A device which raises or lowers the frequency of an input signal. It does this using a complex amplitude modulation like technique. Unlike the pitch shifter, the frequency shifter does not preserve the harmonic relationships between the various tones and harmonics in the input signal. Because of this, sounds processed by a frequency shifter start to sound very unnatural with only a small amount of shift. However, unlike the pitch shifter, it is feasible to implement a frequency shifter with all analog circuitry -- meaning that frequency shifters were available (if expensive) as far back as 1950. Recently, there has been a resurgence of interest in frequency shifters as effects, and currently models intended for use with modular synthesizers are being produced by Modcan and Encore Electronics.