Complete Guide To The Chorus Audio Modulation Effect

When it comes to modulation effects, chorus is certainly one of the most popular. This versatile audio effect can be heard on countless records and helps in making audio tracks sound larger than life.

In this article, we'll discuss the chorus modulation effect and how it's achieved. We'll liken it to natural acoustic chorusing and take a look at a few chorus unit examples to learn about this superb audio effect.

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What Is The Chorus Effect?

The chorus effect is a type of audio modulation effect typically used to thicken up or otherwise add interest to a sound. It works by duplicating an audio signal, varying the pitch of the copy/copies over time, and mixing the signals back together.

A chorus effect's sonic results may range from extra richness and width to added shimmer to a downright wobbly mess or pitch variation. The effect can also be used to push elements further to the back of a mix by making them less defined.

Though pitch variation is essential for chorus effects, there's no actual pitch shifting circuit that translates the signal's frequency content.

Rather, more technically, the chorus effect is a phase-shift modulation effect centred around a delay circuit and a low-frequency oscillator (LFO) that modulates the delay time of that delay circuit.

An LFO is an oscillator with a frequency below the range of human hearing (under 20 Hz). Many chorus units have an LFO frequency range between 0.1 Hz and 6 Hz (though this is certainly not a hard rule).

We'll discuss the LFO frequency range and other chorus parameters in the later section titled Chorus Parameters.

It's the modulation of this delay time that causes the pitch variation.

As the delay time is being shortened, the waveform of the delayed signal is experiencing a slight amount of time compression. This time compression shortens the waveform, thereby increasing the frequency (and pitch) of the waveform.

Conversely, as the delay time is being lengthened, the waveform of the delayed signal is experiencing a slight amount of time expansion. This time stretching elongates the waveform, thereby decreasing the frequency (and pitch) of the waveform.

So it's important that the original signal is mixed in with the phase-shifted/delayed signal(s) to achieve the chorus effect. Otherwise, we'd have a vibrato effect.

We’ll get to the basics of chorus circuits a bit later in the article. For now, let’s briefly touch on the two signal paths that are mixed together to produce the chorus effect:

• A dry/direct path that leaves the signal unaffected.
• A delay circuit path that phase-shifts the signal, which is modulated by an LFO (low-frequency oscillator).

Chorus effects can be mono, stereo, or multi-channel and can be designed with 2 voices (dry and delayed signals) or more than 2 voices. Each voice would have its own signal path that would be mixed with the others at the output.

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A Note On Acoustic Chorusing

This article is largely focused on the chorus audio effect and, therefore, the electrical side of chorusing. However, to improve our understanding, we should consider instances of natural/acoustic chorus.

In the acoustic sense, Chorus is defined as a group of people signing or otherwise playing the same note(s) on the same instruments in unison. By that definition, choruses of people are regularly found in choirs and sections of orchestras.

Note that, within these groups of people, there will often be harmonies performed. Three people singing a harmonized major triad is not necessarily a chorus effect, whereas three people singing the same note definitely is.

Though the concept of an “ideal world” is useful when discussing science and, therefore, sound and audio, let's focus on the fact that our world is definitely not ideal.

What do I mean by this?

To start, acoustic instruments (including the voice) typically won't hold a note completely steady in pitch. There will often be, even if only slightly, some amount of pitch variation as a note is sounded.

On top of that, not all instruments (including the voice) will be perfectly in tune with one another at all times.

In addition to pitch, not all instruments (including the voice) will be in perfect timing with one another.

These slight variations in time and pitch won't affect the perceived note(s) being performed so long as the chorus voices are playing/singing the same note(s). However, it will cause a thickening of the sound.

Of course, having more performers perform the same note(s) will increase the amplitude. Furthermore, the chorusing effect will widen and thicken the sound by introducing these slight variations as each voice within the chorus converges into a single perceived sound source.

Acoustic chorusing can also happen in singular instruments like the piano, where some keys are responsible for striking/sounding multiple strings. The higher keys of an acoustic piano strike 3 strings; the middle keys strike 2 strings, and the lower keys strike one string.

Each string of a key is tuned to the same note. However, the non-idealities of strings and overall piano design will cause some slight variations in pitch between the strings. In the keys that control multiple strings, the piano will exhibit some amount of natural acoustic chorusing.

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The Chorus Circuit Design

Chorus audio effect units are electrical, so it's important to understand the basics of their circuits. Note that we'll focus on analog chorus signal flow in the section, though the same ideas apply to digital chorus and the programs that run chorus software plugins.

Like many phase-shifting modulation effects (vibrato, flanger), chorus units are actually centred around a delay circuit and a low-frequency oscillator (LFO).

More specifically, the heart of chorus circuit is a delay path that has its delay time modulated by an LFO.

