What is a frequency shifter?
Frequency Shifter is 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. This means that frequency shifters were available (even though expensive) as far back as 1950. Recently, there has been a resurgence of interest in frequency shifters as effects. Currently, Modcan and Encore Electronics produce models intended for use with modular synthesizers.
Ring modulation resemblance
Frequency shifting is similar to ring modulation but produces only an upper or a lower spectral side-band whereas ring modulation produces both side-bands. Ring modulation with a sinusoid carrier can create a very dense spectrum. It introduces new partials symmetrically around the carrier, so the number of partials doubles. Since frequency shifting (a.k.a. single side-band modulation, SSB) only shifts the partials that are present in the sound, it does not increase the number of partials. Therefore it may sound a bit hollower or smoother than ring modulation.
If the input sound is harmonic, a frequency shift will alter the ratios of the partials and make the sound inharmonic, unless the shift amount is equal to the frequency of one of the harmonic partials in the original sound. Downshifting may result in negative frequencies. But there is no way of telling if an observed frequency in a signal is positive or negative, so a negative frequency will just flip sign and become positive. For example, suppose the original sound has partials at 50, 150, 250 and 350 Hz. Shifting this signal down by 180 Hz gives new partials at -130, -30, 70 and 170 Hz. Since negative frequencies are just mirrored into positive, the new spectrum has partials at 30, 70, 130 and 170 Hz.
Frequency shifting plugins
In modern frequency shifter devices such as plugins, we can usually find similar controlling parameters. These parameters are Coarse and Fine parameters and they are setting the amount of the frequency shift. Besides that, more complex plugin units offer switches between frequency shifting mode to ring modulation mode. Additionally, we can find a ‘Wide’ parameter which is used to create a stereo effect by inverting the Spread value in one channel so that one channel is shifted up, whilst the other is shifted down.
Frequency shifting can be used for tuning drum sounds. Used in small amounts, frequency shifting can make a nice alternative to pitch shifting when tuning drums.
Additional resources & source texts