Stereo phaser built from multiple variable allpass filters
An elaboration of the algorithm illustrated in the "Stereo Phaser Built From Allpass" DrawingBoard to include
multiple time-varying allpass filters in series. Since the response of a series of allpass filters also exhibits allpass
characteristics, then by connecting a series of time-varying allpass filters in parallel with a dry signal path, an elaborate
time-varying multi- notch filter is obtained. This allows you to build elaborate multi-element phasers.
The heart of this phaser algorithm is the Variable 2-pole allpass filter whose frequency response has a flat
magnitude spectrum but a nonlinear phase response. When the allpass filter is connected in parallel with the original dry
signal (via the Linear Mixer), the net effect is a notch filter. The key to achieving the classical "phaser" effect is to vary the
allpass filter coefficients in time, such that the notch filter varies in time. The Variable 2-pole allpass block is designed for
this purpose. It contains parameters for adjusting the exponential sweep of the the instantaneous centre frequency and
bandwidth of the of the notch. By chaining multiple allpass filters together, then multiple notches with different
characteristics can be efficiently implemented.
The limiter ensures that the output doesn't cause digital clipping arising from constructive interference of the dry signal with
the notched signal.
Try changing all parameter settings of all components to achieve a wide range of interesting feedback phasing effects.
Also, try replacing one (or more) of the "Variable 2-pole allpass" blocks with a "Variable feedback allpass" block(s) in
order to build an elaborate feedback phaser.