The most basic signal flow diagram of a [mono] chorus unit would resemble the following:

Note that there is generally no feedback loop in the delay circuit when producing the chorus effect. However, many chorus units will offer the flanging effect, which benefits from feedback (and requires shorter delay times).

So we know what the chorus effect does. Now let’s consider how it does it.

Starting with the input, we see that the dry signal is fed into the output mixer (to be mixed with the phase-shifted/delayed signal) and the delay line mixer.

The delay circuit (whether analog or digital) delays the dry signal and outputs the “phase-shifted” signal that is fed into the output mixer. The LFO alters the delay time parameter of the delay circuit.

By itself, the delay circuit would output a signal with a vibrato effect. By altering the signal's delay time, the delay circuit can affect the pitch of the signal.

More specifically, as the delay time slows down, the signal is effectively time-stretched, which lowers its frequency content and a drop in its pitch. Conversely, as the delay time speeds up, the signal is effectively time-compressed, which results in a raising of its frequency content and an increase in its perceived pitch.

Mixing these two signals together gives one dry/straight signal and one signal with both time and pitch variations. This is the basis of the chorus effect.

To achieve this effect, the modulated delay time of the delay line should be kept above 10 ms throughout the modulation. Shorter delay times (especially if there is feedback) will cause a flanger-like effect rather than a chorus effect.

Note that the LFO will have a frequency well below 20 Hz (often between 0.1 Hz to 6 Hz) in chorus effects. The waveform of the LFO is often a sine wave, though other shapes are certainly possible (triangle waves are also quite common).

Not all chorus units are mono. Let's consider a few more signal flow diagrams to show stereo and multi-voice chorus units.

Here’s a simplified signal diagram of a stereo chorus unit that flips the phase of the right channel:

Here’s a simplified signal diagram of a stereo chorus unit with 5 voices per channel:

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Chorus Parameters

Chorus is a powerful audio effect/process and has many different sonic flavours. The flexibility of this effect is due not only to the quality of the circuit design (or computer program) but also to the adjustable parameters common to chorus effect units.

Chorus effects unit/plugins will often have many (if not all) of the following controls:

• Speed/Rate
• Depth/Width/Intensity
• Mix
• High-Pass Filter

Speed/Rate

The speed or rate parameter of a chorus unit affects the frequency of the LFO and, therefore, the rate/speed at which the delay time(s) are modulated. As we've discussed, this will, in turn, affect the rate at which the pitch variation occurs in the additional “wet” voice(s).

In general, a slower speed/rate will produce a more subtle effect, while a faster speed/rate will cause increased phase-shifting and more perceived “detune” of the input signal.

Depth/Width/Intensity

The parameter known as depth, width or intensity effectively controls the amplitude of the LFO. From what we've learned, increasing the modulation range of delay times in the delay circuit will affect the amount of perceived pitch and phase shifting in the chorus output.

Because increasing the depth/width/intensity will increase the amount of pitch-variation in the wet signal. It will also cause a more intense chorus effect with a more pronounced detune effect.

Mix

A chorus effect unit's mix control will allow users to mix the direct/dry signal with the modulated delayed/wet signal(s) just before the unit's output.

A 100% wet mix will only output the modulated delayed signal(s). Note that if there is only one copy of the signal, this effect will essentially be considered vibrato.

A 0% wet (or 100% dry) mix will only output the direct signal, and there will be no chorus effect whatsoever.

High-Pass Filter

Low-end frequencies take up a lot of energy and can sometimes be poorly defined.

A high-pass/low-cut filter will remove some amount of the low-end from the modulated delayed signal(s). Phase shifting in the low-end, especially in stereo, can lead to poor results. Any pitch variation in the low-end can also lead to ill-defined bass reproduction.

As an additional note, we naturally hear low-end frequencies omnidirectionally as low-end sound waves are long and difficult to locate. For that reason, along with those listed above, low-end is best mixed in mono. That's another reason to high-pass/low-cut low-end frequencies when we're dealing with stereo chorus units.

An example to back up this claim is the piano, which has three strings per key in the highest range and two strings per key in the middle range. These strings increase the piano's amplitude at higher notes and offer some amount of chorus (they likely won't be in perfect tune). However, as we reach the lowest register, there will only be one string per key, eliminating any chance of “natural chorusing” in the low-end.

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Chorus Effect Unit/Plugin Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 5 different chorus units to help solidify our understanding of this modulation effect.

In this section, we’ll discuss:

• 500 series chorus unit: TB Audio TBDD Stereo Chorus
• 19″ rack mount chorus unit: TC Electronic 1210
• Chorus effect pedal: TC Electronic Stereo Chorus +
• Chorus Eurorack module: Feedback 106 Chorus
• Chorus plugin: Eventide TriceraChorus

TB Audio TBDD Stereo Chorus

The TB Audio TBDD Stereo Chorus is a “modern” 500 Series version of Roland's classic Dimension D rack-mount chorus from the 1970s.

