Guitar pedals explained (Part 3): Modulation

Guitar pedals explained (Part 3): Modulation


Modulation-based guitar pedals modulate one (or more) characteristic of the input signal, such as amplitude, frequency or phase. From just slight phase shifting to orbital flanging, these pedals can definitely change the character of the sound of your guitar.

How the modulation is produced very much depends on the effect. However, all of them change one of the characteristics of the signal. This change is generated (modulated) by a predefined pattern.

To do so, all these kind of pedals work similarly: the input signal is divided in two copies. One of them (dry) stays the same, whereas the other (wet) is modulated. Then both are blended together, creating an amazing effect.

As you know, any sound is composed by a few (or billions) of single sine waves. Each of these single sine waves are defined by three parameters: amplitude, frequency and phase.

  1. The amplitude (measured in volts) of the signal determines the volume of the sound. The higher the amplitude, the louder the sound.
  2. The frequency (measured in Hertz) sets the pitch of the sound. Remember that we usually tune any instrument by using an A at 440Hz as a reference.
  3. The phase of the signal (measured in radians, or degrees) express its time delay when crossing the zero volts amplitude.

Perhaps some of the effects that I include in this part don’t fit very well within this category, but it is sure that all of them ad some kind of modulation to the input signal. These are the most representative modulation guitar pedals:

  • Chorus. Chorus pedals try to make a single instrument like of more than one is playing, just like a voice chorus does.
  • Phaser. Phaser pedals shift the phase of the signal.
  • Flanger. Flanger pedals also shift the phase of the signal, but the effect is a little different than the one phaser do.
  • Tremolo. Tremolo pedals modulate the amplitude of the signal.
  • Vibrato. Vibrato pedals try to emulate the vibrato you can apply to the neck of your guitar naturally, with your left hand.

Chorus pedals

The chorus effect tries to make one instrument to sound like if more than one is playing.

To do so, it takes the input signal, doubles it, and put the doubled signal slightly out of time and tune with the original. This way, when both the original sound and the doubled one are played at the same time, it seems like two instruments are playing together, but not in perfect tuning.

To do so, they apply a short (selectable delay) that gives a more natural taste to two instruments playing together. That also apply a varying pitch shifting at a given rate, to emulate a “bad” but natural slight out-of-tune.

Boss CE2 Chorus
Boss CE2 Chorus

A little history…

Due to the short delay times these effect apply, these effects weren’t available until the seventies. The reason was, as with other effects, the availability of low price chips.

The first commercially available chorus pedal was the Boss CE1 Chorus Ensemble. As soon as it appeared in 1976, it was instantly assumed by the big fishes of the music scene. In fact, this pedal was one of the main factors why The Police sounded like it did. Andy Summers was a big fan of this pedal.

After the CE1, Electro Harmonix brough into the scene two other models that you can still find reissued: the Memory Man Stereo Chorus/Delay and the smaller Small Clone.

Then MXR, DOD, Ibanez and the rest of big manufacturers incorporated a chorus pedal in their catalogs. Today, a lot a boutique pedal makers offer chorus pedals in both analog and digital implementations.

Controls and features

The features of chorus pedals have increased with the time, mainly due to the possibilities that digital signal processing brings to these circuits. Nevertheless, you can expect to be able to control this parameters in any chorus pedal:

  • Level. This knob simply changes the presence of the effect over the dry sound. Turn it counter-colckwise and you will have the dry sound; turn it clockwise all the way up and you’ll feel the presence of
  • Tone. The tone control equalizes the high (or mid) frequencies of the wet signal (i.e. the part of the signal affected by the chorus).
  • Rate. This knob changes the speed (frequency) of the modulation effect.
  • Depth. This control changes the depth of the modulation.

Do I need a chorus pedal?

As I have explained in the Part 2 of this series, you NEED a few (a lot) of different gain pedals. However, modulation pedals are more like a personal choice.

Do you need a chorus pedals? Buy one if you like the effect that it produces, or just because you love the eighties… Check out any song by The Police and you will hear Andy Summers mastering the chorus effect with his guitar.

Chorus pedals sound great with clean sounds, either chords or arpeggios. On the other hand, they can make your distortions sound great too!

You may like the color this pedal can give to your sound when playing arpeggios in pop music. It’s great for some kind of hard jazzy sounds or fusion music too. Mike Stern is a great example of what you can do with this effect when playing impossible solos.

Phaser pedals

Phasers, or phase shifters, where originally designed to emulate the effect of a rotary speaker, like a classic Leslie cabinet.

