Adventures In Audio
Weird and wonderful sounds using the Air Music Tech Chorus plug-in

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Monday December 14, 2020
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This video demonstrates the full range of possibilities of chorussing using the Air Music Tech Chorus plug-in on sine wave and electro-acoustic guitar. Videos in this post are best watched in full-screen mode.

Transcript

The chorus effect takes a signal, delays it slightly, then modulates the delay time up and down. This modulated signal is mixed with the original to create movement and interest in a sound.

Here we will use the Air Music Tech Chorus plug-in, which is simple to use and very effective.

In this exploration we will use a simple 220 Hz sine wave and an electro-acoustic guitar to examine the effects of the controls, and we will take the settings to their extreme values to examine the range of sound textures that are possible, many of which would not be useful musically but could be used for special effects.

As chorus is often best used as a stereo effect, we will use a file that has the same sine wave signal on both channels as the source. In this demonstration we will use a vectorscope as a visual aid to be able to see the stereo effect of the chorus as well as hear it.

Here is the sine wave with the chorus plug-in bypassed.

Notice that in the vectorscope the trace is a vertical straight line. This shows that the signal, being the same on both channels, is mono.

Muting the right channel, so that the signal is only on the left, does this...

Muting the left channel, so that the signal is only on the right, does this...

The upper quadrant of the vectorscope between L and R, and the mirror-image lower quadrant, represent signals that are mono or stereo. If the trace extends to the +S or -S quadrants then it contains information that is out of phase. If the trace is a horizontal line between +S and -S then it is completely out of phase.

Let's start with the controls at their default settings, which clearly Air Music Tech believes to be a generally useful starting point, with the exception that we'll set the depth to zero so there is no delay and therefore no chorus effect. We will then gradually raise the depth from zero to maximum, then return to a setting that would commonly be useful. The calibration of depth in milliseconds, by the way, refers to the maximum delay time.

What we can notice is that at first the tips of the vectorscope trace stay within the boundaries of left and right, but at a certain point they stray into the out-of-phase quadrants. Let's see that again...

Here is the point where the tips of the trace just touch the edges of the stereo zone, which in this example for a 220 Hz sine wave is a depth (maximum delay) of 0.81 milliseconds...

This is interesting, and also of relevance because audio that contains out-of-phase information can cause problems for mono compatibility, and is something that a wise engineer will always check.

Rate

To continue, we'll reset the depth to its default starting point.

Next we will sweep the rate control through its range of settings.

Clearly there is a range of rates that would potentially be musically useful, and higher rates might be useful for special effects, particularly science-fiction sound effects. It's worth bearing in mind that it is widely considered that vibrato in singing should have a rate around 6 Hz. This doesn't mean that you have to set the chorus rate to 6 Hz, which in this example is too high to sound musical, but it's always worth having a reference point in mind when making artistic decisions.

Pre Delay

To continue, we'll set the controls back to their defaults.

Pre delay is an overall delay that is applied to the modulated signal. Again we will sweep the control through its range of settings.

This has less of an effect on a sine wave signal than a real-world music signal so we will apply it to a recording of electro-acoustic guitar, which is a good source for demonstrating any type of time-modulation effect.

As you can hear, if you listen closely, the sound gets richer with longer pre-delay times, and there is probably a point at which the degree of richness is optimum. This will vary according to the instrument or vocal that you are chorussing.

LFO L/R Phase

LFO (low-frequency oscillator) L/R Phase represents the relative phase of the modulating signal in the left and right channels

If the phase is set to zero, then both channels are in step and the result is mono. The overall level modulates but there is no stereo image shift.

If the phase is set to +180 degrees or -180 degrees then when the left channel is at, for instance, 3 milliseconds of delay, the right channel is advanced (with respect to the input signal) by the same amount, 3 milliseconds. This will create a stereo image that swings widely between left and right.

The default setting in this plug-in is +90 degrees, which sounds subjectively the same as -90 degrees. The 90 degree relationship, sometimes known as quadrature, places the modulating signals in the left and right channels one quarter of a wavelength out of step, which seems to be a good starting point for chorussing although all options should be explored.

LFO Waveform

LFO Waveform offers the choice of sine or triangle wave. There is a very subtle difference between the two. The triangle wave, other than going back and forth, has a constant rate of change. The rate of change of the sine wave varies throughout its cycle. The sine wave therefore sounds more 'swirly' than the triangle, both in the subjective texture of the sound, and in the pan of the signal. we will demonstrate this using both the sine wave and electro-acoustic guitar.

Feedback

The feedback control will work on the sine wave, but other than for science-fiction effects (which we'll come to later) does not provide a useful demonstration. So the electro-acoustic guitar in this case is better source material.

The feedback control works very much in combination with pre-delay so we will step through a range of combinations of settings, some of which will work musically better than others.

Mix

Finally the Mix control. Normally in any plug-in the mix controls the amount of effect you want to hear. But in the case of Chorus, it can dramatically change the sound texture.

Here is a sweep of the mix control on electro-acoustic guitar with otherwise default settings.

It's pretty much an 'amount' control. But here it is with a high level of feedback.

Sci-fi sound effects

As mentioned earlier, this chorus plug-in is capable of a useful range of sci-fi oriented sound effects. Here are a few.

More weird effects

Effects can get weirder when the controls are in motion. Here are a few demonstrations.

Summary

The chorus effect can be both musically useful and capable of creating interesting sound textures. These demonstrations have only scratched the surface of the possibilities.

 

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