The flanger effect is similar in its implementation to the chorus. However, its sound is different. Like the chorus, the flanger created by mixing the original signal with a delayed version of itself, where the amount of delay is slowly and periodically changing. However, with the flanger the amount of delay is smaller. With the longer delays of the chorus you of hear two different versions of the sound. With the shorter delays of the flanger the effect is spectral. That is, you hear one sound whose spectrum is changing. This is because adding the delayed sound to itself creates a comb filter which has notches at regularly spaced frequencies. As the amount of delay increases the notches get closer together, and as the delay decreases the notches grow farther apart. This creates the typical whooshing effect of the flanger, which can be mostly heard on wideband sounds such as noise. By adding feedback to the flanger the notches are transformed into peaks, making the effect more dramatic. Since the delay length is changing whenever the mod_depth parameter is greater than zero, the number and location of peaks will be gradually changing.
For the two and four-channel flanger, each channel has its own delay line. The input to the delay line is a mixture of the input signal and the feedback signal from the output of the delay line. The effect out is a mixture of the delay line input and the delay line output. There is a low-frequency sinusoidal oscillator, which is used to change the delay length of each delay line. Since the desired delay time may not be an integer number of samples the delay line output is linearly interpolated between adjacent samples. The output of the effect is adjusted to keep the overall gain caused by the effect the same as the amount of feedback changes. In the reference code this normalization is factored into the feedfwd and delaygain coefficients.
The 5.1 six-channel flanger is the same as the 4-channel chorus, but it has two additional channels. However, the low frequency effect (LFE) does not have any flanging applied to it. This is because the spectral effect caused by the flanger will not be heard well at low frequencies so we do not apply it. The LFE channel does have a gain added to keep the level equivalent to the other channels level.
2013/11/22 Initial version.