Similarly to chorus pedals, they split the input signal in two and modulate one of them in a less intuitive manner than a chorus pedal does. A phaser takes the doubled signal and shifts its phase between 0 and 360 degrees. This phase shift affects differently to each frequency present in the original signal.

When the shifted signal is mixed back with the original signal, some frequencies phase cancel while others add together to create notches and peaks in the frequency response.

Modulating the filter’s phase shift with a low-frequency oscillator (LFO) sweeps those notches and peaks up and down the frequency range over time to create a spacey whoosh and swirl effect.

A little history…

MXR vintage Phase 90
MXR vintage Phase 90

Phaser pedals were created to emulate the effect of the rotary speaker of a Leslie Cabinet.

The first phaser pedal was the Univox Uni-Vibe. Even though it included Chorus and Vibrato controls on it, it was more a phaser pedal, based on optoelectronic circuits. Its circuit included discrete transistors, and some light bulbs and light cells, and an LFO. This is the reason why it was so big.

Big but sweet… Any of the modern clones don’t have its warmness and hypnotic sound. Just try to sound like David Gilmour in “The dark side of the moon”. No way without an original Uni-Vibe.

After the Uni-Vibe, other epic phaser pedals came into scene. The MXR Phase 45 and Phase 90, and the Electro Harmonix Small Stone are still available today at great prices.

These were simple devices (just a single knob to control the phase shifting). Nowadays, every major manufacturer includes one (or more) phasers in their catalogs. Some of them being fully digital, allowing endless features and controls.

Controls and features

There are some powerful phaser pedals with a lot of knobs and switches that are very flexible a versatile. They can provide a wide span of different sounds and effects.

On the other hand, take one of the greatest phasers of all time: MXR phase 90. How many knobs and switches does it have? Just one! And it sounds great…

Most of phaser pedals are very simple, and these are the typical controls you will find in an average phaser:

  • Depth. This control changes the maximum phase shift of the signal. Shifting the phase to higher values (usually turning the knob clockwise) gives a more noticeable effect.
  • Rate. Change the speed at which the phase varies from zero to the maximum value set by the Depth control with this knob.

Do I need a phaser pedal?

In my opinion, yes.

Not just because you can obtain orbital sounds with it… Small phase shifts at low effect levels make slight changes in the guitar tone that are great too.

But for those loving the effect of Leslie-like rotary speakers, or trippy sounds from the late sixties and seventies, this pedal can provide them with great times.

What if I ask you what is the best album of all times? I’m sure that “The dark side of the moon” by Pink Floyd is in your top 10 list (if not in your number one). David Gilmour taught everyone how to use a phaser (using an Univox Uni-vibe). Take “Breathe” as an example.

It sounds great, doesn’t it? Go and get a phaser!!

Flanger pedals

They are based on the same principle of modulation than phasers, with a similar sweep and motion to its sound.

However, they usually apply more dramatic frequency-altering effects going on within that motion, by imposing more control over the inverse points of the out-of-phase relationship, what results in a more oppressive effect.

They are more complex devices, requiring more involved circuitry and therefore more control knobs than phasers.

Electro Harmonix Deluxe Electric Mistress
Electro Harmonix Deluxe Electric Mistress

A little history…

Long time before the first flanger pedal appeared as a stompbox, the effect was generated manually in the studios.  How? By running identical recordings synchronized on two separate reel-to-reel recording machines, and placing a finger against the flange of one to slow it slightly; then releasing again to let the reel speed up again and chase the unadulterated machine.

As “simple” as that.

Once again, the proliferation of transistors made it possible to reproduce the flanging effect with an electronic circuit inside a guitar pedal.

However, flanger circuits are pretty complex so that they emulate manual flanging in a reliable way, and it wasn’t until the end of the seventies where the most preferred devices hit the market, allowing guitarists to widespread those trippy sounds.

Flanger pedals like A/DA Flanger, Electro Harmonix Electric Mistress and MXR Flanger set the standards for flanger pedals, and started forcing guitarist to spend a few hundred dollars (in the seventies) to get one of those stompboxes.

Controls and features

As I said before when discussing about phasers, there are flangers with a lot of knobs and switches, which can take you to another planet.

