The strength of onsets in each frequency bin with the power spectrogram after Sobel filtering. a Popular music (120 BPM), b guitar backing performance (110 bpm). Red ballets, red triangles, blue ballet denote tactuses of the pieces, absent notes at tactuses, error candidates of tactuses. In this paper, a frame is equivalent to 0.0116 sec. Detailed parameter values about time frame are shown in Section 3.1.