductors often have to deal with the problem that the tuning pitch of a choir tends to decrease gradually over time. For that reason, we present an algorithm which measures and displays the tuning frequency evolution for polyphonic choir music over time. Basically, it analyzes the short-time Fourier spectrogram, picks out the most important peaks, and determines their frequencies. From these frequencies, the algorithm determines the concert-A pitch that leads to the smallest least-squares error when the measured frequencies are sorted into a semitone grid. The algorithm has been implemented on the Android platform and works in real time.
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