PURPOSE:To prevent a hawling by changing the musical interval pitch of an input signal which is inputted to a musical interval pitch changing device from a microphone, etc., and outputting said signal after amplifying it. CONSTITUTION:The signal inputted from the microphone, etc., is inputted to the musical interval pitch changing device through a variable subtracter, and the musical interval pitch is properly changed. The output of the musical interval pitch changing device is amplified by an amplifier, is outputted, and is given to a speaker. The input signal is inputted to a controller too. The output of the musical interval pitch changing device is given to a memory device as well and is stored. The controller refers to the input signal and the stored output of the musical interval pitch changing device, and if a similar signal is contained in the input signal, said controller removes said signal by the subtracter. The stored contents of a memory device is renewed at a suitable interval. In such way, the hawling is completely prevented. By only the musical interval pitch changing device, the hawling can be prevented.
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