The device of the invention extracts a fundamental frequency from signals coming from a played musical instrument. From this is synthesized a waveform with the same fundamental frequency which can be given an arbitrary form, so that an audical impression of e.g. a violin, a trumpet or a guitar can be given to sound produced by the waveform. The waveform is produced by making a pulse train with frequency n times the fumdamental frequency, leading the pulse train to a counter activating cyclically and sequentially n different outputs. The outputs are summed with different and adjustable weights, and the waveform is determined by adjusting the n weights. The number n can be any number. An embodiment is shown with n=16.