Modulation effects are used to enhance the original signal by adding motion and depth. The most common effects in this category are chorus, flanger and phaser. In general, modulation effects tend to be used in conjunction with other effects such as reverb or delay.
Modulation effects are a subcategory of time-based effects. They work by delaying the incoming signal by a few milliseconds and often incorporate a Low Frequency Oscillator (LFO) which modulates the processed signal. This signal is then mixed back in with the dry one in varying amounts to produce the desired effect.
The Chorus effect adds a short delay to the original signal. The delay time is then modulated using an LFO before the processed signal is mixed back in with the original signal to produce the output. Chorus is a classic effect which tends to work best on clean, un-distorted guitar tones.
The Flanger effect is similar to that of a Chorus, but uses a much shorter delay time. Flangers usually offer the option of mixing the processed signal back into the input of the delay line using a ‘Feedback’ control. Sonically, a flanger will produce a more audible effect and tends to work best in instances where a particular section of a track is to be enhanced.
The Phaser effect consists of adding a slightly offset copy of the incoming signal and mixing it back in with the original. The duplicate signal is passed through a series of all-pass filters which shifts its phase and thus it creates a series of peaks and troughs in the frequency spectrum. The position of these peaks and troughs of the affect waveform is usually modulated over time using an LFO which creates an audible sweeping effect. Phasers are a common guitar pedal effect that gives a clean electric guitar a "shimmering" quality. They are also widely used on acoustic guitar, among many other instruments.
Other Modulation Effects include ensemble effects, ring modulators, vibrato and tremolo effects, phase altering effects such as stereo spreaders. Multi-effects units such as the Eventide H3000 series usually include a combination of all the above and offer extensive routing capabilities, ideal for achieving complex effects.