Abstract: A method and apparatus for transforming audio input signals with variable amplitude and narrow bandwidth into outputs with relatively fixed amplitude and variable bandwidth responsive to input signal amplitude, by the steps of dividing the range of amplitude into an odd number of equal intervals, detecting the interval of amplitude instantaneously occupied by said signal, shifting said instantaneous signal to the central interval, and smoothing the resulting discontinuous signal with a continuous and bilaterally tapered transfer function. This method permits bandwidth expansion and control of the output bandwidth of musical tones produced in electronic musical instruments in response to control of the input amplitude of the tones.