Modulated Short Delay Effects: Chorus & Flanger Explained 

Chorus

In music, a chorus effect occurs when individual sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one. While similar sounds coming from multiple sources can occur naturally (as in the case of a choir or string orchestra), it can also be simulated using an electronic effects unit or signal processing device.

The chorus effect is created by adding a slightly delayed, pitch-modulated version of a sound to the original sound, in roughly equal proportions. The intention is to create the illusion that two or more instruments are playing the same part at the same time.

The chorus is widely used on  clean electric guitar and keyboard pad, it can yield very dreamy or ambient sounds.

Chorus, however, has the effect of 'de-localising' a sound: It sounds rich and wide, but you don't really know where it's coming from, and the psycho-acoustic outcome is that it sits further back in the mix.

Chorus Device Parameters

Rate: The rate dictates how fast the modulation happens. This parameter is described as a frequency (usually 0.1 to 10 Hz). The frequency actually doesn't refer to a pitch; rather, it describes how many times per second (Hz) the oscillation happens. The oscillation is controlled by the depth parameter.

Depth: The depth parameter controls the amount of pitch modulation that’s produced by the chorus. The settings are often arbitrary (you can get a range of 1 to 100). This range relates to a percentage of the maximum depth to which the particular chorus can go, rather than an actual level.

Delay: The pre delay setting affects how far out of time the chorus’s sound is in relation to the original. This setting is listed in milliseconds, and the lower the number, the closer the chorused sound is to the original in time.

Feedback: The feedback control sends the affected sound from the chorus back in again. This allows you to extend the amount of chorusing that the effect creates. This setting can also be called stages in some systems.

Effect Level: This could also be called mix in some systems. The effect level controls how much effect is sent to the aux return bus. This allows you to adjust how affected the sound becomes.

Flanger

Flanging is an audio effect produced by mixing two identical signals together, with one signal delayed by a small and gradually changing period, usually smaller than 20 milliseconds. 

This produces a swept comb filter effect: peaks and notches are produced in the resultant frequency spectrum, related to each other in a linear harmonic series. Varying the time delay causes these to sweep up and down the frequency spectrum. A flanger is an effects unit dedicated to creating this sound effect.




Part of the output signal is usually fed back to the input (a "recirculating delay line"), producing a resonance effect which further enhances the intensity of the peaks and troughs. The phase of the fed-back signal is sometimes inverted, producing another variation on the flanging sound.

The underlying technology is almost identical to that of chorus, except that chorus tends to use slightly longer delay times and doesn't feed any of the output signal back to the input. A flanger creates its deep, almost resonant whooshing effect by feeding some of the delay output signal back to the input, and although this has no counterpart in manual tape flanging, the effect is pretty dramatic. If you were to look at the spectral characteristics of the output signal, you'd see a whole series of strong peaks and notches in the response; these move across the audio spectrum under the control of the modulating LFO.

Flanging works best on harmonically rich sounds, but it is also strong enough to show up clearly on clean guitar, drums, or even vocals. Very often its done differently in the left and right speaker to give it a back-and-forth kind of swirly, wide stereo presence.

Flanger effect Parameters

Delay : This parameter changes the time it takes for the second signal to play after the original. Most times the highest setting will not be more than 20 milliseconds, as mentioned above. In some cases, the delay setting can be to to a negative value to create interesting ambient effects as well.

Depth : This parameter functions as the control for the ''warble'' of the flange effect. That is, the severity of the changes in pitch.

Width : This parameter is somewhat similar to Depth, but has a noticeable difference to the ear. It controls the speed at which the peaks and valleys of the flange are reached.

Rate : This parameter controls the rate at which the warbles repeat themselves. The faster the speed, the faster your audio signal will go through the complete flange process.

LFO : This parameter enable you to alter the output of the wave in accordance to the flange. The four possible settings are sine, square, saw, and triangle.

Feedback : Feedback loops the output signal back into the input, creating a possibly infinite amount of noise. This results in many strange effects, especially in higher settings. If you're using headphones, be sure to keep the volume low before you experiment with this parameter.



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Sources:

http://www.soundonsound.com/sos/jan98/articles/learnprocessors.htm
http://www.eumus.edu.uy/eme/ensenanza/electivas/csound/materiales/book_chapters/30multieffects/30multieffects.html