Modulation effects are those that change over time. Parameters of the effect are tied to a Low Frequency Oscillator (LFO).
If you don’t know what a Low Frequency Oscillator is, think of a clock and a light bulb. When the hand is on the 12 the light is all the way Off, as the hand moves past 1 the light begins to turn on; when the hand is on the 6 the light is all the way On and starts to turn off again as it moves past 7 back to Off at 12. This cycle repeats indefinitely, and you usually control how fast the clock spins. This is basically what is happening internally with each of these effects.
Tremolo is a very simple effect that very often gets confused with Vibrato. It doesn’t help that there are plenty of instances of the effect being labeled incorrectly by amp and pedal manufacturers. A Tremolo is a LFO-controlled Gain. Simply put, the Tremolo effect is the same as a guitar player turning the Volume knob up and down very rapidly. Usually, the user can adjust the speed of the LFO. Take a look at the BOSS AUDIO TR2 Tremolo Pedal, or Moog Minifooger Analog Tremolo Pedal.
The Vibrato effect is a LFO-controlled pitch shifter. With each cycle of the LFO, the effect raises the pitch by a small amount and then lowers it again. It is very similar to the way a guitar player naturally adds vibrato by quickly pulling and releasing a string while holding a note. The user can often control the speed of the Vibrato and sometimes the amount of pitch shift. Take a look at the MXR M68 Uni-Vibe Chorus/Vibrato Pedal, or DigiTech-ventura-vibe Rotary/Vibrato Pedal.
The Phaser works by splitting the signal into two parts and using an LFO to cause the second signal to be more and more out of phase with the original signal. When the two signals are combined, certain frequencies are cancelled out based on how much the second signal is out of phase at the time. This frequency cancellation creates a “whooshing” sound that when pushed too hard can begin to sound like an airplane taking off and landing. Take a look at the MXR M101 Phase 90, or Boss PH-3 Phase Shifter Guitar Effects Pedal.
The Flanger is an effect that sounds very similar to a Phaser but it is different and more complex. Instead of using phase inversion to create frequency cancellation, a Flanger uses a Delay. This creates a much more even comb-filtering effect over the entire frequency range; a LFO changes the delay time and therefore the comb-filtered frequencies. The Flanger effect covers the entire frequency spectrum and it is a very strong and “heavy handed” sound that dramatically changes the tone of the guitar. Take a look at the Source Audio SA240 Mercury Flanger Effect Pedal, or Boss BF-3 Flanger Guitar Effects Pedal.
Chorus is the most complex of the Modulation effects. It splits the signal into at least three parts and then adds Delay, Phase, Pitch Shift, and Panning to create a sound that really emulates multiple instruments or vocals playing at the same time. Chorus can create very full, open, and ambient sounds and can, at times, create an underwater type feeling. Take a look at the MXR M234 Analog Chorus Pedal, or Electro-Harmonix Small Clone Chorus Pedal.
The Ring Modulator gets its name from four diodes that are arranged in a ring shape and then paired with a carrier signal to produce its effect. The Ring Modulator was originally created for use in telecommunications, but it also made its way into other industries, including audio effects. The circuit multiplies certain frequencies in the original signal which gives the effect its characteristic metallic sound. The sound is quite reminiscent of a Resonator on a Steel Guitar, but the effect is much more pronounced and extreme. Take a look at the Moog MF Minifooger Ring Modulator, or Electro-Harmonix Ring Thing Modulator Guitar Effects Pedal.