A simple note onset detection system for music is presented in this work. To detect onsets, a 1/12 octave filterbank is simulated in the frequency domain and the band derivatives in time are considered. The first harmonics of a tuned instrument are close to the center frequency of these bands and, in most instruments, these harmonics are those with the highest amplitudes. The goal of this work is to make a musically motivated system which is sensitive on onsets in music but robust against the spectrum variations that occur at times that do not represent onsets. Therefore, the system tries to find semitone variations, which correspond to note onsets. Promising results are presented for this real time onset detection system.