PURPOSE: To increase the detection accuracy of the basic frequency of the musical sound signal and ensure a simple and accurate tuning, by providing the LPF that cut-off frequency shifts toward the lower level according to the input signal at the front stage of a comparator.

CONSTITUTION: The input signal is converted into the digital signal by a comparator, and then the periodical properties of growth is calculated to obtain the basic frequency of the input. And then the musical interval is obtained from the reference sound of the frequency recided from outside to be displayed. In this case, an LPF is installed at the front stage of the comparator. With application of the input, the LPF decreases the current flowing to the LED of the photocouplers PC1 and PC2 after the terminal voltage of the capacitor C3 in the wave detecting circuit is inversed and amplified through the amplifier A2. As a result, the resistance increases for the cells of both the PC1 and 2 to cause a shift of the cut-off frequncy of the LPF toward the lower level, thus eliminating the higher harmonic component.