Comb Filter

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Overview

Comb Filter implements stereo feedforward or feedback comb filters with optional cross-linked feedback between sides and optional per-side phase shift, low-pass filtering on feedback, and soft-clip waveshaping with adjustable saturation. It can be run as an SFX or as an adjustable reverb, and at some settings can produce sounds similar to flangers or phase-shifters. The filter resonant frequency, the feedback gain (Depth), and the phase shift can all be modulated independently for each side, with adjustable range of modulation.

The left and right sides of the filter operate independently, unless either or both channel Link buttons are selected, enabling cross-channel feedback linking. The phase adjustment affects only one channel at a time, as its function is to shift one channel by a time delay relative to the other, and it would make no sense to allow both to be adjusted simultaneously.

The Depth controls can operate in a simple percentage of gain mode, or in a mode where the feedback gain is based on an RT60 reverberation decay time, as set by the Depth Mode toggle button. This affects both channels, so it's not possible to run one side in linear gain mode and the other in RT60 mode. In general, you'll likely want to engage the LPF gain filters if you're running in RT60 mode, to achieve a more realistic reverb effect.

Controls

Except where noted, the following controls exist independently for each channel. They're described as they appear on the module interface, from top to bottom.

• The Control Coupling multistate toggle determines which set of controls, left or right, controls which channel(s). The default, Ind, for Independent, gives control of each channel to its own controls. The other two options give control of both channels to either the left channel controls, or the right channel controls, with the inactive controls disabled. As with all Cyberwerks multistate toggles, clicking repeatedly on it cycles through the states, Alt-clicking cycles in reverse order, and Ctrl-clicking brings up a shorcut menu containing all the button's states.

• The Frequency (Freq) knobs set the tuning of the comb filters in Hertz. Lower frequencies will tend to sound more echo-like, and higher ones more complex and, perhaps, metallic.

• The Depth knobs control the amount of feedback (or feedforward) in the comb filters. In linear mode, this is specified as a direct percentage, while in RT60 mode it's specified as a reverb time in milliseconds.

• The Depth Mode toggle button determines whether the Depth controls operate in linear or RT60 mode. The single button controls both left and right channels.

• The Frequency CV jacks and knobs allow the comb filter frequencies to be top-down modulated by an externally-supplied, zero-centered, bipolar waveform in the range [-5V, 5V]. The knobs specify the percentage amount [0%, 100%] of the frequency range the modulation will range over.

• The Depth CV jacks and knobs allow the comb filter feedback amounts to be top-down modulated by an externally-supplied, zero-centered, bipolar waveform in the range [-5V, 5V]. In linear mode, the knobs specify the percentage amount [0%, 100%] of the frequency range the modulation will range over, while in RT60 mode they specify the percentage amount [0%, 100%] of the reverb time the modulation will range over.

• The Phase Offset CV jacks and knobs allow the Phase Offset (see below) to be top-down modulated by an externally-supplied zero-centered bipolar waveform in the range [-5V, 5V]. The knobs specify the percentage amount [0%, 100%] of the total Phase Offset the modulation will range over. Note that if no Phase Offset is specified for a channel, neither channel will be modulated, no matter what CV signal is input. Taking the Phase Offset controls into account (see below), it should be clear that at most only one channel will be modulated at any one time.

• The Feedback Type toggle buttons specify for each channel whether the comb filter will be a feedback or feedforward type.

• The Phase Offset (Pha) toggle buttons and knobs set the amount one channel will lag the other one, over the range [0%, 100%]. The buttons are programmed so that only one can be active at any given time, although both channels' knobs can be used to set values. This is because it only makes sense to have one channel delayed relative to the other one, not both delayed at the same time, even if by different amounts.

• The Low-pass Filter (LPF) toggle buttons and knobs enable/disable the LPF filters in the feedback/feedforward loops of the comb filters and allow their gain to be adjusted over the range [1%, 95%].

• The Feedback Link (Link) toggle buttons and knobs enable/disable feedback links between channels, and allow the feedback gain to be adjusted over the range [1%, 95%].

• The Soft-Clip Waveshaper (Clip) toggle buttons and knobs enable/disable post-filter soft-clip waveshaping, and allow the saturation amount to be adjusted over the range [1, 0.02], where 1 represents no saturation, and 0.02 represents extreme saturation.

• The Input/Output VU Meters show in real time the logarithmic values present at the inputs and outputs (see below).

• The Input (In) jacks and attenuverters accept and adjust input to the two filter channels. These values are shown in the Input VU Meters.

• The Output (Out) jacks and attenuverters output and adjust the two filter channels' processed signals. These values are shown in the Output VU Meters.