Waveform (top) and harmonic peaks (bottom) comparing human and robotic playing at string A₃. The spectrum frequency-axis is given as normalised by the fundamental frequency (so that the peaks align with each harmonic number). The spectrum of the human-played tone is plotted in gray for better visualisation. The vertical lines separate the harmonic peaks used for the errors E_a,1, E_a,2 and E_a,3.