Basic building block for constructing a controllable phaser effect. The audio output is a filtered version of the audio input, employing a time-varying 2-pole all-pass filter whose coefficients are dynamically adjusted according to the control signal inputs. IMPORTANT: In order to achieve a controllable phaser effect, this component must be connected in parallel with the original signal, thereby creating a time-varying notch filter. (On its own, it simply acts as a time-varying all-pass filter and will not produce a phaser effect.) The characteristics of the resulting notch filter depend on the control inputs to the time-varying all-pass filter. The first control input controls the instantaneous center-frequency (in Hz) of the notch; the second control input controls the instantaneous bandwidth (in Hz) of the notch. The depth of the notch is adjusted via the slider in the Parameter Window. Maximum depth (lower limit of slider) implies a deep notch; minimum depth (upper limit of slider) implies a shallow notch. The depth is fixed in time. NOTE: A multi-stage (i.e. multi-notch) phaser can be obtained by connecting multiple copies of this component in series, then connecting this cascade in parallel with the original signal. (This works because a cascaded series of all-pass filters retains the all-pass characteristics, which results in a set of notches when connected in parallel with the original signal.)
This component is implemented using a 2-pole IIR all-pass filter with time-varying coefficients. See [Zo] for further information on 2-pole filters for audio applications. When connected in parallel with the original signal, the resulting notch filter will have a time-varying center-frequency and bandwidth, as required for creating the phaser effect. See [HRM] for general information on audio effects.
