Cheat Sheet:Modulation Effects

Cheat Sheet delivers concise, explicit information about specific recording/audio-related tasks or processes. This installment describes the parameters found in modulation effects such as flanging and chorusing.
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Modulation effects vary the delay time around a particular initial delay time, usually specified in milliseconds. Shorter delay times (0–15ms) are common with flanging, longer delay times (10–20ms) with chorusing. Attempting to change this value in real time often causes zippering, so it’s generally a set-and-forget parameter.


Some chorusing processors have several delay lines, each with its own set of controls. This can create a more complex, evolving sound compared to using a single delay line. Each set of delay lines and controls is called a voice. Chorus units typically have 1, 2, 4, or 8 voices.


In modulation effects with multiple voices, you’ll generally find a Pan control for each voice to place it as desired in the stereo field.


There are many possible modulation sources. An LFO provides a cyclic change at a particular (usually slow) rate. Other sources include envelope follower (tracks the dynamics of the input signal), footpedal, MIDI continuous controller, etc.


Also called Modulation Width. This sets the difference between the minimum and maximum delay time. With chorusing, this is usually a relatively small difference, such as between 12 and 15ms. Flanging covers a much wider range, such as 0–15ms. High depth settings coupled with high rate settings tend to give a “warped record” effect, with unwanted pitch changes. Either lower the depth, or slow the rate.


This determines how the Low Frequency Oscillator-induced modulation varies over time. Examples: A square wave switches between two different delay times. A triangle wave varies linearly from a maximum to minimum delay time, then back again. A random wave changes the delay time randomly.


Also called LFO Speed or LFO Frequency. The Rate sets the frequency of the modulation LFO. Typically, faster rates are used for chorusing, and slower ones for flanging.


Choruses with multiple voices often have multiple LFOs (even a stereo chorus might have one LFO for each channel), and the Phase parameter varies the phase between the LFOs. Example: You might set two LFOs 180 degrees out of phase to modulate the channels in an equal and opposite manner. Increasing phase tends to increase the apparent stereo field width, as there’s less correlation between the right and left channels.


LFOs can usually be either free-running (the frequency is fixed at a user-set rate) or in the case of a software plug-in (or hardware unit with MIDI input), sync to the tempo of a host program or master clock. Tempo sync values are expressed as rhythmic values (1/4 note, 1/8 note, 1/8 note triplet, dotted 1/2 note, etc.)


This provides a mathematical integration of the LFO waveform (rough translation: it rounds off square edges). It’s used mainly with a random waveform to change the standard “stairstep” shape into a smoother curve, thus producing smoother delay time changes.


Sends audio from the output back to the input, creating a more resonant sound. There may be separate feedback controls for each channel or voice.


Also called Feedback Invert. Positive feedback feeds the audio back in-phase and creates a “sharper” sound. Negative feedback feeds back audio out-of-phase and creates a “hollower” type of sound, like a resonant tube.


With stereo effects, this feature feeds back audio from one channel to the other channel’s input. This can create a more complex effect than simply feeding back the output of one channel to its own input. Feedback and Cross-Feedback can sometimes both be present.


Also called High Cut, Rolloff, etc. This inserts a filter in the feedback path that reduces high frequencies, creating a somewhat more subdued feedback sound. Additional filtering may be available to tailor the low and mid frequencies as well.


Sets the ratio of processed to unprocessed sound. Typically with flanging, these are mixed equally to provide the most dramatic flanging effect. With chorusing, the delayed signal is often mixed a bit lower than the dry signal to prevent the chorus effect from “taking over.” When set for wet sound only, it may be possible to obtain vibrato effects by shifting pitch cyclically without mixing it with the dry signal.


In addition to feedback phase, a flanger might have a Mix Phase switch that allows throwing the wet signal out-of-phase. This produces a different type of tonality for flanging and to a less extent, chorusing.


Also called Tape mode. With throughzero flanging, the delayed sound is mixed with a very slightly delayed dry sound. This allows the delayed signal to pass through a point where there is zero delay time between the dry and processed signals.