But the flanger effect is so great that they can sound orbital with just a few controls. Take as an example the Electric Mistress by Electro Harmonix… In an average flanger pedals, you will find three knobs:

  • Level. Flanger pedals usually have a level knob, so you can change how the dry and wet parts of the signal are mixed together.
  • Depth. The depth control changes the maximum time that the wet part of the signal is delayed.
  • Rate. The rate knob varies the speed (or frequency) of the time shift from zero to the maximum delay set by the depth control.

Do I need a flanger pedal?

I think that the flanger effect is something that either you love or hate it.

You can have a great time playing with your flanger pedal, but if you are in a band, be sure that your guitar will be ahead everything else if you use this effect.

But it could be great though… Add distortion to your flanger, and you will love the metallic sound you will get. Check out “Are you gonna go my way” by Lenny Kravitz. That is how you should use a flanger…

Tremolo pedals

By modulating the amplitude of the signal (i.e. volume level) you can obtain a helicopter-like pulsing effect at different speeds.

A little history…

DeArmond Tremolo
DeArmond Tremolo

In the guitar world, Tremolo first appeared as an amp-based device. It basically consisted on an tube-based circuit that cut (at different speeds and amplitudes) the signal entering the power tubes in the output stage.

The very first Tremolo pedal as a stompbox was the DeArmond Tremolo Control. I recommend reading this great article with the brief history of tremolo for more information about how this effect evolved over time.

Nowadays you can find great tremolo pedals. You will see some (like the Demeter TRM-1 Tremulator) with the few controls you could find in a vintage tube amplifier like the Fender Tremolux. On the other hand, other units are way out more complex, but also very versatile (check out the Empress Effects Tremolo 2).

Controls and features

Vintage tube amps implementing tremolo had very basic functionality. You were only able to change both the depth and frequency of the amplitude modulation.

There are modern tremolo pedals that have many possibilities to experiment with. However, most of them implement (perhaps with different names) the following controls:

  • Wave. Some tremolo pedals have a knob (or a switch) to change the shape with which the amplitude is modulated. It can be squared, saw teeth, sine wave, etcetera.
  • Depth. The depth changes the dynamic range of the amplitude variation. With the depth control all the way down, you won’t notice the effect. Turn it all the way up, and the amplitude difference will be maximum, just like an on-off effect.
  • Rate. This knob changes the frequency of the amplitude modulation

Do I need a tremolo pedal?

If your amplifier has a built-in vibrato, you don’t need a tremolo pedal.

But, would you like to have more controls over the vibrato, like modifying the attack, changing the modulation shape, etc. you’d like to check out one of the most modern stompboxes.

On the other hand, if your amp doesn’t have tremolo and you like old school, vintage effects, you should add a tremolo pedal to your pedalboard. They are very cool…

Vibrato pedals

Vibrato is another great example of a pedal that emulates a natural effect.

Think of an orchestra playing classical music. You will see any single player applying vibrato to the instrument.

By slightly modifying the note above or below its original pitch you obtain a warbling effect, similar to the one you obtain when bending a note of your guitar.

Univox Uni-Vibe
Univox Uni-Vibe

A little history…

Vibrato was also first adopted in tube amps, but it was also one of the first guitar pedals available for guitarists to play with.

As I’ve said before, the Univox Uni-Vibe was more a phaser than either a chorus or vibrato. However, it is also a kind of a vibrato unit. Guess where does its name come from?

Today, most of the vibrato pedals you will find are based on the old Uni-Vibe. Some of them claim to be clones (or slightly modified versions of it) like the JHS Warble-Tron and MXR M68 Univibe. Other pedals are re-designs of the Uni-Vibe, like this beauty: the Effectrode Tube-Vibe, a tube-based Vibrato unit, probably the best vibrato pedal in the market.

Controls and features

Just like modern flangers, state-of-the-art vibrato pedals may contain a few knobs that allow you to play with different kind of vibratos.

In any case, you can find, at least, the same controls that you had in a vintage Univox Uni-Vibe:

  • Level. Like in all pedals, the Level knob changes the presence (i.e.) volume of the effect.
  • Rate. The rate control changes the frequency at witch the vibrato is produced. You can generate vibratos at higher frequencies than your left hand (at least mine) can produce…
  • Depth. The Depth knob sets the maximum pitch of the vibrato.

Do I need a vibrato pedal?

Not really. You can do vibrato with your left hand, or with your whammy bar. But, as I always say, you love guitar pedals… Do you like Pink Floyd? What about “The dark side of the moon”? Listen the guitar at “Breathe”. Liked it?

Mmmm, perhaps you start thinking on buying one now…


Here is the complete list of posts of this “pedals explained” series: