What is chorus and how does it work?
Frequency or pitch.
Before we jump into understanding how chorus works, and creating chorus effects, it’s handy to have a basic idea what makes an audio signal sound like its does. One characteristic of a musical sound is that it is more or less cyclical.
This means that the sound waves repeat a regular pattern moving between positive and negative cycles of the signal. The amount of time it takes for this signal to complete one cycle is the signal’s frequency in Hertz (Hz).
Another characteristic of all the components making up a sound wave is the phase between these components.
What is Phase?
Phase in audio is the position of a sound wave at a particular point in time, and our interested will be in the amplitude of the signal at that point.
Think of our sound wave on a graph. The phase of a wave is its position along the x-axis (time) relative to the start of the wave’s cycle.
When discussing the phase of a sound wave, we refer to it in of degrees (similar to how we think about a position the circumference of a circle). One full wave cycle is defined as 360º and each multiple of 360º marks the beginning of a new wave cycle. To keep things simple, we’ll use a sine wave to illustrate this concept.
Phase difference.
When looking at phase relationship between two or more waveforms of the same frequency, we can say that these signals are either in-phase (0 degrees), out-of-phase (180 degrees), or anything in between. It’s these phase relationships between audio signals that creates the audio effects that we call chorus, flanging, and phasing.
In the diagram below we have two audio sine waves of the same frequency, that are out of phase by 90 degrees.
How phase affects sounds.
There are two phase relationships to keep in mind:
a) Constructive (in-phase)
b) Destructive (out-of-phase) interference which is more likely to occur when identical signals are layered together, as is the case with chorus, flangers, and phasers.
Constructive interference: in-phase
When two signals of the same pitch are in-phase, it means that they are perfectly aligned in time, and if they are the same amplitude we will have a voltage difference of 0 volts between them.
Think of when you record a track and then you duplicate it without making any changes to differentiate them from one another.
This is an example of constructive interference: the identical waveforms will add together to produce a signal with amplitude equal to the sum of the individual waveforms (+6 dB of gain on a meter).
Destructive interference: out-of-phase
When two waveforms are perfectly out-of-phase (180 degrees), it means one waveform’s peak (highest point) is being subtracted from the other waveform. If the two are of identical volume and pitch they will cancel each other out.
We can see the results of in Phase and Out of phase signals below. The signals of the left are in phase and add to each other, the signals of the right are out of phase and are cancelling each other.
Destructive interference: comb filtering
Comb filtering is using destructive interference in a complex audio signal where an identical copy of the signal is time shifted slightly. This delay can be anywhere from 0.1ms to 10ms. Depending on the time delay, this will result in a frequency response that looks like a comb when the two signals are mixed together. The teeth the “comb” represent the cancellation of certain parts of the spectrum, resulting in the “whooshing” sound we hear sweeping back and forth in flangers.
The diagram below shows how we achieve the flanging effect:
And this is what the end result is in terms of our output frequency spectrum.
What does all this have to do with Chorus?
“Chorusing” is meant to recreate the (usually) subtle differences in pitch and timing that occur when a number musicians or vocalists create the same note, but there are natural variations in pitch and timing.
We can describe the sound of “chorus as a sort of doubling effect that adds thickness, shimmer, and helps to create an instrument sound “larger” than it would on its own.
How does the chorus effect work?
You create a ‘chorus’ effect by creating a copy (or better, multiple copies) of a signal, the slightly varying the pitch, introducing a small delay to each copy, and then mixing these copies with the original signal.
Rather than using pitch modulation to modulate the pitch, like a vibrato effect, a chorus achieves subtle changes in pitch by modulating the delay time of the waveform with a low frequency oscillator (LFO).
These moderate differences in time and pitch cause a constantly changing
phase-shift between the original signal and the copied signals.
Timing is important.
A chorus effect will use typically use time delays in the order of 15 to 35 mS so that the ear is tricked into perceiving these delayed copies a separate instruments, and the small pitch shifts will add the rich “shimmering” sound.
We don’t want to use a delay that’s too short otherwise we are going to get more of a comb filter effect, or a delay that’s too long as then we are starting to get a “reverb” effect.
Creating a simple chorus effect in SynthEdit:
In this structure I’m using four of the stock Delay2 modules with the delay times set in the properties panel as;
15 ms (0.015), 18 ms (0.018), 20 ms (0.02) and 25 ms (0.025).
I have used two TD LFO_A_ST “stereo” LFOs with their phase plugs set to;
1.0 V and 2.5 V for the top LFO and -1.0 V and -2.5 V for the bottom LFO so that the modulation of each delay is always always slightly shifted in time. The feedback on each delay module can be left set to 0, or you could add a slider control common to all modules feedback plugs so that you can set a small amount.
Converting the design to stereo is just a matter of doubling up on the modules for left and right channels, though you’ll probably want to have the controls common to both channels. The modulation depth doesn’t need to be very great, setting the Max value on the slider to 3 V will give more than enough of a pitch variation.
Note: You will need to set the Interpolate Out setting to “On” for all the delay modules as we are modulating the delay time.