This chorus unit features two identical signal paths for stereo processing. They're made from the same new old stock analog bucket-brigade device (BBD) delay chips used in the original Roland model.

The input can be set to mono or stereo, and the Blend knob mixes the phase-shifted/delayed signal with the direct signal at the output. Levels can be metered with the ten-segment LED meter.

This unit has 4 modes/presets and an off button. There's not a whole lot of tweaking to do with this chorus unit, but it still sounds great.

TC Electronic 1210

The TC Electronic 1210 is defined as a “spatial expander and stereo chorus plus flanger.” It's largely based on TC's world-famous Stereo Chorus/Flanger pedal.

This rack-mounted unit has two complete chorus/flanger circuits for stereo processing, though the device is also mono-compatible. It has balanced line level XLR inputs and outputs and instrument level 1/4″ inputs and low-impedance 1/4″ outputs (2 of each for stereo).

The inputs can be set as stereo or separate mono.

In addition to the spatial expansion capabilities (based on the Haas effect) and the possible flanger effects, the 1210 also produces a superb chorus effect in mono or stereo.

Each of the channels features an input sensitivity control and bypass switch. The two channels can be stereo-linked or set independently.

The intensity control for each channel has a single knob and 4 modes:

• Mode 1: controls the amount of direct signal.
• Mode 2: controls the amount of phase-shifted signal.
• Mode 3: controls the amount of signal regeneration (feedback) which is used only for flanger.
• Mode 4: controls the amount of inverses signal regeneration (inversed feedback) which is used only for flanger.

In addition to intensity, each channel has speed, width and delay parameters (keep the delay short for chorus).

TC Electronic Stereo Chorus +

The versatile TC Electronic Stereo Chorus + is one of the most famous chorus pedals ever. It, too, is based on the TC's world-famous Stereo Chorus/Flanger pedal. It's actually the successor of that pedal.

The TC SCF (Stereo Chorus + Pitch Modulator & Flanger) pedal has been making a name for itself since 1976 as one of the go-to chorus units for musicians worldwide.

Let’s have a look at the Stereo Chorus+ controls:

• Speed knob: controls the sweep rate from 0.1 Hz (1 sweep per 10 seconds) or 10 Hz (10 sweeps per second).
• Intensity knob: mode dependent:
  • Chorus mode: blends between chorus and clean signal.
  • Flanger mode: controls the strength of the comb filter phasing of the signal.
  • Pitch Modulation mode: blends vibrato and chorus.
• Mode toggle switch: switches between Chorus, Flanger and Pitch Modulation modes.
• Width knob: determines how wide a modulation (sweep) is produced.

A simple momentary switch can be connected via the External Bypass jack to control the bypass function remotely if need be.

Feedback 106 Chorus

The Feedback 106 Chorus is a super-simple chorus module for Eurorack modular synthesizers.

This unit has a mono input and stereo (left and right channel) outputs. There are an input level control and a bypass switch.

The chorus effect comes in two flavours (type I and type II), accessible via a toggle switch. Each chorus type is designed with XVIVE MN3009 analog bucket-brigade device chips.

Eventide TriceraChorus

The Eventide TriceraChorus is an audio plugin inspired by the classic tri-chorus units and stompbox choruses of the 1970s and early 1980s.

The TriceraChorus combines emulation of BBD-style analog tri-chorusing with its own proprietary Eventide MicroPitch detuning to produce a thick analog chorus sound in a digital plugin.

This plugin is designed with three voices (left, centre and right). The left and right voices are set to have a -120º and +120º phase shift relative to the centre voice.

Each voice is modulated by the LFO, which has a dedicated rate (frequency) control that can be synced to the DAW tempo or tapped in via the tap control. Independent Depth controls determine the amount by which the LFO modulates each voice for each voice.

Detuning controls help to widen the sound further, and a few parameters can be controlled via the envelope follower. Chorus mix, minimum delay time and tone controls are also available for tweaking in this powerful chorus plugin.

Related Questions

What is the difference between chorus and flanger? Chorus and flanger are based on the same basic design: a modulated delay circuit that causes phase modulation in the mixed output signal. The chorus effect sounds like multiple voices with slight detuning, while the flanger effect sounds like a sweeping comb filter. The major differences in design are:

• Flangers typically only use two voices (wet and dry). Chorus may have multiple wet voices (though a 2-voice effect is also common).
• Flangers have shorter delay times (below 10 ms), while choruses have longer delay times (above 10 ms). There is crossover between these values.
• Flangers utilize a feedback loop in the delay circuit while choruses do not.

What is the vibrato effect in audio? Vibrato is a fast but slight up-and-down variation in pitch. Vibrato is used in signing and in instruments to add character and improve tone.