Modulation Effects (Part 1)

The modulation will give your sound a sense of motion, dimension, and depth. Modulators (sometimes called "control signals") allow you to control how and at what rate the sound changes over time. Besides, they can even introduce new spectral content to the sound (see blog: Intro to FM synthesis). Below is an example of one type of modulation called "amplitude modulation."

Modulation effects process an input signal dynamically over time. Of course, today, we have a multitude of modulation effects. Primary modulation effects, in essence, operate on the principle of copying an incoming signal and modulating that copy as it is mixed with the unaffected signal. This induces "phase shifts," which affect the overall sound.

LFOs (low-frequency oscillators) in modulation effects are similar to LFOs in synthesizers. However, they produce frequencies that are too low to hear. For this reason, they are used to modify other signals periodically.   LFO parameters include….

Rate (sometimes called "frequency" or "speed") - determines how frequently the oscillator will cycle the selected waveform.

Depth (sometimes called "modulation index" or "intensity") - determines how much the modulator will affect the destination.

Waveshape - determines the contour of change. Here are the most basic waveshapes. Square - "on/off" type change sine - smooth, gradual change (described by y = Asinx) sawtooth - abrupt, one-directional change   Modulation Effects usually contain these parameters as well...

Dry/Wet -  determines the modulated signal (wet) ratio to the original signal (dry).—usually a percentage.

Feedback - determines how often the output will be multiplied and "fed back" into the input. Usually, it is a percentage or integer value.

Here are a few traditional Modulation Effects…..

Chorus

The "chorusing" effect adds a rich, thick movement to the sound and can significantly change the overall feeling. Chorus mixes a slightly delayed and modulated copy with the original signal. Delay times for chorusing are between 5-20ms.

"Mono Chorus" - works with a mono input or sums stereo inputs to mono before running them through the modulated delay.

"Stereo Chorus" - affects both the left and right stereo inputs separately and outputs the two channels individually. Two channels can share the same LFO, which often induces phase shifts up to 180 degrees. This means while the left channel cycles up, the right channel cycles down, which causes periodic cancellations and amplification. This may be a desirable effect, especially if you want to emulate a wider stereo image.

"Multi-Voice Chorus" -  uses several delays to create more complex sounds with even wider stereo images. Some devices allow each delayed signal to be modulated independently by separate LFOs. This provides a lot of control but increases CPU.

"Complex & Multi-band Chorus" - A more modern chorus. These units can produce some unique and interesting textures. Unlike the chorus effects mentioned above, these devices are somewhat advanced. They can divide a sound into separate filtered "bands" and chorus them independently. Some of the most advanced units allow users to chorus up to 6 frequency bands. Remember, if the delay times are too short, phasing occurs. This effect is not always pleasant to the ear.  

Flanger

Flanging is similar to chorusing but occurs with a shorter delay time (1-10ms). It is named after an old-school effect that happens when you gently push down on the "flange" of an analog tape recording. It creates a spacey, underwater texture you've heard thousands of times. In extreme settings, it can generate exotic robot-like effects. The parameters are similar to those in a chorus. However, some flangers have additional parameters like……

"Jet" -  determines how intense an additional tremolo effect (amplitude modulation) is on the "flanged" signal.

"Jet phase" - determines the phase offset of the tremolo generator compared to the modulator causing the flanging.  

Phaser

Like choruses and flangers, a phaser splits the signal into two identical copies, modulates one, and mixes them. The main difference is that a phaser does not usually delay the copied signal. Instead, it shifts the phase. The end result is a signal containing periodic cancellations (anti-nodes) and amplifications (nodes) at specific frequencies due to the interactions between the movement that is "in phase" and the signal that is "out-of-phase." Since phase, time, and frequency are all related mathematically, phasers share some of the same parameters as the other modulation effects we have discussed. However, few parameters are unique to phasers….

Stages or modes -  a parameter in some phaser effects for implementing the delays.

Phase difference - determines the difference between the "starting point" of the left channel's waveform and the right channel's waveform. This is a straightforward and practical way to accomplish some sense of stereo expansion.

Practical Use and Audio Examples

The chorus creates the illusion of two or more people playing together. In short, the best way to demonstrate it is on a solo instrument. Check out this fast guitar phrase before and after chorusing. Notice the feeling the effect evokes. I modulated the delay, bandwidth, and dry/wet parameters to make it a bit more interesting.

Clean Guitar

Guitar with Chorus

Flanger, unlike chorus & phaser, creates a pretty dramatic effect. If you were to look at the spectral characteristics of a flanged output signal, you'd see peaks that shift around because of the LFO. So, let's listen to how it sounds on plucked synth, accompanied by distortion and some reverb.

Plucked Synth

Plucked Synth with Flanger and Distortion

Phaser is commonly used on various sources, including acoustic guitar, piano, synth pads, etc...

Consequently, it creates a resonant, other-worldly, swirling effect. For this example, I will use it on a short Rhodes type of sound.

Clean Rhodes

Clean Rhodes with Phaser