Basic building block for constructing a phaser effect. The output is a filtered version of the input, employing a time-varying 2-pole all-pass filter. IMPORTANT: In order to achieve a 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 settings of the time-varying all-pass filter, adjustable via the Parameter Window, as summarised in the following table. Since the intended use is in parallel with the original signal to create a notch, the all-pass parameters are described in terms of the resulting notch. Stereo signals have separate adjustments for left and right channels. Parameter Purpose "Lower" slider Sets the lower limit for the center-frequency of the sweeping notch (within the allowable range extending from 0 up to the Nyquist frequency (i.e. half the sample rate)). "Upper" slider Sets the upper limit for the center-frequency of the sweeping notch (within the allowable range extending from the lower limit up to the Nyquist frequency). "Rate" slider Sets the rate (in Hz) at which the notch center-frequency sweeps through its profile from the lower limit to the upper limit. The sweep follows an exponential rise from the lower limit to the upper limit, followed by an exponential decay from the upper limit to the lower limit. "Skew" slider Adjusts the time-asymmetry of the shape of the exponential frequency sweep. A value of 0.5 corresponds to a symmetric profile (i.e. the rise profile is a mirror image of the decay profile). "Width" slider Adjusts the bandwidth of the notch as a fraction of the instantaneous center-frequency. As the center-frequency sweeps through its exponential profile, the bandwidth thereby adjusts accordingly. "Depth" slider Adjusts the depth of the notch filter. 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. See [Zo] for further information on 2-pole filters for audio applications. The all-pass filter has time-varying coefficients (following an exponential rising and falling profile), such that 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. The filter coefficient time-history for the exponential variation is stored in a wavetable, and is scanned using a table-lookup oscillator algorithm (see [Moore] chapter 3.1.7 for further information). See [HRM] for general information on audio effects.
